14.4: Structure of Bone - Biology

14.4: Structure of Bone - Biology

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Roasted Bone Marrow

Do you recognize the food item in the top left of this photo in Figure (PageIndex{1})? It’s roasted bone marrow, still inside the bones. It’s considered a delicacy in some cuisines. Marrow is a type of tissue found inside many animal bones, including our own. It’s a soft tissue that in adults may be mostly fat. You’ll learn more about bone marrow and other tissues that make up bones when you read this concept.

Bones are organs that consist primarily of bone tissue, also called osseous tissue. Bone tissue is a type of connective tissue consisting mainly of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and hard mineral crystals makes bone tissue hard without making it brittle.

Bone Anatomy

There are several different types of tissues in bones, including two types of osseous tissues.

Types of Osseous Tissue

The two different types of osseous tissue are compact bone tissue (also called hard or cortical bone) tissue and spongy bone tissue (also called cancellous or trabecular bone).

Compact bone tissue forms the extremely hard outside layer of bones. Cortical bone tissue gives bone its smooth, dense, solid appearance. It accounts for about 80 percent of the total bone mass of the adult skeleton. Spongy bone tissue fills part or all of the interior of many bones. As its name suggests, spongy bone is porous like a sponge, containing an irregular network of spaces. This makes spongy bone much less dense than compact bone. Spongy bone has a greater surface area than cortical bone but makes up only 20 percent of bone mass.

Both compact and spongy bone tissues have the same types of cells, but they differ in how the cells are arranged. The cells in the compact bone are arranged in multiple microscopic columns, whereas the cells in the spongy bone are arranged in a looser, more open network. These cellular differences explain why cortical and spongy bone tissues have such different structures.

Other Tissues in Bones

Besides cortical and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum. You can see these tissues in Figure (PageIndex{2}).

  • Bone marrow is a soft connective tissue that is found inside a cavity, called the marrow cavity. There are two types of marrow in adults, yellow bone marrow, which consists mostly of fat, and red bone marrow. All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones. Red bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.
  • Periosteum is a tough, fibrous membrane that covers the outer surface of bones. It provides a protective covering for cortical bone tissue. It is also the source of new bone cells.

Bone Cells

As shown in Figure (PageIndex{3}), bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.

  • Osteoblasts are bone cells with a single nucleus that make and mineralize bone matrix. They make a protein mixture that is composed primarily of collagen and creates the organic part of the matrix. They also release calcium and phosphate ions that form mineral crystals within the matrix. In addition, they produce hormones that also play a role in the mineralization of the matrix.
  • Osteocytes are mainly inactive bone cells that form from osteoblasts that have become entrapped within their own bone matrix. Osteocytes help regulate the formation and breakdown of bone tissue. They have multiple cell projections that are thought to be involved in communication with other bone cells.
  • Osteoclasts are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the minerals in bone and release them into the blood.
  • Osteogenic cells are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.

Bone is a very active tissue. It is constantly remodeled by the work of osteoblasts and osteoclasts. Osteoblasts continuously make new bone, and osteoclasts keep breaking down bone. This allows for minor repair of bones as well as homeostasis of mineral ions in the blood.

Microscopic Anatomy of The Compact Bone

The basic microscopic unit of bone is an osteon (or Haversian system). Osteons are roughly cylindrical structures that can measure several millimeters long and around 0.2 mm in diameter. Each osteon consists of lamellae of compact bone tissue that surround a central canal (Haversian canal). The Haversian canal contains the bone's blood supplies. The boundary of an osteon is called the cement line. Osteons can be arranged into woven bone or lamellar bone. Osteoblasts make the matrix of bone which calcifies hardens. This entraps the mature bone cells, osteocytes, in a little chamber called lacunae. The osteocytes receive their nutrition from the central (Haversian) canal via little canals called canaliculi. All of these structures plus more are visible in Figure (PageIndex{4}).

Types of Bones

There are six types of bones in the human body based on their shape or location: long, short, flat, sesamoid, sutural, and irregular bones. You can see an example of each type of bone in Figure (PageIndex{5}).

  • Long bones are characterized by a shaft that is much longer than it is wide and by a rounded head at each end of the shaft. Long bones are made mostly of compact bone, with lesser amounts of spongy bone and marrow. Most bones of the limbs, including those of the fingers and toes, are long bones.
  • Short bones are roughly cube-shaped and have only a thin layer of cortical bone surrounding a spongy bone interior. The bones of the wrists and ankles are short bones.
  • Flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum (breast bone).
  • Sesamoid bones are embedded in tendons, the connective tissues that bind muscles to bones. Sesamoid bones hold tendons farther away from joints so the angle of the tendons is increased, thus increasing the leverage of muscles. The patella (knee cap) is an example of a sesamoid bone.
  • Sutural bones are very small bones that are located between the major bones of the skull, within the joints (sutures) between the larger bones. They are not always present.
  • Irregular bones are those that do not fit into any of the above categories. They generally consist of thin layers of cortical bone surrounding a spongy bone interior. Their shapes are irregular and complicated. Examples of irregular bones include the vertebrae and the bones of the pelvis.

Feature: Reliable Sources

Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often cure many life-threatening conditions, including leukemia, lymphoma, sickle cell anemia, and thalassemia. If a bone marrow transplant is successful, the new bone marrow will start making healthy blood cells and improve the patient’s condition.

Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:

  1. How does one become a potential bone marrow donor?
  2. Who can and who cannot donate bone marrow?
  3. How is a bone marrow donation made?
  4. What risks are there in donating bone marrow?


  1. Describe osseous tissue.
  2. Why are bones hard but not brittle?
  3. Compare and contrast the two main types of osseous tissue.
  4. What non-osseous tissues are found in bones?
  5. List four types of bone cells and their functions.
  6. Identify six types of bones, and give an example of each type.
  7. True or False. Spongy bone tissue is another name for bone marrow.
  8. True or False. Periosteum covers osseous tissue.
  9. Compare and contrast yellow bone marrow and red bone marrow.
  10. Which bone is mostly made of cortical bone tissue?

    A. Pelvis

    B. Vertebrae

    C. Femur

    D. Carpal

  11. a. Which type of bone cell divides to produce new bone cells?

    b. Where is this cell type located?

  12. Where do osteoblasts and osteocytes come from, and how are they related to each other?

  13. Which type of bone is embedded in tendons?

  14. True or False. Calcium is the only mineral in bones.

Explore More

Watch this entertaining and fast-paced Crash Course video to further explore bone structure:

Check out this video to learn more about bone remodeling:

14.4: Structure of Bone - Biology

Bones are made of a combination of compact bone tissue for strength and spongy bone tissue for compression in response to stresses.

Learning Objectives

Distinguish between compact and spongy bone tissues

Key Takeaways

Key Points

  • Compact bone is the hard external layer of all bones that protects, strengthens, and surrounds the medullary cavity filled with marrow.
  • Cylindrical structures, called osteons, are aligned along lines of the greatest stress to the bone in order to resist bending or fracturing.
  • Spongy or cancellous bone tissue consists of trabeculae that are arranged as rods or plates with red bone marrow in between.
  • Spongy bone is prominent in regions where the bone is less dense and at the ends of long bones where the bone has to be more compressible due to stresses that arrive from many directions.

Key Terms

  • trabecula: a small mineralized spicule that forms a network in spongy bone
  • epiphysis: the rounded end of any long bone
  • osteocyte: a mature bone cell involved with the maintenance of bone
  • osteon: any of the central canals and surrounding bony layers found in compact bone

Bone Tissue

Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue. Osteocytes, the living cells of bone tissue, form the mineral matrix of bones. There are two types of bone tissue: compact and spongy.

Compact Bone Tissue

Compact bone (or cortical bone), forming the hard external layer of all bones, surrounds the medullary cavity (innermost part or bone marrow). It provides protection and strength to bones. Compact bone tissue consists of units called osteons or Haversian systems. Osteons are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi which transport blood. They are aligned parallel to the long axis of the bone. Each osteon consists of lamellae, layers of compact matrix that surround a central canal (the Haversian or osteonic canal), which contains the bone’s blood vessels and nerve fibers. Osteons in compact bone tissue are aligned in the same direction along lines of stress, helping the bone resist bending or fracturing. Therefore, compact bone tissue is prominent in areas of bone at which stresses are applied in only a few directions.

Components of compact bone tissue: Compact bone tissue consists of osteons that are aligned parallel to the long axis of the bone and the Haversian canal that contains the bone’s blood vessels and nerve fibers. The inner layer of bones consists of spongy bone tissue. The small dark ovals in the osteon represent the living osteocytes.

Spongy Bone Tissue

Compact bone tissue forms the outer layer of all bones while spongy or cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons. Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Red bone marrow is found between the trabuculae. Blood vessels within this tissue deliver nutrients to osteocytes and remove waste. The red bone marrow of the femur and the interior of other large bones, such as the ileum, forms blood cells.

Arrangement of trabeculae in spongy bone: Trabeculae in spongy bone are arranged such that one side of the bone bears tension and the other withstands compression.

Spongy bone reduces the density of bone, allowing the ends of long bones to compress as the result of stresses applied to the bone. Spongy bone is prominent in areas of bones that are not heavily stressed or where stresses arrive from many directions. The epiphysis of a bone, such as the neck of the femur, is subject to stress from many directions. Imagine laying a heavy-framed picture flat on the floor. You could hold up one side of the picture with a toothpick if the toothpick were perpendicular to the floor and the picture. Now, drill a hole and stick the toothpick into the wall to hang up the picture. In this case, the function of the toothpick is to transmit the downward pressure of the picture to the wall. The force on the picture is straight down to the floor, but the force on the toothpick is both the picture wire pulling down and the bottom of the hole in the wall pushing up. The toothpick will break off right at the wall.

The neck of the femur is horizontal like the toothpick in the wall. The weight of the body pushes it down near the joint, but the vertical diaphysis of the femur pushes it up at the other end. The neck of the femur must be strong enough to transfer the downward force of the body weight horizontally to the vertical shaft of the femur.

What Are Cranial Bones?

Cranial bone development starts in the early embryo from the neural crest and mesoderm cells. The cranial bones develop by way of intramembranous ossification and endochondral ossification. Endochondral ossification replaces cartilage structures with bone, while intramembranous ossification is the formation of bone tissue from mesenchymal connective tissue.

The cranial bones of the skull join together over time. They must be flexible as a baby passes through the narrow birth canal they must also expand as the brain grows in size. The gaps between the neurocranium – before they fuse at different times – are called fontanelles.

Craniosynostosis is the result of the cranial bones fusing too early. Often, only one or two sutures are affected. This causes a misshapen head as the areas of the cranium that have not yet fused must expand even further to accommodate the growing brain.

The 8 cranial bones are the:

The sphenoid and ethmoid bones are sometimes categorized as part of the facial skeleton. This is because these bones contribute to both areas.

Bones: All you need to know

Bones are more than just the scaffolding that holds the body together. Bones come in all shapes and sizes and have many roles. In this article, we explain their function, what they are made of, and the types of cells involved.

Despite first impressions, bones are living, active tissues that are constantly being remodeled.

Bones have many functions. They support the body structurally, protect our vital organs, and allow us to move. Also, they provide an environment for bone marrow, where the blood cells are created, and they act as a storage area for minerals, particularly calcium.

At birth, we have around 270 soft bones. As we grow, some of these fuse. Once we reach adulthood, we have 206 bones.

The largest bone in the human body is the thighbone or femur, and the smallest is the stapes in the middle ear, which are just 3 millimeters (mm) long.

Bones are mostly made of the protein collagen, which forms a soft framework. The mineral calcium phosphate hardens this framework, giving it strength. More than 99 percent of our body’s calcium is held in our bones and teeth.

Bones have an internal structure similar to a honeycomb, which makes them rigid yet relatively light.

Bones are composed of two types of tissue:

1. Compact (cortical) bone: A hard outer layer that is dense, strong, and durable. It makes up around 80 percent of adult bone mass.

2. Cancellous (trabecular or spongy) bone: This consists of a network of trabeculae or rod-like structures. It is lighter, less dense, and more flexible than compact bone.

  • osteoblasts and osteocytes, responsible for creating bone
  • osteoclasts or bone resorbing cells
  • osteoid, a mix of collagen and other proteins
  • inorganic mineral salts within the matrix
  • nerves and blood vessels
  • bone marrow
  • cartilage
  • membranes, including the endosteum and periosteum

Below is a 3D map of the skeletal system. Click to explore.

Bones are not a static tissue but need to be constantly maintained and remodeled. There are three main cell types involved in this process.

Osteoblasts: These are responsible for making new bone and repairing older bone. Osteoblasts produce a protein mixture called osteoid, which is mineralized and becomes bone. They also manufacture hormones, including prostaglandins.

Osteocytes: These are inactive osteoblasts that have become trapped in the bone that they have created. They maintain connections to other osteocytes and osteoblasts. They are important for communication within bone tissue.

Osteoclasts: These are large cells with more than one nucleus. Their job is to break down bone. They release enzymes and acids to dissolve minerals in bone and digest them. This process is called resorption. Osteoclasts help remodel injured bones and create pathways for nerves and blood vessels to travel through.

Bone marrow

Bone marrow is found in almost all bones where cancellous bone is present.

The marrow is responsible for making around 2 million red blood cells every second. It also produces lymphocytes or the white blood cells involved in the immune response.

Extracellular matrix

Bones are essentially living cells embedded in a mineral-based organic matrix. This extracellular matrix is made of:

Organic components, being mostly type 1 collagen.

Inorganic components, including hydroxyapatite and other salts, such as calcium and phosphate.

Collagen gives bone its tensile strength, namely the resistance to being pulled apart. Hydroxyapatite gives the bones compressive strength or resistance to being compressed.

Bones serve several vital functions:

Bones serve several vital functions:


Bones provide a frame to support the body. Muscles, tendons, and ligaments attach to bones. Without anchoring to bones, muscles could not move the body.

Some bones protect the body’s internal organs. For instance, the skull protects the brain, and the ribs protect the heart and lungs.


Cancellous bone produces red blood cells, platelets, and white blood cells. Also, defective and old red blood cells are destroyed in bone marrow.


Storing minerals: Bones act as a reserve for minerals, particularly calcium and phosphorous.

They also store some growth factors, such as insulin-like growth factor.

Fat storage: Fatty acids can be stored in the bone marrow adipose tissue.

pH balance: Bones can release or absorb alkaline salts, helping blood to stay at the right pH level.

Detoxification: Bones can absorb heavy metals and other toxic elements from the blood.

Endocrine function: Bones release hormones that act on the kidneys and influence blood sugar regulation and fat deposition.

Calcium balance: Bones can raise or reduce calcium in the blood by forming bone, or breaking it down in a process called resorption.

Bone regeneration strategies

At present, the “conventional standard” healing of patients suffering from long or imperfect bone treatment is to implement bone grafting, by means of either an autograft or an allograft. Though, there are problems to bone grafting. Subsequently, a more maintainable, long-term healing plan is necessary. To that end, bone graft replacements are being concocted to aid damaged fracture treatment. Based on the gravity of the trauma, the main strategies are established for bone repair:

Synthetic substitutes alone

Scaffolds combined with active molecules

Nanomedicine for healing of bone trauma and defects

Cell-based combination products with cells from various sources

Biomimetic fibrous and nonfibrous substitutes

Biomaterial-based 3D cell-printing substitutes

Bioactive porous polymer/inorganic composite

Magnetic field and nano-scaffolds with stem cells.

Suitable material selection of damaged bone substitute

Demineralized bone matrix (DBM) was produced with osteoconductive and osteoinductive properties (Giannoudis et al. 2005). The preferred choice for DBM synthesis was reported cortical bone as a result of osteoinductive with a lesser antigenic possibility in comparison to cancellous bone (Burg et al. 2000). Utilization of viscous spongy cellulose revealed that it was extremely suitable for bone regeneration. The implanted cellulose in the femoral bone of rats showed that osteoconduction was mostly happened (Ekholm et al. 2005). Recently, bacteria strains secretion-derived bacterial cellulose was introduced as an emerging player in tissue engineering because of its extremely good cytocompatibility and physiochemical properties. Therefore, its modified compounds were used for bone regeneration (Stumpf et al. 2018). Synthetic media have similarly been evaluated as acellular bone tissue engineering materials. Hence, poly- l -lactide (PLLA) films were used to repair 1-cm trauma in the radius bone of mature rabbits, and histologic results indicated that the cortical bone was redeveloped over the defect (Zhang et al. 2006). Poly-ε-caprolactone-co-lactide was used as a different potential filler material in bone defect, and investigated in non-osseous usage. To evaluate the absorption and biocompatibility of this copolymer, it was used in femoral defect of rat (Helminen et al. 2002). Photocrosslinkable polyanhydrides constituents demonstrated the convinced benefits for orthopedic regeneration. In this case, the photopolymerizable component enhanced microfabrication probability of porous scaffolds. Also, mechanical investigations proved reliability of these polymers for tissue engineering applications (Pakulska 2016). Calcium phosphate and silicate-based bioceramics were prominently featured among used biomaterials for bone regeneration (Samavedi et al. 2013 Diba et al. 2014 Dziadek et al. 2017). For example, nanostructured monticellite (CaMgSiO4) ceramic and its composites (with HA) showed good in vitro bioactivity, biocompatibility, and antibacterial properties for bone tissue engineering application (Chen et al. 2008 Kalantari et al. 2017 Kalantari et al. 2018a, b, c, 2019 Kalantari and Naghib 2019).

Scaffolds combined with active biomolecules

The approaches can be categorized in three classifications: (a) application of recombinant growth factors and a combination of growth factors associated with a natural medium or calcium phosphate substantial carrier, (b) application of proteins to target cellular receptors, and (c) application of small molecules that target the signaling pathway (Ansari et al. 2018). Figure 3 presents the general schematic presentation of the scaffold loaded with active biomolecules for accelerate bone repair. The main growth factors have already been used in clinics included BMP-2, BMP-7, and rhPGDF-BB (Ho-Shui-Ling et al. 2018). The growth factors effect on bone progenitors by interrelating with their particular receptors, which activate the chemical signaling pathways result in bone development. Several studies have previously been done on BMP-2 associated with a type I collagen porous structure as delivery service, for application in open tibial fractures and in spinal defects (Bessa et al. 2008). In a study, it is combined with a titanium or PEEK structure for application in anterior lumbar interbody fusion (Vaidya et al. 2008). It is well known that bone restoration in response to BMP-2 is dose dependent. High doses of BMP-2 may result in osteolysis. So, the potent activating characteristic of bone repair of BMP-2 needs to be further optimized (Bruder et al. 1994). Latest documents in a rat femoral bone fracture, applied PLGA as a polymeric carrier as a nano reservoir for BMP-2 delivery (Zheng et al. 2010). The result revealed that it is conceivable to adjust the dose of BMP-2 in vivo. This controlled dose influenced the dimensions of bone formation, and enhanced the kinetics of bone renovation by means of delivery of BMP-2.

Schematic presentation of macro/micro/nano-porous scaffold loaded with active biomolecules for accelerate bone regeneration (Yi et al. 2016)

BMP-7 growth factor is osteoinductive which is studied for the clinical application (Bostrom and Seigerman 2005). Collagen type I incorporated with BMP-7 in the paste form is used for recalcitrant long bone and spine surgical treatment. Investigations on a sheep model indicated that the BMP-7 paste may be able to incorporate with a porous scaffold to initiate long bones regeneration (Haidar et al. 2009). rhPDGF-BB in form of device/drug product was employed for hindfoot and ankle fusion (Solchaga et al. 2012). PDGF, by functioning on PDGF receptors, motivates the employment, migration and proliferation of cells containing mesenchymal stem cells and stimulates the neovascularization by growing vascular endothelial cells at the location of bone repair.

Presently, a β-TCP/PDGF scaffold was employed to provide osteoconductivity for bone repair. In a study on patients devoted to hindfoot or ankle arthrodesis, healed with rhPDGF-BB/β-TCP, gave rise to comparable fusion extents, minus pain, and less side effects in comparison to healing with autograft (Bateman et al. 2005).

Other growth factors like GDF-5 are kind of BMP group that encourages bone formation (Nickel et al. 2005). Several researches have confirmed rhGDF-5 has the potential of bone induction tissue growth. A bone substitute rhGDF-5/(β-TCP) was applied for dental implants and medical cure of periodontal syndrome. The in vitro results indicating that rhGDF-5 has the potential to promote gene expression and production of the ECM proteins such as collagen type II and aggrecan (Poehling et al. 2006).

Peptides, they have the ability to access cellular receptors, are substitutes for recombinant growth factors which are able to produce easily. Bioactive B2A (B2A2-K-NS) synthetic polypeptide applied to augment spinal fusion (Omrani et al. 2016). HAP/β-TCP incorporated with B2A granules were investigated for foot and ankle fusion.

In vitro results indicate that B2A induces chondrogenic differentiation and improves the in vivo healing of injured cartilage in an osteoarthritis model (Ho-Shui-Ling et al. 2018). Collagen is a major protein of the ECM and contributes in osteoblast attachment and activity (Ansari and Moztarzadeh 2012). P-15 is a 15 amino-acid protein obtained from collagen and promotes the differentiation of mesenchymal stem cells. P-15 has been applied in combination with bone inorganic for spinal fusion, non-union fractures and joint reconstruction with acceptable results (Bhatnagar et al. 1999). Besides, small molecules applied as controllers of bone bulk. Parathyroid hormone has a vital character in controlling calcium phosphate digestion. PTH entrapped in a normal fibrin medium, incorporated with a mechanical ceramic constituent (HAP/TCP granules), may be responsible for structural stability and osteoconduction for the period of healing (Portale et al. 1984).

Microorganism-derived polyhydroxyalkanoate (PHA) scaffolds have emerged as polymeric promising biomaterials with excellent potential for bone tissue engineering applications because of their good biodegradability, biocompatibility and vascularization, and unique physiochemical properties. They induced cell adhesion and growth on their porous structure for bone regeneration (Lim et al. 2017). Lalzawmliana et al. (2019) reported that mesoporous bioactive glass (MGB) scaffolds as third-generation biomaterials were used for regeneration of critical bone defects, and MGB scaffolds should have large interconnected pores for improving growth, adhesion and proliferation of osteoblast cells and assisting in angiogenesis. In recent studies, effect of three-dimensional (3D) scaffolds and their fibrous order on biocompatibility were investigated. The results showed that 3D aligned nanofibrous scaffolds provided cell behaviors better than two-dimensional (2D) scaffolds, because of their more accommodation for the attached cells, and loading more bioactive molecules for promotion of cell growth, proliferation, migration and differentiation. Nevertheless, a big challenge was mentioned for them that related to their static status. For example, they cannot remodel their stiffness, surface chemistry and roughness in an in situ and dynamic situation, so cannot mimic the function of main tissue (Jin et al. 2018). Rather et al. reviewed the dual functional strategies to spread osteogenesis coupled angiogenesis through different scaffolds. Vascularization played the important role to carry oxygen, nutrients and essential molecules and growth factors into damaged tissue. Then, the angiogenesis and osteogenesis communicated harmoniously together for bone regeneration, because many studies confirmed the crosstalk between bone progenitor cells and endothelial cells. Therefore, the scaffolds containing osteoinductive and angioinductive factors released various types of molecules to stimulate osteogenesis and angiogenesis (Rather et al. 2019). The studies showed that many scaffolds were investigated in the field of bone repair with the purpose of bettering cell growth, adhesion and proliferation, osteogenic differentiation, vascularization, and mechanical properties, but Roseti et al. (2017) reported that the further depth studies will be needed for using the bone tissue engineering scaffolds in clinical application.

Nanomedicine for healing of bone trauma and defects

One of the most important drawbacks in the healing of open fractures is infection. The failure of the tissue obstruction among the rupture location and the external milieu result in bone bacteriological infection and contamination (Broos and Sermon 2004).

Besides, it was proved that Staphylococcus aureus (is a Gram-positive, round-shaped bacterium that is a member of the firmicutes, and it is a usual member of the microbiota of the body) may attack intracellular sites contained by osteoblasts, result in complications in microbial eradication and amplified vulnerability to osteomyelitis subsequent contamination (Join Lambert et al. 2005). The infected fractures need management including medical debridement, antibiotics, and skeletal stabilization. Frequently, antibiotic-contained cement made of polymethylmethacrylate (PMMA) is implanted to harmonize the antibacterial activity (Schade and Roukis 2010). The growth and differentiation of osteoblasts and osteoclasts are controlled by growth factors, cytokines produced in the bone-marrow ECM, and adhesion structures that facilitate cell–cell and cell–ECM communications (Lu et al. 2012).

Numerous categories of antibacterial nanoparticles (NPs) and nano-sized carriers for antibiotic delivery have been confirmed to be applicable in curing infectious diseases, containing antibiotic-resistant ones, in vitro and in vivo. For instance, several NPs are able to join to the membrane of microorganisms by electrostatic interface and destruct the unity of the microorganism membrane (Banerjee et al. 2011). Another mechanism is that designed NPs are able to produce massive oxidative stress to microorganisms through free radical construction such as reactive oxygen species (ROS) and destroy their contamination hazard as it is shown in Fig. 4 (Long et al. 2006).

Toxicity mechanisms of NPs and their ions (e.g., silver and zinc) against bacteria by induce oxidative stress by means of the production of reactive oxygen species (ROS). The ROS is able to conclusively break bacteria (e.g., their membrane, DNA, and mitochondria) culminating in bacterial death (Hajipour et al. 2012)

One of the most frequently used NPs for diminution of infection risk in orthopedic distress is silver (Ag) NPs. Their extensive antibacterial performance is proved. They are extensively applied to remove various bacteria containing S. aureus, Bacillus subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escherichia coli.

Polymethylmethacrylate (PMMA)-based bone cement, consisting of Ag NPs (about 50 nm), completely prevented the propagation of Staphylococcus epidermidis, methicillin-resistant S. epidermidis (MRSE), and methicillin-resistant S. aureus (MRSA), while PMMA bone cement contained 2% of gentamicin sulfate avoided only the propagation of S. epidermidis (Abid et al. 2017). In additional research, an antibacterial HA NPs scaffold was created, and antibacterial properties were attained by the addition of Ag NPs.

In another study, selenium (Se) NPs were employed to coat a bioactive glass-based structure (SiO2 and molar ratio of P to Ca = 1/5) fabricated by the foam replica technique. The results indicated that this scaffold has antibacterial activity (Fathi-Achachelouei et al. 2019)..

Cell-based combination products with cells from various sources

Tissue-particular cells such as osteoblasts maybe employed as the cellular constituent of bone transplants. Several kinds of stem cells have been mostly used through the construction of bone grafts (Omrani et al. 2019). Multipotent adult mesenchymal stem cells (MSCs) show unlimited differentiation capacity into various kinds of cell lineages, containing osteoblasts and chondrocytes (Baksh et al. 2004). Adult MSCs perform as an inducible backup potency for tissue restoration following damage, and have been considered broadly for bone fracture regeneration. MSCs obtained from numerous diverse sources containing bone marrow, synovial membrane, skeletal muscle, and adipose tissue. Cell-based therapy with allogenic BMSCs implants is operative in bone regeneration in different animal bone defect models. In initial clinical studies, autologous BMSCs have been cultured on bio-ceramic scaffold to heal big bone defects. Local transplantation at the defect situate of MSCs led to widespread fusion at 5–7 months after surgery (Cancedda et al. 2003).

Another study indicated bone repair in rabbit skull defects healed with autologous, osteogenically induced adipose-derived stem cells (ADSCs) transplanted onto fibronectin-coated polylactic acid scaffold (Di Bella et al. 2008). Additional study showed cranial bone defect regeneration in canine by means of osteogenically induced ADSCs transplanted onto a coral structure (Aimaiti et al. 2011). In a different research, calvarial defects treated through autologous ADSCs/fibrin glue/autologous cancellous bone graft. After 2 months, new bone mineralization and complete calvarial integrity were observed (Lendeckel et al. 2004).

Multipotential synovial membrane-derived MSCs (SMSCs) stromal cells can operate as a healing substitute for focal cartilage damages and have capability to differentiate to osteogenesis. Stimulatingly in a current research, SMSCs from knee joints presented greater osteogenic and adipogenic potential than SMSCs of hip joints (Kristjánsson et al. 2013).

Dental pulp-derived stem cells (DPSCs) have lately being discovered for bone tissue engineering. DPSCs present the low percentages of morbidity, widespread differentiation capacity into chondrogenic, and osteogenic cell lines, and expression of bone markers in vitro and in vivo (d’Aquino et al. 2008). Alginate microsphere DPSCs carrier has the osteogenic potential through detecting improved mineralization and upregulated intensities of osteogenic genes (Moshaverinia et al. 2012). DPSCs seeded/collagen-HA-poly( l -lactide-co-ɛ-caprolactone) scaffold confirmed effective ECM mineralization of osteoblast (Akkouch et al. 2014). Some tissues for instance placenta, umbilical cord blood (UCB) and umbilical cord tissue are different sources of MSCs (Jin et al. 2013). The regenerative capability of RGD-functionalized microporous calcium phosphate cements (CPC) contained UC MSCs and BM MSCs were compared in a rat bone defect model. The results showed comparable great bone inorganic compactness, new bone formation and vascularization in vitro and in vivo (Gan et al. 2018).

Pluripotent human embryonic stem cells (hESCs) are obtained from human blastocysts. Effective differentiation of hESCs into the osteogenic lineage has been confirmed in frequent reports mutually in vitro and in vivo. Actually, following osteogenic stimulation, hESCs demonstrated to retain molecular and fundamental characteristics similar to bone cells by means of the creation of mineralized bone in vitro. Osteogenic cells derived from ESCs seeded on poly( d , l -lactic-co-glycolic acid)/HA scaffold showed substantial in vivo bone construction in immunodeficient mice through subcutaneously seeding (Tang et al. 2012).

Induced pluripotent stem cells (iPSCs) have been originated from adult somatic cells such as skin fibroblast (Kim et al. 2010). IPSCs have the capacity to differentiate to all cell types. IPSCs obtained from embryonic source have the ability to produce MSC-like cells in vitro which presented the capability of more differentiating property to osteoblast cells, whereas similarly indicating osteogenic capacity comparable to that of BMSCs in vivo. Furthermore, in vivo investigations have confirmed that MSC-like cells obtained from iPSCs present the capability to develop mature mineralized construction similar to bone structure (Wu et al. 2017).

Endochondral bone tissue engineering using progenitor cells such as chondroprogenitors has been lately demonstrated. Several researches presented that articular chondrocytes are able to be stimulated to endochondral ossification and generate TGFβ-1 and BMP-2 (Perez et al. 2018). In a study, chondrocyte cell seeded on BMP-2-loaded polycaprolactone (PCL) scaffold which subcutaneously implanted in vivo result in bone formation (Lee and Shin 2007).

Biomimetic fibrous and nonfibrous substitutes

Bone tissue has a mineralized construction. Biomimetic composite substitute with a mineral constituent were used broadly for bone repair. The mineral component induces structural integrity and osteoconductive properties to the scaffold. HA is frequently used for the reason that has the potential to simulate the natural minerals part of bone. Besides, other calcium phosphate or bioglass were similarly used for their biocompatibility. Using dioxane/water as a solvent, nano-HA/PLLA nanofibers composite scaffolds through TIPS (thermally induced phase separation) technique were fabricated. The high surface area of the nanofibrous permits further the HA to be exposed, which is appropriate for bone tissue regeneration (He et al. 2009).

In another study, HA was incorporated into electrospun nanofibers, then utilized a gelatin-apatite precipitate homogenized in an organic solvent with polylactide-co-caprolactone (PLCL). For the duration of the precipitation reaction, the Ca/P proportion was reserved to 1.67 to guarantee stoichiometric apatite fabrication. Just the lowest concentration of gelatin-apatite leads to a growth in normal strength (Kim et al. 2006).

Lately, electrodeposition method has been developed that decreases the mineralization time. To demonstrate the flexibility of the technique, electrodeposition has been effectively made on both electrospun PLLA fibers and phase-separated PLLA fibers. Consequently, electrodeposition confirmed to be a fast and operative method to mineralize a bone tissue scaffold (Wei and Ma 2006). Collagen, in the form of injectable hydrogels, membranes, or sponges, extensively employed for bone tissue regeneration. Individually, as composite with calcium phosphate structures such as HA Several instances include, collagen/HA/chitosan or collagen/HA/alginate hydrogels (Teng et al. 2008).

Biomaterial-based 3D cell-printing substitutes

3D printing employs 3D images of the bone trauma anatomy, usually acquired from computed tomography (CT) scans, using a calculating software, to fabricate a bone graft substitutes (BGS) structure that matches to a bony defect (Burleson and DiPaola 2019). The personalized bone graft substitute form is fabricated using a 3D printer to control the BGS mechanical features and substantial parameters. The composition optimization confirms an improved correspondence among the BGS and the patient’s anatomy, permitting the regeneration. Metallic replacements manufactured by titanium are the further most extensively used. Titanium plates are usually employed to immobilize bone parts in jaw operations. 3D printing is similarly being studied for orthopedic purposes: for acetabular ruptures, ankle defects and further bone defects due to bone fracture, spurt fissure of spine, bone cancer and orbital ground repair. The tailored spongy implant printed using Ti6Al4V presented outstanding physicochemical features and biological function such as biocompatibility, osteogenic property, and bone regeneration (Alvarez and Nakajima 2009). Bioceramics and biopolymers such as polyetheretherketone (PEEK) are currently custom designed, and are presently being investigated at the pre-clinical phase (Yan et al. 2015). PCL/HA composite is being studied for the repair of gingival recession concomitant with bone and gingival tissue repair (Osathanon et al. 2017).

In a recent study, a mandible bone was repaired via human amniotic fluid-derived stem cell (hAFSC)-laden hydrogel, a mixture of PCL and tricalcium phosphate (TCP), and pluronic F127 (Fig. 5b). The PCL/TCP and hAFSCs mixed with the combination of hydrogel were reproduced in a type I design with a Pluronic F127 impermanent support (Fig. 5c). Subsequently induction of osteogenic differentiation for 28 days (Fig. 5d), they stained the constructions with Alizarin Red S staining at the surface of the 3D bone constructions showed calcium deposition in the hAFSC laden hydrogel (Fig. 5e).

Mandible bone regeneration. a 3D CAD model identified a mandible bony defect from human CT image data. b Visualized motion program was generated to construct a 3D architecture of the mandible bone defect using CAM software. c 3D printing process using integrated organ printing system. d Photograph of the 3D-printed mandible bone defect construct, which was cultured in osteogenic medium for 28 days. e Osteogenic differentiation of hAFSCs in the printed construct was confirmed by Alizarin Red S staining, indicating calcium deposition (Jang et al. 2018)

Bioactive porous polymer/inorganic composites

The artificial and biodegradable, polymer/inorganic bioactive part compounds are used as bone tissue engineering supports as a result of their formability, bioactive performance and regulating biodegradation kinetics (Rezwan et al. 2006).

Two categories of biodegradable biopolymers are presented: the natural polymers containing polysaccharides such as starch, alginate, chitin/chitosan, hyaluronic acid, proteins for instance collagen, fibrin gels, silk and, as reinforcement, a diversity of bio-fibers including lignocellulosic natural fibers, and the synthetic polymers are used as 3D scaffolds in bone tissue engineering, are saturated poly-α-hydroxy esters, containing polylactic acid (PLA) and poly glycolic acid (PGA), as well as polylactic-co-glycolide (PLGA) copolymers (Gentile et al. 2014).

PPF (polypropylene fumarate) has been used as an injectable bone substitute scaffold for conducted tissue regeneration. It was similarly utilized as a substrate for osteoblast cultures. The growth of composite substrates adjoining polypropylene fumarate and mineral elements, such as HA or bioglasses, in contrast with the broad investigation works devoted to PLGA and PLA composites (Chen et al. 2012).

Aliphatic polyesters PHAs manufactured through bacteria under unstable progress situations. They are commonly biodegradable (through hydrolysis), biocompatible and thermoprocessable (Lizarraga-Valderrama et al. 2016). These fascinating properties make them suitable for biomedical applications in particular tissue engineering. PHA, principally poly-3-hydroxybutyrate (PHB), copolymers of 3-hydroxybutyrate and 3 hydroxyvalerate (PHBV), poly-4-hydroxybutyrate (P4HB), copolymers of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx) and poly-3-hydroxyoctanoate (PHO) were confirmed to be appropriate for bone tissue regeneration (Ke et al. 2017).

Degradation products of bioglasses, especially the 45S5 Bioglass structure, regulate the gene activation that manages osteogenesis and the fabrication of growth factors (Xynos et al. 2001). HA and silicon have a vital character in the bone mineralization and gene expression, which requires greater than before attention in the substitution of silicon for calcium into HA structure (Arvidson et al. 2011). In vivo results have revealed that bone remineralization into silicon-doped HA particles has been significant larger than that pure HA. Bioactive glasses lately have been used as scaffold, filler or coatings of polymers and, as porous constituents, which contains melt-derived and sol–gel-derived bioglasses (Wang and Yeung 2017).

In vivo and in vitro evaluation of crystalline or amorphous calcium phosphates, in bulk, coating, powder, or porous form, induce the attachment, differentiation, and proliferation of osteoblasts and mesenchymal stem cells (Rezwan et al. 2006).

Magnetic field and nano-scaffolds with stem cells

Innovative approaches are using magnetic nanoparticles (MNPs) and magnetic fields to improve bone repair efficiency containing osteogenic improvements by means of magnetic fields, MNPs and magnetic approaches to develop the cells, scaffolds and growth factor conveyances including cell tagging, targeting, designing, and gene modifications (Panseri et al. 2012). The process of scaffolds containing MNPs using magnetic fields and stem cells to improve bone redevelopment were recognized as including the motivation of signaling trails containing MAPK, integrin, BMP and NF-κB (Gonçalves et al. 2016). Static magnetic fields (SMFs), pulsed electromagnetic fields (PEMFs), rotating magnetic fields (RMFs) and alternating electromagnetic fields have the potential to improve the defect healing, bone mineral density, attachment of implants among bone tissue (Xia et al. 2018b). Combination of magnetic fields with growth factors and signaling factor, magnetically aided freezing and defrosting of stem cells, magnetically aided scaffold and coating constructions are able to improve bone restoration. Animal research presented that SMFs with moderate intensity improved the bone mineral compactness and bone repair (Fitzsimmons et al. 1995).

SMFs may possibly modify cell functions such as the attachment, morphology, proliferation, differentiation, apoptosis, gene expression, in particular osteogenic differentiation for different kinds of cells, containing BMSCs, human osteosarcoma cell lines MG63, human adipose-derived MSCs, and dental pulp stem cells (DPSCs) due to electrodynamic interactions and magneto mechanical interactions (Xia et al. 2018b). Superparamagnetic iron oxide nanoparticles (SPIONs) are encouraging for targeted drug delivery, tissue engineering, hyperthermia, gene therapy, imaging and cell tracking purposes. SPIONs without a magnetic field can improve the tissue repair productivity, be responsible for dynamic mechanical motivations for bone regeneration, encourage osteogenic differentiation of BMSCs, and develop bone healing in vivo (Santhosh and Ulrih 2013).

In a study, gelatin/SPION-scaffold were implanted in the incisor sockets of rat model which improved bone restoration in comparison to gelatin porous structure control without SPIONs. It is notable that the endocytic SPIONs stimulated the osteogenic and angiogenic performance of the cells result in better bone regeneration (Gu et al. 2013). The aggregation of SPIONs as a result of magnetic fields may change their biological impression. Definitely, decrease in cell uptake followed for the reason that agglomeration of the particles as a result of substantial variations in mutually the size and zeta potential. Furthermore, external magnetic fields may affect the biological properties of SPIONs such as therapeutic/toxic effects. In a research, (Fe 2+ /Fe 3+ )-doped HA (FeHA) nanoparticles in cultures with osteoblast-like cells in the absence, or presence, of an SMF were investigated. Application of external magnetic field to FeHA lead to a substantial cell growth, proliferation and more osteoblastic activity as a result of the tremendous biological impacts of HA and the partial iron content. Consequently, the variations in the biological characteristic and endocytosis of the cells, created by the MNPs using external magnetic field, may pointedly improve the cell performance and bone renewal abilities (Tampieri et al. 2014). Stem cells have excessive potential for tissue repair. Magnetically labeled cells have the potential application for bone tissue regeneration, containing cell targeting and cell patterning. SPIONs are able to operate as an ideal labeling and tracer tool for MSCs. MNP intake into the cells and make them manageable and manipulated by external magnetic field. In a study implantation of magnetically labeled MSCs were employed to regenerate serious chronic osteochondral traumas, exposure to an external magnetic field, considerably produced new chondrogenic tissues (Li et al. 2018).

Dip coating technique was used to fabricate magnetic HA/collagen scaffolds. The magnetic scaffolds support the attachment and proliferation of hBMSCs, and motivate osteoblastic differentiation. The results were along with a different study on MNP-HA magnetic scaffolds.

In another study, nanofibrous γ-Fe2O3/HA/polylactic acid was fabricated. This scaffold improved the proliferation of osteoblastic by reason of the SPION integration (Kim et al. 2006). In a current research, an injectable calcium phosphate/SPIONs cement has been developed by mixing with a SPION. Osteogenic differentiation and bone matrix mineral synthesis by the cells was similarly improved two folded in comparison to samples without SPIONs (Xia et al. 2018a). Fe3O4 nanoparticle/bioactive glass/polycaprolactone (Fe3O4/MBG/PCL) scaffolds was fabricated using a 3D printing method. The results indicate that cell growth on the Fe3O4/MBG/PCL scaffolds was greater than non-magnetized control sample (Zhang et al. 2014). In vivo results using rabbit model confirmed that the PCL/FeHA scaffolds were enhanced bone regeneration in comparison to non-magnetized control. SPIONs/nHA and PLA nanofibrous scaffold was implanted in the lumbar transverse defects in rabbit model and then SMF were applied. The MNP scaffold with application of an SMF persuaded more osteogenesis, new bone formation and remodeling in the rabbit defects (Hu et al. 2018). Along with stem cells and scaffolds, growth factors delivering are a vital method in bone tissue regeneration. MNPs have the potential for using as a delivery tool for biological mediators for instance drugs, chemotherapeutics, antibodies, peptide, oligonucleotides, and growth factors through magnetic fields. For example, gene delivery using MNPs possibly will be multifunctional, performing utilities that contain the identification, healing and visualization of the disease at the same time. Consequently, magnetic-based gene delivery is extremely promising method for stem cell therapy (McCarthy et al. 2007).

Bone/biomaterials interface studies

For the effective integration of implants or scaffolds for tissue regeneration, cell adhesion to biomaterials is a vital necessity. Adjusting cells–scaffolds communications seems of most important to affect succeeding cell biological progressions for instance attachment, proliferation and differentiation (Tormos 2016). Numerous reports show that the adhesion of integrins in bone cells including osteoblasts and osteoclasts to the extracellular matrix is vital through the bone repair (Puleo and Nanci 1999). Bone extracellular matrix proteins arbitrate the biological function of cells through moderating their milieu. Motivation of the attachment, proliferation and differentiation of the bone cells are determined by the part of the superficial characteristics, chemical composition, electrostatic charge, texture, geometrical configuration, roughness and smoothness, of the replacement (Venkatesh and Sen 2017). The ceramic biomaterials may be abrasive and consequently, it is crucial to avoid them in uncontrolled damage neighboring to articular surfaces. Bioglass ionic extracts and surface exchanges stimulate the proliferation and differentiation of osteoblasts and the fabrication of the primary phenotypic biomarkers (Abid et al. 2017, 2016).

The cell activities are regulated by biodegradable polymers with properties such as chemical structure, polymer ratio of PLA or PGA for example, molecular weight and crystallinity. Polymer degradation products for instance catalysts, additives, byproducts and residual monomers that led to an inflammatory reaction and influence the cell attachment, cell survival and proliferation (Puppi et al. 2010). Composite structures present tolerable physiological and mechanical performance such as the characteristics and morphology of cortical and trabecular bone. Signaling factors can be included to bone composites to stimulate cell behavior and favor bone repair. Several factors impact on the release of growth factors, for example the surface charge and chemistry of composite, geometry, dimensions, porosity, wettability, crystallinity, the rate of degradation. The growth factor release could be regulated by diffusion, exterior motivation, enzymatic/chemical response (Muzzarelli 2011). The cell attachment to biomaterial and their following performances can be influenced by surface features for instance topography, hydrophobicity, charge, chemistry and special surface energy (Von Recum and Van Kooten 1996). These all affect the conformation, alignment and amounts of adhesion proteins including vitronectin or fibronectin that facilitate the interfaces among cells and biomaterial (Place et al. 2009). Lithography, colloidal particle adsorption, micro-contact printing, novel polymer preparations and self-assembled monolayers are all employed to analyze the interactions among cells and biomaterials at the micro- and nanometer scale (Ma et al. 2007). These methods may be utilized to regulate the topographic properties, micrometer or nanometer ridges, grooves pits, islands, holes. Some studies have revealed improved bone–biomaterial interactions with a high surface roughness. In a study, PLA-polystyrene films with porous of about 45 nm caused human fetal osteoblasts to proliferate expressively more and attach greatly better than a flat PLA surface (Kochesfahani 2016). Current investigations have employed in vitro self-assembling monolayers containing PEG, OH, COOH, NH2 and CH3 groups to assess the consequence of surface chemistry and hydrophilicity on protein adsorption and cell performance such as the attachment strength of MC3T3-E1 preosteoblasts and the medium mineralization. The results show mineralization by cells on OH and NH2 surfaces is associated with improved alpha 5 beta 1 integrin adhesion and FAK stimulation (Keselowsky et al. 2003). Besides, the experiments approved that osteoblasts adhered and proliferated further on positively charged hydrogels in comparison to neutral or negatively charged ones (Liu et al. 2014).


The word baculum meant "stick" or "staff" in Latin and originated from Greek: βάκλον , baklon "stick". [10]

The baculum is used for copulation and varies in size and shape by species. Its evolution may be influenced by sexual selection, and its characteristics are sometimes used to differentiate between similar species. [11] A bone in the penis allows a male to mate for a long time with a female, [12] [13] which can be a distinct advantage in some mating strategies. [14] [15] The length of the baculum may be related to the duration of copulation in some species. [16] [17] In carnivorans and primates, the length of the baculum appears to be influenced by postcopulatory sexual selection. [18] In some bat species, the baculum can also protect the urethra from compression. [19]

Mammals having a penile bone (in males) and a clitoral bone (in females) include various eutherians:

  • Order Primates, although not in lorises, [20]humans, spider monkeys, or woolly monkeys[21]
  • Order Rodentia (rodents), [22] though not in the related order Lagomorpha (rabbits, hares, etc.) [23]
  • Order Eulipotyphla[24] (insectivores, including shrews and hedgehogs)
  • Order Carnivora[9] (including members of many well-known families, such as ursids (bears), [25]canids (dogs), [1]pinnipeds (walruses, seals, sea lions), [6]procyonids (raccoons etc.), [26]mustelids (otters, weasels, skunks and others)). [27] The baculum is usually longer in the Canoidea than in the Feloidea, although fossas have long bacula and giant pandas have short bacula. [9]
  • Order Chiroptera (bats). [28][29][30]

Evidence suggests that the baculum was independently evolved 9 times and lost in 10 separate lineages. [24] The baculum is an exclusive characteristic of placentals and closely related eutherians, being absent in other mammal clades, and it has been speculated to be derived from the epipubic bones more widely spread across mammals, but notoriously absent in placentals. [33]

Among the primates, marmosets, [ clarification needed ] weighing around 500 grams (18 oz), have a baculum measuring around 2 millimetres (0.079 in), while the tiny 63 g (2.2 oz) galago has one around 13 millimetres (0.51 in) long. The great apes, despite their size, tend to have very small penis bones, and humans are the only ones to have lost them altogether. [15]

In some mammalian species, such as badgers [35] [36] and raccoons (Procyon lotor), the baculum can be used to determine relative age. If a raccoon's baculum tip is made up of uncalcified cartilage, has a porous base, is less than 1.2 g (0.042 oz) in mass, and measures less than 90 mm (3.5 in) long, then the baculum belongs to a juvenile. [26]

Unlike most other primates, humans lack an os penis or os clitoris, [37] [38] but the bone is present, although much reduced, among the great apes. In many ape species, it is a relatively insignificant 10–20 mm (0.39–0.79 in) structure. Cases of human penis ossification following trauma have been reported, [39] and one case was reported of a congenital os penis surgically removed from a 5-year-old boy, who also had other developmental abnormalities, including a cleft scrotum. [40] Clellan S. Ford and Frank A. Beach in Patterns of Sexual Behavior (1951), p. 30 say, "Both gorillas and chimpanzees possess a penile bone. In the latter species, the os penis is located in the lower part of the organ and measures approximately three-quarters of an inch in length." [4] In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa. An "artificial baculum" or penile implant is sometimes used to treat erectile dysfunction in humans. [41]

The first recorded attempts to explain the lack of baculum in humans might be more than two thousand years old: the Biblical 'rib' that was 'taken' from Adam may actually be the baculum. [42] However, the evolutionary history of this anatomical loss remains enigmatic to this day. [43] Several scientific hypotheses for the loss in humans have been proposed.

In The Selfish Gene, Dawkins [44] proposed honest advertising as the evolutionary explanation for the loss of the baculum. The hypothesis states that if erection failure is a sensitive early warning of ill health (physical or mental), females could have gauged the health of a potential mate based on their ability to achieve erection without the support of a baculum.

The tactile stimulation hypothesis proposes that the loss of the baculum in humans is linked to the female choice for tactile stimulation: a boneless penis would be more flexible, facilitating a larger range of copulatory positions and whole body movement, giving females greater general physical stimulation. [45]

The elongation-loss hypothesis proposed that the evolution of bipedalism and sperm competition resulted in an increased evolutionary pressure for a longer penis, possibly with a larger supporting baculum. Following this, progression towards mating systems where males controlled access to females (monogamy or polygyny) may have favoured selection against the baculum, possibly as a large penis with a large baculum would have been a discomfort to males and/or vulnerable to injuries. [46]

The mating system shift hypothesis proposes that the shift towards monogamy as the dominant reproductive strategy may have reduced the intensity of copulatory and post-copulatory sexual selection, and made the baculum obsolete. [47]

Humans "evolved a mating system in which the male tended to accompany a particular female all the time to try to ensure paternity of her children" [15] [ better source needed ] which allows for frequent matings of short duration. Observation suggests that primates with a baculum only infrequently encounter females, but engage in longer periods of copulation that the baculum makes possible, thereby maximizing their chances of fathering the female's offspring. Human females exhibit concealed ovulation, also known as hidden estrus, meaning it is almost impossible to tell when the female is fertile, so frequent matings would be necessary to ensure paternity. [15] [48] [49]

Strengths and weaknesses of these hypotheses were revised in a 2021 study, which also proposed an alternative hypothesis: that conspecific aggression, in combination with the development of self-awareness, may have played a role in the loss. If the presence of a baculum exacerbated the prevalence and severity of penile injuries resulting from blunt trauma to a flaccid penis, increasing ability to foresee the consequences of their actions would also enable hominins to realise that these injuries are a useful tool in male-male competition. This behavioural innovation, planned conspecific aggression with the goal of temporary exclusion of competitors from the breeding pool, would create an environment in which a genetic mutation for a penis without a baculum (or with an unossified baculum) would strongly increase the fitness of the mutant phenotype. Along with the hominin propensity for social learning and cultural transmission, this hypothetical scenario may explain why this phenotype became fixed in all human populations. [50]

An alternative view is that its loss in humans is an example of neoteny during human evolution late-stage fetal chimpanzees lack a baculum. [51]

The existence of the baculum is unlikely to have escaped the notice of pastoralist and hunter-gatherer cultures.

It has been argued that the "rib" (Hebrew צֵלׇע ṣēlā', also translated "flank" or "side") in the story of Adam and Eve is actually a mistranslation of a Biblical Hebrew euphemism for baculum, and that its removal from Adam in the Book of Genesis is a creation story to explain this absence (as well as the presence of the perineal raphe – as a resultant "scar") in humans. [52]

In hoodoo, the folk magic of the American South, the raccoon baculum is sometimes worn as an amulet for love or luck. [53]

Oosik Edit

Oosik is a term used in Native Alaska cultures to describe the bacula of walruses, seals, sea lions and polar bears. Sometimes as long as 60 cm (24 in), fossilized bacula are often polished and used as a handle for knives and other tools. The oosik is a polished and sometimes carved baculum of these large northern carnivores.

Oosiks are also sold as tourist souvenirs. In 2007, a 4.5 ft-long (1.4 m) fossilized penis bone from an extinct species of walrus, believed by the seller to be the largest in existence, was sold for $8,000. [54]

United States Congressman for Alaska, Don Young, is known for possessing an 18 inch walrus oosik, and once brandished it like a sword during a congressional hearing. [55]

Cell Types in Bones

Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. Osteoblasts are bone cells that are responsible for bone formation. Osteoblasts synthesize and secrete the organic part and inorganic part of the extracellular matrix of bone tissue, and collagen fibers. Osteoblasts become trapped in these secretions and differentiate into less active osteocytes. Osteoclasts are large bone cells with up to 50 nuclei. They remove bone structure by releasing lysosomal enzymes and acids that dissolve the bony matrix. These minerals, released from bones into the blood, help regulate calcium concentrations in body fluids. Bone may also be resorbed for remodeling, if the applied stresses have changed. Osteocytes are mature bone cells and are the main cells in bony connective tissue these cells cannot divide. Osteocytes maintain normal bone structure by recycling the mineral salts in the bony matrix. Osteoprogenitor cells are squamous stem cells that divide to produce daughter cells that differentiate into osteoblasts. Osteoprogenitor cells are important in the repair of fractures.

In Summary: Structure of Bones

Compact bone tissue is composed of osteons and forms the external layer of all bones. Spongy bone tissue is composed of trabeculae and forms the inner part of all bones. Four types of cells compose bony tissue: osteocytes, osteoclasts, osteoprogenitor cells, and osteoblasts.

The wrong model

All Hollywood script writers worth their salt know that a successful plot is built around a boy meeting a girl, losing the girl and then winning the girl back. Change a few nouns, and you pretty much have the story of the double helix: Crick and Watson 'discover' the structure, lose the 'discovery' when it turns out to be wrong, and then win back the discovery by coming up with a better model. Where they erred was in rushing to triumphalism with the first, incorrect, model: "we suddenly received a call, in '51 I think it was, from Crick - from Maurice. Crick had got in touch with Maurice to say that he hoped he didn't mind, but they had built - him and Jim had built - a model of DNA as a double helix, following the results that we had deduced in structure 'B'. And would we like to go to Cambridge to see it?"

According to Gosling, it was clear to him then that the King's data, such as it was at the time, had fed into Watson and Crick's model. Nevertheless, Gosling, Franklin and Wilkins, together with their King's colleagues Bill Seeds and Geoffrey Brown, took the Liverpool Street train "with a heavy heart" to Cambridge. But, upon arrival, it was immediately apparent that Watson and Crick had made some elementary mistakes, in both senses of the word.

"We arrived in the lab to be shown the model and to the absolute relief of Rosalind and myself - I don't know about Wilkins, what he thought at the time, because I was dealing with my own thoughts and not observing other people - the boys had built a model with the phosphate linkages going up the middle of the thing, which gave it, of course, rigidity, and so you could hang all the nucleotides and things off the ends of the ionic chain. That must be wrong, because we knew that the water went into that phosphate-oxygen group, and there was an ionic linkage there between the sodium - it was the sodium salt of DNA - and the phosphate group, and you got eight molecules of water going in, quite a lot of water that would go in and come out very easily, as we had shown. So it meant that whatever the structure was, those phosphate groups had to be on the outside. And so we were delighted, and Bragg was embarrassed because it wasn't done to actually work on another man's problem."

Watson and Crick's recklessness, in playing their cards so early, was pounced upon by Franklin, who "tore the model apart point by point." As Gosling notes with some amusement, Crick was later to comment that Franklin's demolition of the model was the only time he ever saw Jim Watson at a loss for words. "And I can believe it!"

So did Franklin deliver her criticism with obvious relish, or did she play it straight? "Oh, no, with obvious relish! She reminded me very much of a particular lady in the University of St Andrews Physics Department that I worked in when I left King's, in which she'd turn up at seminars by new PhD students or the like and she would tear their suggestions apart. 'You're wrong, and you're wrong for the following reasons, one, two, three, four. '"

Gosling had, like Franklin, realized instantly that the model was wrong but did not join her in skewering its inadequacies. Why not? "I left it to her. I didn't need to discuss it at all, I mean she was. she was on top of her form! My word, no."

Perhaps the famously negative portrayal of Franklin in Jim Watson's book 'The Double Helix' was payback for this moment? "Yes. Oh, I'd never thought of it, but yes, that's true. The humiliation. He must have felt - that's the word - he must have felt humiliated. Who the hell is this woman telling me. Yes, you can see it more clearly looking back, can't you?"

Rather than focus on Jim Watson's humiliation, however, Gosling was at the time "just happy that it meant that Rosalind and I could go back to the Strand and just get on with doing the mathematics. And it was ours to take as long as we liked."

Watson and Crick's misplaced haste really did seem to have handed the game to King's. When Wilkins reported what had happened to Randall, he was "furious and stormed off to Cambridge to see Sir Lawrence, and Lawrence was apologetic and actually forbade the lads from doing any more work on DNA, and that it was a King's problem and that Crick had plenty to do on hemoglobin and that he should concentrate on getting his PhD."

Skeletal system 1: the anatomy and physiology of bones

The skeletal system is formed of bones and cartilage, which are connected by ligaments to form a framework for the remainder of the body tissues. This article, the first in a two-part series on the structure and function of the skeletal system, reviews the anatomy and physiology of bone. Understanding the structure and purpose of the bone allows nurses to understand common pathophysiology and consider the most-appropriate steps to improve musculoskeletal health.

Citation: Walker J (2020) Skeletal system 1: the anatomy and physiology of bones. Nursing Times [online] 116: 2, 38-42.

Author: Jennie Walker is principal lecturer, Nottingham Trent University.

  • This article has been double-blind peer reviewed
  • Scroll down to read the article or download a print-friendly PDF here (if the PDF fails to fully download please try again using a different browser)
  • Read part 2 of this series here


The skeletal system is composed of bones and cartilage connected by ligaments to form a framework for the rest of the body tissues. There are two parts to the skeleton:

  • Axial skeleton – bones along the axis of the body, including the skull, vertebral column and ribcage
  • Appendicular skeleton – appendages, such as the upper and lower limbs, pelvic girdle and shoulder girdle.


As well as contributing to the body’s overall shape, the skeletal system has several key functions, including:

  • Support and movement
  • Protection
  • Mineral homeostasis
  • Blood-cell formation
  • Triglyceride storage.

Support and movement

Bones are a site of attachment for ligaments and tendons, providing a skeletal framework that can produce movement through the coordinated use of levers, muscles, tendons and ligaments. The bones act as levers, while the muscles generate the forces responsible for moving the bones.


Bones provide protective boundaries for soft organs: the cranium around the brain, the vertebral column surrounding the spinal cord, the ribcage containing the heart and lungs, and the pelvis protecting the urogenital organs.

Mineral homoeostasis

As the main reservoirs for minerals in the body, bones contain approximately 99% of the body’s calcium, 85% of its phosphate and 50% of its magnesium (Bartl and Bartl, 2017). They are essential in maintaining homoeostasis of minerals in the blood with minerals stored in the bone are released in response to the body’s demands, with levels maintained and regulated by hormones, such as parathyroid hormone.

Blood-cell formation (haemopoiesis)

Blood cells are formed from haemopoietic stem cells present in red bone marrow. Babies are born with only red bone marrow over time this is replaced by yellow marrow due to a decrease in erythropoietin, the hormone responsible for stimulating the production of erythrocytes (red blood cells) in the bone marrow. By adulthood, the amount of red marrow has halved, and this reduces further to around 30% in older age (Robson and Syndercombe Court, 2018).

Triglyceride storage

Yellow bone marrow (Fig 1) acts as a potential energy reserve for the body it consists largely of adipose cells, which store triglycerides (a type of lipid that occurs naturally in the blood) (Tortora and Derrickson, 2009).

Bone composition

Bone matrix has three main components:

  • 25% organic matrix (osteoid)
  • 50% inorganic mineral content (mineral salts)
  • 25% water (Robson and Syndercombe Court, 2018).

Organic matrix (osteoid) is made up of approximately 90% type-I collagen fibres and 10% other proteins, such as glycoprotein, osteocalcin, and proteoglycans (Bartl and Bartl, 2017). It forms the framework for bones, which are hardened through the deposit of the calcium and other minerals around the fibres (Robson and Syndercombe Court, 2018).

Mineral salts are first deposited between the gaps in the collagen layers with once these spaces are filled, minerals accumulate around the collagen fibres, crystallising and causing the tissue to harden this process is called ossification (Tortora and Derrickson, 2009). The hardness of the bone depends on the type and quantity of the minerals available for the body to use hydroxyapatite is one of the main minerals present in bones.

While bones need sufficient minerals to strengthen them, they also need to prevent being broken by maintaining sufficient flexibility to withstand the daily forces exerted on them. This flexibility and tensile strength of bone is derived from the collagen fibres. Over-mineralisation of the fibres or impaired collagen production can increase the brittleness of bones – as with the genetic disorder osteogenesis imperfecta – and increase bone fragility (Ralston and McInnes, 2014).


Bone architecture is made up of two types of bone tissue:

Cortical bone

Also known as compact bone, this dense outer layer provides support and protection for the inner cancellous structure. Cortical bone comprises three elements:

The periosteum is a tough, fibrous outer membrane. It is highly vascular and almost completely covers the bone, except for the surfaces that form joints these are covered by hyaline cartilage. Tendons and ligaments attach to the outer layer of the periosteum, whereas the inner layer contains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) responsible for bone remodelling.

The function of the periosteum is to:

  • Protect the bone
  • Help with fracture repair
  • Nourish bone tissue (Robson and Syndercombe Court, 2018).

It also contains Volkmann’s canals, small channels running perpendicular to the diaphysis of the bone (Fig 1) these convey blood vessels, lymph vessels and nerves from the periosteal surface through to the intracortical layer. The periosteum has numerous sensory fibres, so bone injuries (such as fractures or tumours) can be extremely painful (Drake et al, 2019).

The intracortical bone is organised into structural units, referred to as osteons or Haversian systems (Fig 2). These are cylindrical structures, composed of concentric layers of bone called lamellae, whose structure contributes to the strength of the cortical bone. Osteocytes (mature bone cells) sit in the small spaces between the concentric layers of lamellae, which are known as lacunae. Canaliculi are microscopic canals between the lacunae, in which the osteocytes are networked to each other by filamentous extensions. In the centre of each osteon is a central (Haversian) canal through which the blood vessels, lymph vessels and nerves pass. These central canals tend to run parallel to the axis of the bone Volkmann’s canals connect adjacent osteons and the blood vessels of the central canals with the periosteum.

The endosteum consists of a thin layer of connective tissue that lines the inside of the cortical surface (Bartl and Bartl, 2017) (Fig 1).

Cancellous bone

Also known as spongy bone, cancellous bone is found in the outer cortical layer. It is formed of lamellae arranged in an irregular lattice structure of trabeculae, which gives a honeycomb appearance. The large gaps between the trabeculae help make the bones lighter, and so easier to mobilise.

Trabeculae are characteristically oriented along the lines of stress to help resist forces and reduce the risk of fracture (Tortora and Derrickson, 2009). The closer the trabecular structures are spaced, the greater the stability and structure of the bone (Bartl and Bartl, 2017). Red or yellow bone marrow exists in these spaces (Robson and Syndercombe Court, 2018). Red bone marrow in adults is found in the ribs, sternum, vertebrae and ends of long bones (Tortora and Derrickson, 2009) it is haemopoietic tissue, which produces erythrocytes, leucocytes (white blood cells) and platelets.

Blood supply

Bone and marrow are highly vascularised and account for approximately 10-20% of cardiac output (Bartl and Bartl, 2017). Blood vessels in bone are necessary for nearly all skeletal functions, including the delivery of oxygen and nutrients, homoeostasis and repair (Tomlinson and Silva, 2013). The blood supply in long bones is derived from the nutrient artery and the periosteal, epiphyseal and metaphyseal arteries (Iyer, 2019).

Each artery is also accompanied by nerve fibres, which branch into the marrow cavities. Arteries are the main source of blood and nutrients for long bones, entering through the nutrient foramen, then dividing into ascending and descending branches. The ends of long bones are supplied by the metaphyseal and epiphyseal arteries, which arise from the arteries from the associated joint (Bartl and Bartl, 2017).

If the blood supply to bone is disrupted, it can result in the death of bone tissue (osteonecrosis). A common example is following a fracture to the femoral neck, which disrupts the blood supply to the femoral head and causes the bone tissue to become necrotic. The femoral head structure then collapses, causing pain and dysfunction.


Bones begin to form in utero in the first eight weeks following fertilisation (Moini, 2019). The embryonic skeleton is first formed of mesenchyme (connective tissue) structures this primitive skeleton is referred to as the skeletal template. These structures are then developed into bone, either through intramembranous ossification or endochondral ossification (replacing cartilage with bone).

Bones are classified according to their shape (Box 1). Flat bones develop from membrane (membrane models) and sesamoid bones from tendon (tendon models) (Waugh and Grant, 2018). The term intra-membranous ossification describes the direct conversion of mesenchyme structures to bone, in which the fibrous tissues become ossified as the mesenchymal stem cells differentiate into osteoblasts. The osteoblasts then start to lay down bone matrix, which becomes ossified to form new bone.

Box 1. Types of bones

Long bones – typically longer than they are wide (such as humerus, radius, tibia, femur), they comprise a diaphysis (shaft) and epiphyses at the distal and proximal ends, joining at the metaphysis. In growing bone, this is the site where growth occurs and is known as the epiphyseal growth plate. Most long bones are located in the appendicular skeleton and function as levers to produce movement

Short bones – small and roughly cube-shaped, these contain mainly cancellous bone, with a thin outer layer of cortical bone (such as the bones in the hands and tarsal bones in the feet)

Flat bones – thin and usually slightly curved, typically containing a thin layer of cancellous bone surrounded by cortical bone (examples include the skull, ribs and scapula). Most are located in the axial skeleton and offer protection to underlying structures

Irregular bones – bones that do not fit in other categories because they have a range of different characteristics. They are formed of cancellous bone, with an outer layer of cortical bone (for example, the vertebrae and the pelvis)

Sesamoid bones – round or oval bones (such as the patella), which develop in tendons

Long, short and irregular bones develop from an initial model of hyaline cartilage (cartilage models). Once the cartilage model has been formed, the osteoblasts gradually replace the cartilage with bone matrix through endochondral ossification (Robson and Syndercombe Court, 2018). Mineralisation starts at the centre of the cartilage structure, which is known as the primary ossification centre. Secondary ossification centres also form at the epiphyses (epiphyseal growth plates) (Danning, 2019). The epiphyseal growth plate is composed of hyaline cartilage and has four regions (Fig 3):

Resting or quiescent zone – situated closest to the epiphysis, this is composed of small scattered chondrocytes with a low proliferation rate and anchors the growth plate to the epiphysis

Growth or proliferation zone – this area has larger chondrocytes, arranged like stacks of coins, which divide and are responsible for the longitudinal growth of the bone

Hypertrophic zone – this consists of large maturing chondrocytes, which migrate towards the metaphysis. There is no new growth at this layer

Calcification zone – this final zone of the growth plate is only a few cells thick. Through the process of endochondral ossification, the cells in this zone become ossified and form part of the ‘new diaphysis’ (Tortora and Derrickson, 2009).

Bones are not fully developed at birth, and continue to form until skeletal maturity is reached. By the end of adolescence around 90% of adult bone is formed and skeletal maturity occurs at around 20-25 years, although this can vary depending on geographical location and socio-economic conditions for example, malnutrition may delay bone maturity (Drake et al, 2019 Bartl and Bartl, 2017). In rare cases, a genetic mutation can disrupt cartilage development, and therefore the development of bone. This can result in reduced growth and short stature and is known as achondroplasia.

The human growth hormone (somatotropin) is the main stimulus for growth at the epiphyseal growth plates. During puberty, levels of sex hormones (oestrogen and testosterone) increase, which stops cell division within the growth plate. As the chondrocytes in the proliferation zone stop dividing, the growth plate thins and eventually calcifies, and longitudinal bone growth stops (Ralston and McInnes, 2014). Males are on average taller than females because male puberty tends to occur later, so male bones have more time to grow (Waugh and Grant, 2018). Over-secretion of human growth hormone during childhood can produce gigantism, whereby the person is taller and heavier than usually expected, while over-secretion in adults results in a condition called acromegaly.

If there is a fracture in the epiphyseal growth plate while bones are still growing, this can subsequently inhibit bone growth, resulting in reduced bone formation and the bone being shorter. It may also cause misalignment of the joint surfaces and cause a predisposition to developing secondary arthritis later in life. A discrepancy in leg length can lead to pelvic obliquity, with subsequent scoliosis caused by trying to compensate for the difference.


Once bone has formed and matured, it undergoes constant remodelling by osteoclasts and osteoblasts, whereby old bone tissue is replaced by new bone tissue (Fig 4). Bone remodelling has several functions, including mobilisation of calcium and other minerals from the skeletal tissue to maintain serum homoeostasis, replacing old tissue and repairing damaged bone, as well as helping the body adapt to different forces, loads and stress applied to the skeleton.

Calcium plays a significant role in the body and is required for muscle contraction, nerve conduction, cell division and blood coagulation. As only 1% of the body’s calcium is in the blood, the skeleton acts as storage facility, releasing calcium in response to the body’s demands. Serum calcium levels are tightly regulated by two hormones, which work antagonistically to maintain homoeostasis. Calcitonin facilitates the deposition of calcium to bone, lowering the serum levels, whereas the parathyroid hormone stimulates the release of calcium from bone, raising the serum calcium levels.

Osteoclasts are large multinucleated cells typically found at sites where there is active bone growth, repair or remodelling, such as around the periosteum, within the endosteum and in the removal of calluses formed during fracture healing (Waugh and Grant, 2018). The osteoclast cell membrane has numerous folds that face the surface of the bone and osteoclasts break down bone tissue by secreting lysosomal enzymes and acids into the space between the ruffled membrane (Robson and Syndercombe Court, 2018). These enzymes dissolve the minerals and some of the bone matrix. The minerals are released from the bone matrix into the extracellular space and the rest of the matrix is phagocytosed and metabolised in the cytoplasm of the osteoclasts (Bartl and Bartl, 2017). Once the area of bone has been resorbed, the osteoclasts move on, while the osteoblasts move in to rebuild the bone matrix.

Osteoblasts synthesise collagen fibres and other organic components that make up the bone matrix. They also secrete alkaline phosphatase, which initiates calcification through the deposit of calcium and other minerals around the matrix (Robson and Syndercombe Court, 2018). As the osteoblasts deposit new bone tissue around themselves, they become trapped in pockets of bone called lacunae. Once this happens, the cells differentiate into osteocytes, which are mature bone cells that no longer secrete bone matrix.

The remodelling process is achieved through the balanced activity of osteoclasts and osteoblasts. If bone is built without the appropriate balance of osteocytes, it results in abnormally thick bone or bony spurs. Conversely, too much tissue loss or calcium depletion can lead to fragile bone that is more susceptible to fracture. The larger surface area of cancellous bones is associated with a higher remodelling rate than cortical bone (Bartl and Bartl, 2017), which means osteoporosis is more evident in bones with a high proportion of cancellous bone, such as the head/neck of femur or vertebral bones (Robson and Syndercombe Court, 2018). Changes in the remodelling balance may also occur due to pathological conditions, such as Paget’s disease of bone, a condition characterised by focal areas of increased and disorganised bone remodelling affecting one or more bones. Typical features on X-ray include focal patches of lysis or sclerosis, cortical thickening, disorganised trabeculae and trabecular thickening.

As the body ages, bone may lose some of its strength and elasticity, making it more susceptible to fracture. This is due to the loss of mineral in the matrix and a reduction in the flexibility of the collagen.

Diet and lifestyle factors

Adequate intake of vitamins and minerals is essential for optimum bone formation and ongoing bone health. Two of the most important are calcium and vitamin D, but many others are needed to keep bones strong and healthy (Box 2).

Box 2. Vitamins and minerals needed for bone health

Key nutritional requirements for bone health include minerals such as calcium and phosphorus, as well as smaller qualities of fluoride, manganese, and iron (Robson and Syndercombe Court, 2018). Calcium, phosphorus and vitamin D are essential for effective bone mineralisation. Vitamin D promotes calcium absorption in the intestines, and deficiency in calcium or vitamin D can predispose an individual to ineffective mineralisation and increased risk of developing conditions such as osteoporosis and osteomalacia.

Other key vitamins for healthy bones include vitamin A for osteoblast function and vitamin C for collagen synthesis (Waugh and Grant, 2018).

Physical exercise, in particular weight-bearing exercise, is important in maintaining or increasing bone mineral density and the overall quality and strength of the bone. This is because osteoblasts are stimulated by load-bearing exercise and so bones subjected to mechanical stresses undergo a higher rate of bone remodelling. Reduced skeletal loading is associated with an increased risk of developing osteoporosis (Robson and Syndercombe Court, 2018).


Bones are an important part of the musculoskeletal system and serve many core functions, as well as supporting the body’s structure and facilitating movement. Bone is a dynamic structure, which is continually remodelled in response to stresses placed on the body. Changes to this remodelling process, or inadequate intake of nutrients, can result in changes to bone structure that may predispose the body to increased risk of fracture. Part 2 of this series will review the structure and function of the skeletal system.

Key points

  • Bones are key to providing the body with structural support and enabling movement
  • Most of the body’s minerals are stored in the bones
  • Diet and lifestyle can affect the quality of bone formation
  • After bones have formed they undergo constant remodelling
  • Changes in the remodelling process can result in pathology such as Paget’s disease of bone or osteoporosis

Bartl R, Bartl C (2017) Structure and architecture of bone. In: Bone Disorder: Biology, Diagnosis, Prevention, Therapy.

Danning CL (2019) Structure and function of the musculoskeletal system. In: Banasik JL, Copstead L-EC (eds) Pathophysiology. St Louis, MO: Elsevier.

Drake RL et al (eds) (2019) Gray’s Anatomy for Students. London: Elsevier.

Iyer KM (2019) Anatomy of bone, fracture, and fracture healing. In: Iyer KM, Khan WS (eds) General Principles of Orthopedics and Trauma. London: Springer.

Moini J (2019) Bone tissues and the skeletal system. In: Anatomy and Physiology for Health Professionals. Burlington, MA: Jones and Bartlett.

Ralston SH, McInnes IB (2014) Rheumatology and bone disease. In: Walker BR et al (eds) Davidson’s Principles and Practice of Medicine. Edinburgh: Churchill Livingstone.

Robson L, Syndercombe Court D (2018) Bone, muscle, skin and connective tissue. In: Naish J, Syndercombe Court D (eds) Medical Sciences. London: Elsevier

Tomlinson RE, Silva MJ (2013) Skeletal blood flow in bone repair and maintenance. Bone Research 1: 4, 311-322.

Tortora GJ, Derrickson B (2009) The skeletal system: bone tissue. In: Principles of Anatomy and Physiology. Chichester: John Wiley & Sons.

Waugh A, Grant A (2018) The musculoskeletal system. In: Ross & Wilson Anatomy and Physiology in Health and Illness. London: Elsevier.


The body is deeper in front or in the back and is prolonged downward anteriorly to overlap the upper and front part of the third vertebra.

It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the longus colli muscles.

Dens Edit

The dens, also called the odontoid process or the peg, are the most pronounced projecting feature of the axis. The dens exhibit a slight constriction where it joins the main body of the vertebra. The condition where the dens are separated from the body of the axis is called os odontoideum and may cause nerve and circulation compression syndrome. [1] On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed and gives attachment to the apical odontoid ligament. Below the apex, the process is somewhat enlarged and presents on either side a rough impression for the attachment of the alar ligament these ligaments connect the process to the occipital bone.

The internal structure of the odontoid process is more compact than that of the body. The odontoid peg is the ascension of the atlas fused to the ascension of the axis. The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. It is a non-weight bearing joint. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the margins of the foramen magnum. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the notochord.

Other features Edit

The pedicles are broad and strong, especially in the front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces.

The laminae are thick and strong. They play a large role in the stability of the cervical spine alongside the laminae of C7. [2]

The vertebral foramen is large, but smaller than the atlas.

The transverse processes are very small, and each ends in a single tubercle. Each process is perforated by the transverse foramen, which is directed obliquely upward and laterally.

The superior articular surfaces are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes.

The inferior articular surfaces have the same direction as those of the other cervical vertebrae.

The superior vertebral notches are very shallow, and lie behind the articular processes. The inferior vertebral notches lie in front of the articular processes, as in the other cervical vertebrae.

The spinous process is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity.

Variation Edit

Contact sports are contraindicated for individuals with anomalous dens, as any violent impact may result in a catastrophic injury. [3] This is because a malformed odontoid process may lead to instability between the atlas and axis (the C1 and C2 cervical vertebrae).

Development Edit

The axis is ossified from five primary and two secondary centres.

The body and vertebral arch are ossified in the same manner as the corresponding parts in the other vertebrae, viz., one centre for the body, and two for the vertebral arch.

The centres for the arch appear about the seventh or eighth week of fetal life, while the centres for the body appear in about the fourth or fifth month.

The dens, or odontoid process, consist originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed.

During about the sixth month of fetal life, two centres make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage.

The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age.

In this cartilage, rudiments of the lower epiphyseal lamella of the atlas and the upper epiphyseal lamella of the axis may sometimes be found.

The apex of the odontoid process has a separate centre that appears in the second and joins about the twelfth year this is the upper epiphyseal lamella of the atlas.

In addition to these, there is a secondary centre for a thin epiphyseal plate on the undersurface of the body of the bone.

Fracture of dens Edit

Fractures of the dens, not to be confused with Hangman's fractures, are classified into three categories according to the Anderson Alonso system:

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