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33.2B: Connective Tissues: Loose, Fibrous, and Cartilage - Biology

33.2B: Connective Tissues: Loose, Fibrous, and Cartilage - Biology


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Connective tissue is found throughout the body, providing support and shock absorption for tissues and bones.

Learning Objectives

  • Distinguish between the different types of connective tissue

Key Points

  • Fibroblasts are cells that generate any connective tissue that the body needs, as they can move throughout the body and can undergo mitosis to create new tissues.
  • Protein fibers run throughout connective tissue, providing stability and support; they can be either collagen, elastic, or reticular fibers.
  • Loose connective tissue is not particularly tough, but surrounds blood vessels and provides support to internal organs.
  • Fibrous connective tissue, which is composed of parallel bundles of collagen fibers, is found in the dermis, tendons, and ligaments.
  • Hyaline cartilage forms the skeleton of the embryo before it is transformed into bone; it is found in the adult body at the tip of the nose and around the ends of the long bones, where it prevents friction at the joints.
  • Fibrocartilage is the strongest of the connective tissues; it is found in regions of the body that experience large amounts of stress and require a high degree of shock absorption, such as between the vertebrae.

Key Terms

  • chondrocyte: a cell that makes up the tissue of cartilage
  • motile: having the power to move spontaneously
  • fibroblast: a cell found in connective tissue that produces fibers, such as collagen

Connective Tissues

Connective tissues are composed of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is composed of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast, an immature connective tissue cell that has not yet differentiated. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues, while others may have specialized cells. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has a proportionally-less-dense matrix.

The organic portion, or protein fibers, found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length, returning to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers, the third type of protein fiber found in connective tissues, consist of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected.

Loose (Areolar) Connective Tissue

Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. Loose connective tissue has some fibroblasts, although macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel, helping to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.

Fibrous Connective Tissue

Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly-arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).

Cartilage

Cartilage is a connective tissue. The cells, called chondrocytes (mature cartilage cells), make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called “lacunae. ”

A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during some pre-birth developmental stages. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.

Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage, as do portions of the larynx, or voice box. In contrast, fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals, which must withstand a tremendous amount of stress. Cartilage can also transform from one type to another. For example, hyaline cartilage found in movable joints, such as the knee and shoulder, often becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage, resulting in “stiff” joints.


Connective Tissue

Connective tissue is found between other tissue types and organs. It contains high quantities of water, several types of cells, and a fibrous extracellular matrix. The connective tissue of an organ is usually referred to as the stroma. This tissue type can have very different structures according to the proportions of its components. Specialized forms includes bone, cartilage, fat, and even blood.


Diagrammatic Representation of Classes of Cartilage | Human | Biology

In this article we will discuss about the diagrammatic representation of classes of cartilage in human body.

Diagrammatic Representation of Hyaline Cartilage:

It is made up of cartilage cells and a clear homogeneous ground substance completely devoid of any fibrous (loose) tissue. (Fig. 1.44)

Hyaline means glass and in fresh condition it appears as a translucent bluish- white mass. The cartilage cell or chondrocytes occupy small empty spaces called lacunae in the matrix. These cells are large with rounded angles and the contiguous surfaces are flattened due to pressure of the adjacent cells.

The nucleus is large and is provided with two or more nucleoli. When the cells are arranged in group of two, four, etc., in a single lacuna it is said to be cell nest. Each group of cells arises from the multiplication of single parent cartilage cell.

The cytoplasm is very rich in glycogen and may contain fat droplets and mitochondria. In electron micro­scope, the cytoplasm shows large rough-surfaced endoplasmic reticulum, Golgi apparatus with large vacuoles.

The intercellular ground substance surrounding, the cells is laid out in concentric rings and take deeper stain. This deeply staining portion around the cells is called the capsule.

It is the solid intercellular substance of the cartilage or bone. The matrix of bone is harder than that of the cartilage only for the deposition of calcium salts in the former. In the hyaline cartilage, this intercellular substance matrix is abundant and appears homogeneous in the fresh condition but with ordinary fixation it shows collagen fibres and amorphous intercellular substance. Collagen fibres are seen in these sections under polarizing microscope.

The ground substance is highly basophilic due to presence of chondromucoprotein, a polymer of a muco- protein together with chondroitin-4 sulphate and chondroitin-6-sulpahte. When boiled, cartilage is slowly dissolved and gives rise to cartilage glue—the chondrin containing gelatin, chondromucoproteins and other albuminous substances.

The hyaline cartilage is enclosed by a tough layer of dense connective tissue capsule which is known peri­chondrium. It consists of loose (fibrous) and dense (chondrogenic) layers with indistinguishable fibrocytes.

It is found in the articular end of the bones (articular cartilage), between the epiphysis and the diaphysis of the growing long bones (epiphyseal cartilage), at the anterior end of the ribs (costal cartilage). The cartilage of the nose, external auditory meatus, larynx, trachea and bronchial tubes also belong to this class.

The costal cartilages are covered by the perichondrium. It serves the same purposes as periosteum of the bones. The nutrition and oxygen to the cartilage are supplied through the blood vessels of the perichondrium. Thicker cartilages are pervaded here and there by fine channels carrying blood vessels.

Diagrammatic Representation of Fibrocartilage:

This type of cartilage is present where great tensile strength with flexibility and rigidity are required. It is capable of withstanding shearing forces. The cells are large, arranged in groups and placed inside lacunae. Fibrocartilage contains more collagen in its intercellular substance than hyaline cartilage and lacks perichondri­um. Between the cell groups, bundles of white fibrous tissue are seen. (Fig. 1.45)

It is found in the intervertebral discs, the menisci of knee joints, mandibular joints, pubic sym­physis, linings of many tendon grooves in bones, in the attachments of some tendons, etc.

Diagrammatic Representation of Elastic Cartilage:

It is present in the area where support with flexibility is required. It is yellow in colour and con­tains many elastic fibres. It differs from hyaline cartilage only for the presence of its enormous elastic fibres in the matrix. The matrix in addi­tion to the elastic fibres contains collagen fibres. (Fig. 1.46)

It is found in the external ear (pin­na), Eustachian tube, and epiglottis and in some of the laryngeal cartilages.


Connective Tissue

The human body is composed of just four basic kinds of tissue: nervous, muscular, epithelial, and connective tissue. Connective tissue is the most abundant, widely distributed, and varied type. It includes fibrous tissues, fat, cartilage, bone, bone marrow, and blood. As the name implies, connective tissues often bind other organs together, hold organs in place, cushion them, and fill space.

Connective tissue is distinguished from the other types in that the extracellular material (matrix) usually occupies more space than the cells do, and the cells are relatively far apart. Fat is an exception, having cells in close contact with each other but with large, nonliving, intracellular lipid droplets, fat contains much more nonliving material than living material.

The matrix of connective tissue typically consists of fibers and a featureless ground substance. The most abundant fiber in connective tissues is a tough protein called collagen. Tendons, ligaments, and the white stringy tissue (fascia) seen in some cuts of meat are composed almost entirely of collagen, as is leather, which consists of the connective tissue layer (dermis) of animal skins. Collagen also strengthens bone and cartilage. Elastic and reticular fibers are less abundant connective tissue proteins with a more limited distribution.

The ground substance may be liquid, as in blood gelatinous, as in areolar tissue rubbery, as in cartilage or calcified and stony, as in bone. It consists mainly of water and small dissolved ions and organic molecules, but the gelatinous to rubbery consistency of some tissues results from enormous protein-carbohydrate complexes in the ground substance. The hard consistency of bone results mainly from calcium phosphate salts in the ground substance.

Some of the cells of connective tissue are fibroblasts (which produce collagen fibers and are the only cell type in tendons and ligaments) adipocytes (fat cells) leukocytes (white blood cells, also found outside the

Connective tissue type and characteristics Functions Locations
Areolar (loose) connective tissue. Loose array of random fibers with a wide variety of cell types Nourishes and cushions epithelia, provides arena for immune defense against infection, binds organs together, allows passage for nerves and blood vessels through other tissues Under all epithelia outer coverings of blood vessels, nerves, esophagus, and other organs fascia between muscles pleural and pericardial sacs
Adipose tissue (fat). Large fat-filled adipocytes and scanty extracellular matrix. Stores energy, conserves body heat, cushions and protects many organs, fills space, shapes body Beneath skin around kidneys, heart, and eyes breast abdominal membranes (mesenteries)
Dense irregular connective tissue. Densely spaced, randomly arranged fibers and fibroblasts. Toughness protects organs from injury provides protective capsules around many organs Dermis of skin capsules around liver, spleen, and other organs fibrous sheath around bones
Dense regular connective tissue. Densely spaced, parallel collagen fibers and fibroblasts. Binds bones together and attaches muscle to bone transfers force from muscle to bone Tendons and ligaments
Cartilage (gristle). Widely spaced cells in small cavities (lacunae) rubbery matrix. Eases joint movements resists compression at joints holds airway open shapes outer ear moves vocal cords forerunner of fetal skeleton growth zone of children's bones External ear, larynx, rings around trachea, joint surfaces and growth zones of bones, between ribs and sternum, intervertebral discs
Bone (osseous tissue). Widely spaced cells in lacunae much of matrix in concentric onionlike layers hard mineralized matrix. Physically supports body, provides movement, encloses and protects soft organs, stores and releases calcium and phosphorus   Skeleton
Blood. Erythrocytes, leukocytes, and platelets in Transports nutrients, gases, wastes, hormones, Circulates in cardiovascular system

bloodstream in fibrous connective tissues) macrophages (large phagocytic cells descended from certain leukocytes) erythrocytes (red blood cells, found only in the blood and bone marrow) chondrocytes (cartilage cells) and osteocytes (bone cells).

The table above lists representative locations and functions of the major types of connective tissue. Further details on connective tissue can be found in textbooks of histology and human anatomy.


Bone And Connective Tissue In The Vertebrate Skeleton

Connective tissue

Connective tissue is abundant in the body: it is characterized by few cells, extensive intercellular mineral and/or protein, and a rich blood supply. It supports, protects and binds other cells together. Connective tissue comprises ‘connective tissue proper’, cartilage, bone and blood.

Connective tissue Proper

Connective tissue proper’ typically has elongated fibroblast cells and a more or less fluid intercellular material. There are five types:

(1) loose areolar tissue is found around organs, in mucous membranes and under the skin hyaluronic acid is in the intercellular matrix with some fibers and cells

(2) adipose tissue is areolar tissue with cells specialized for fat storage: it can grow throughout life

(3) dense connective tissue has collagen fibers: it is found in tendons, ligaments, muscle fasciae, etc.

(4) elastic connective tissue has branching elastic fibers: it is found in arterial walls, trachea, vocal cords, etc.

(5) reticular connective tissue has a network of fibers: it forms the loose frame of the liver, spleen, lymph nodes, etc.

Cartilage Cartilage has a gel matrix containing chondrocytes and collagen and elastic fibers. It grows from within and without.

  • Hyaline cartilage has a glassy consistency it is found at the ends of bones, in the nose and the respiratory tract, and is supportive.
  • Fibrocartilage is very strong. Strengthened with collagen, it connects bones (e.g. in the pelvis) and forms intervertebral disks (the relic of the notochord in most vertebrates).
  • Elastic cartilage maintains the shapes of the external ear (pinna) and the larynx.

Bone

Bone is the main component of the skeletal system. Functions of bone (with cartilage) include:

  • support (framework of body)
  • protection (e.g. skull)
  • leverage (limbs)
  • mineral storage (especially calcium and phosphorus)
  • storage of blood cell synthetic apparatus (red bone marrow: land vertebrates only)
  • storage of energy (fat in yellow bone marrow: land vertebrates only).

Bone histology

Bone comprises many, widely separated cells surrounded by the intercellular matrix of bone tissue. Four principal bone cell types are found.

(1) Osteoprogenitor cells are stem cells which give rise to osteoblasts: they are found in the periosteum surrounding bones and in the endosteum membrane lining the inner bone cavity, also in canals in bone carrying blood vessels.

(2) Osteoblasts (on bone surfaces) secrete bone mineral and collagen.

(3) Osteocytes are osteoblasts within the bone matrix which they have built up around themselves they maintain the functioning of bone.

(4) Osteoclasts (cells in the monocyte/macrophage series) resorb excess bone material. The bone matrix has collagen fibers and crystalline mineral (mainly hydroxyapatite, a hydrated calcium phosphate with some carbonate, fluoride, sulfate and magnesium salts). The protein collagen makes up about 33% of bone: the minerals crystallize around the protein framework. During aging, the proportion of protein in bone decreases, making it more brittle. Note that bone is a living tissue.

Bone morphology

A typical long bone (e.g. femur) comprises a shaft (diaphysis) and ends (epiphyses) connected by metaphyses. The cylindrical structure of a long bone concentrates most material around the periphery (as seen in cross-section), thus giving maximum resistance to bending moments. A thin layer of hyaline cartilage covers the epiphyses and reduces joint friction. The bone is covered by a periosteum: this has an outer fibrous layer (with nerves and blood vessels) and an inner osteogenic layer with bone-forming cells. In the middle (in higher, land vertebrates) is the marrow cavity, separated from the bone material by the endosteum.

Compact bone

  • Compact bone has few cavities within it it covers spongy bone (see below) and is particularly thick in the shaft. It has a concentric ring structure .
  • Vessels and nerves from the periosteum penetrate the bone via perforating canals and connect with nerves and vessels of the marrow cavity and the central Haversian canals which run longitudinally through the bone.
  • Around the Haversian canals are concentric lamellae, rings of hard ‘bone’. Between the lamellae are small spaces, lacunae, containing osteocytes.
  • Radiating from the lacunae are canaliculi which contain processes from the osteocytes and extracellular fluid the canaliculi interconnect with each other and the Haversian canals.
  • The Haversian canal plus the surrounding lamellae, lacunae, osteocytes and canaliculi form an osteon.

Spongy bone

Spongy (cancellous) bone lacks true osteons it comprises an irregular lattice of thin plates (trabeculae). Within the trabeculae are lacunae with osteocytes. Spongy bone is rich in blood vessels and marrow lies within the network of trabeculae. The trabeculae are aligned parallel to the main forces of compression and tension in the bone.

Ossification

  • Bone forms by osteogenesis (ossification), starting when mesenchymal cells in the embryo transform into osteoprogenitor cells which differentiate into osteoblasts and osteoclasts.
  • Intramembranous ossification occurs within fibrous membranes of the embryo, fetus and child (e.g. skull bones). Clusters of osteoblasts accumulate on a fibrous matrix and deposit calcium salts.
  • Endochondrial ossification occurs within a cartilage model, the primary ossification center being the shaft. Breakdown of cartilage results in cavitation: cavities merge to form a marrow cavity.
  • Osteoblasts deposit mineral and protein, eventually replacing cartilage at the epiphyses where only the cartilaginous articular end-sheets remain together with the epiphyseal plates between the epiphyses and the shaft: the latter are the sites for bone growth.

Bone growth

  • Bones elongate by appositional growth at the epiphyseal plates under hormonal control (e.g. growth hormone). New cartilage cells are generated on the epiphyseal side of the plate, and the older cartilage cells are destroyed and replaced by bone on the shaft side of the plate (thus the plate has a constant thickness but the length of the shaft increases).
  • Growth in diameter occurs when osteoblasts from the periosteum add new bone to the outer surface of the bone while osteoclasts erode bone material inside the shaft and so enlarge the marrow cavity.

Modeling

  • There is a homeostatic balance between formation and resorption of bone material, called remodeling. Calcium, phosphorus and vitamins C and D are essential.
  • Overall bone growth is controlled by growth hormone from the anterior pituitary calcitonin from the thyroid inhibits osteoclast activity parathyroid hormone (parathormone, PTH) increases osteoclast activity sex steroids promote a burst of bone growth during puberty followed by the destruction of the cartilage cells of the epiphyseal plates and epiphyseal fusion, with a cessation of bone growth in early adulthood.
  • Bone mechanically stressed in exercise can increase its strength due to the generation of piezoelectric currents (from mineral crystals) which stimulate osteoblasts. In aging, calcium is withdrawn from bones with a tendency towards osteoporosis less collagen makes aged bones more brittle.

Joints

Joints are points of contact between bones (or cartilage and bone) allowing for varying degrees of movement, from none to freely movable.


Cartilage is a connective tissue. The cells, called chondrocytes (mature cartilage cells), make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called "lacunae. "

A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine stains . Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during some pre-birth developmental stages. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.


Muscle tissue

Muscle tissue is responsible for the movement of the body, with specialised cells that have the ability to contract. Unsurprisingly, it takes up most of the amount of energy in animals. There are three types of muscle tissue: skeletal, smooth and cardiac.

Skeletal muscle is responsible for voluntary movements, and its attached to the skeletal system by tendons. Muscle cells forming the skeletal muscle are long and arranged in fibres. Smooth muscle is responsible for contractions that cannot be voluntarily controlled, such as in the walls of the digestive system and arteries. Cardiac muscle forms the wall of the heart and contracts to allow the heart to pump blood around our body. Its properties are similar to that of skeletal muscle, but its actions are not voluntary.


MCQs on Connective Tissue

1. Find the correct statement
(a) Areolar tissue is a loose connective tissue
(b) Tendon is a specialized connective tissue
(c) Cartilage is a loose connective tissue
(d) Adipose tissue is a dense connective tissue

2. In comparison to human erythrocytes, frog’s erythrocytes are
(a) smaller and fewer
(b) nucleated and without haemoglobin
(c) enucleated but with haemoglobin
(d) nucleated and with haemoglobin

3. Tip of the nose and external ears have
(a) areolar tissue
(b) ligament
(c) cartilage
(d) bone

4. Mast cells contain
(a) heparin and histamine
(b) heparin and calcitonin
(c) serotonin and melanin
(d) vasopressin and relaxin

5. The largest extracellular material present in the
(a) Stratified epithelium
(b) Striated muscle
(c) Myelinated nerve fibres
(d) Areolar tissue

6. Peptidoglycan present in the cartilage is
(a) ossein
(b) chondroitin
(c) cartilagin
(d) casein

7. Antibodies are secreted by
(a) adipose cells
(b) reticular cells
(c) plasma cells
(d) mast cells

8. Histamine is secreted by
(a) histiocytes
(b) lymphocytes
(c) fibroblasts
(d) mast cells

9. Ligament is
(a) modified yellow elastic fibrous tissue
(b) inelastic white fibrous tissue
(c) modified white fibrous tissue
(d) none of the above

10. Collagen is
(a) carbohydrate
(b) lipid
(c) fibrous protein
(d) globular protein


BIO 140 - Human Biology I - Textbook

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Chapter 13

Connective Tissue Supports and Protects

  • Identify and distinguish between the types of connective tissue: proper, supportive, and fluid
  • Explain the functions of connective tissues

As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix . The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris.

Functions of Connective Tissues

Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body.

Embryonic Connective Tissue

All connective tissues derive from the mesodermal layer of the embryo (see [link] ). The first connective tissue to develop in the embryo is mesenchyme , the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton&rsquos jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.

Classification of Connective Tissues

The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix (Table). Connective tissue proper includes loose connective tissue and dense connective tissue . Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue &mdashbone and cartilage&mdashprovide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue , in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.

Table 1: Connective Tissue Examples

Connective Tissue Proper

Fibroblasts are present in all connective tissue proper (Figure 1). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.

Figure 1: Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Cell Types

The most abundant cell in connective tissue proper is the fibroblast . Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix. As you might expect, a fibrocyte , a less active form of fibroblast, is the second most common cell type in connective tissue proper.

Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population.

The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue.

The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the body&rsquos defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues.

The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system.

Connective Tissue Fibers and Ground Substance

Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body.

Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column.

Reticular fiber is also formed from the same protein subunits as collagen fibers however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ).

All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance.

Loose Connective Tissue

Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.

Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure 2 ). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term &ldquobaby fat.&rdquo In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.

Figure 2: This is a loose connective tissue that consists of fat cells with little extracellular matrix. It stores fat for energy and provides insulation. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.

Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 3). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means &ldquolittle net.&rdquo

Figure 3: This is a loose connective tissue made up of a network of reticular fibers that provides a supportive framework for soft organs. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Dense Connective Tissue

Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic.

In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (Figure 4).

Dense Connective Tissue

Figure 4: (a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM × 1000, LM × 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

Disorders of the&hellip

Connective Tissue: Tendinitis

Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your opponent. As you toss the ball high in the air, a burning pain shoots across your wrist and you drop the tennis racket. That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now.

After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing. She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered from tendinitis related injuries.

What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. On rare occasions, a sudden serious injury will cause tendinitis. Most often, the condition results from repetitive motions over time that strain the tendons needed to perform the tasks.

Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest risk of tendinitis. You hear of tennis and golfer&rsquos elbow, jumper's knee, and swimmer&rsquos shoulder. In all cases, overuse of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon does not have abundant blood supply and heals slowly.

While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the court to hit that slice-spin serve over the net.

Supportive Connective Tissues

Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs.

Cartilage

The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes , or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.

The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 5). Hyaline cartilage , the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.

Figure 5: Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM × 300, LM × 1200, LM × 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

Bone

Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone&rsquos rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.

Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength.

Fluid Connective Tissue

Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 6). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body.

Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.

Figure 6: Blood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Chapter Review

Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis.

Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers that are thick, strong, flexible, and resist stretch reticular fibers that are thin and form a supportive mesh and elastin fibers that are thin and elastic.

The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers.


Epithelial Tissue

*Some of this content will be reviewed or built-upon in lecture videos, while some of the epithelial tissue content will be covered in the assignment and interactive activity as self-study material.

Epithelial tissues line all surfaces of the body. This includes the surfaces exposed to the outside world, the surface of organs, and the openings within hollow organs. Epithelium also forms much of the glandular tissue of the body.

Functions

Epithelial tissues have several functions. You will learn about the different functions of specific types of epithelium later in this module. However, you should know these general functions of epithelial tissue.

  • Protection: Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear.
  • Selective permeability: The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes.
    • Diffusion: selective, simple transport of substances through a thin layer of tissue
    • Absorption (transcellular transport): absorption of substances through the cell, where it is processed to some degree before being released into the blood and/or underlying tissue. This is called transcellular transport because it is going through the cell.

    Characteristics & Features

    All epithelia share some important structural and functional features:

    • This tissue is highly cellular, with little or no extracellular material present between cells.
    • Epithelial tissues are avascular, meaning they do not contain blood vessels. Instead, they receive nutrients by diffusion or absorption from the underlying tissues or occasionally from substances on the surface.
    • Many epithelial tissues are capable of regeneration, or rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.
    • The epithelial cells exhibit polarity, with differences in structure and function between the exposed or apicalsurface of the cell, the free surface away from underlying tissue, and the basalsurface attached to the underlying tissue.

    The basement membrane sits just below the basal surface of epithelial cells and anchors the cells to the underlying tissue. It is formed of two layers: the basal lamina, which attaches to the basal aspect of the cells, and the reticular lamina, which is attached to the underlying connective tissue.

    Apical Features

    Two microscopic extensions can be found on the apical surface of some cells.

    • Cilia: extensions from the apical surface of the cell that beat in unison to move fluids as well as trapped particles across the surface of the epithelium (surface parallel transport).
    • Microvilli: extensions that serve to increase the surface area of the apical aspect of the cell for absorption. More surface area means more space for substances to contact the apical surface and be absorbed into the cell.

    Intercellular Junctions

    Epithelial cells are closely connected and are not separated by intracellular material. These cells are held together or interact with each other via junctions, or connections between the cells. Three of these junctions hold the cells together, while one allows for cell-to-cell communication.

    Types of Cell Junctions: The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.

    • Tight junction: Holds cells together so there is no extracellular space between them. Tight junctions prevent substances from moving between the cells, instead forcing them to go through the cells. This enables the epithelia to act as a selective barrier.
    • Adhering junction (adherens): Acts like a belt holding the epithelial cells together for support and stability of the tissue.
    • Desmosome: Holds cells together like a push button on a jacket to provide support and stability of the tissue.
    • Gap junction: Forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.

    Classification of Epithelial Tissues

    *This information will not be reviewed in videos. Be sure to understand what is written here in the text and the table. Your assignment and the interactive activity on the Canvas page will help you review this material.

    Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed ((Figure)).

    • Shape:
      • Squamous: Flattened and thin
      • Cuboidal: Like a cube or box, wide as it is tall
      • Columnar: Like a rectangular cube, taller than it is wide.
      • Simple: A single layer of cells, with every cell resting on the basal lamina of the basement membrane
      • Stratified: More than one layer of cells, with only the basal layer resting on the basal lamina of the basement membrane. This can be just a few layers or dozens of layers depending on the location.
      • Pseudostratified: Only one layer of cells, but the height of the cells vary, giving the appearance of more than one layer (pseudo- = false). Only some of the cells have an apical surface that reaches the free surface of the epithelium.

      Cells of Epithelial Tissue: Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells.

      Thinner epithelium allows for faster and easier transport of substances across the surface, so it functions to allow transcellular (through the cell) transport via simple diffusion. The thicker the epithelium is (cuboidal to columnar), the more space there is for intracellular machinery needed to produce substances for secretion or process substances that have been absorbed before releasing it into the underlying tissue. Stratified epithelium has multiple layers, meaning there are more layers to lose before damaging the underlying tissue. Therefore, stratified epithelium always has a protective function.

      There are several different kinds of epithelium based on different combinations of the shape and number of layers of cells. Note that the name of stratified epithelium is determined by the shape of the cell at the most superficial layer, furthest from the underlying connective tissue. You are responsible for knowing the information in the following table: