Connective tissue. Composition. Components

Connective tissue is Connective tissue is composed of three components; cells, Obers, and ground substance. The fibers and ground substance together are known as the extracellular matrix. The functions of connective tissue are multiple and varied, including structural support, an immunologic barrier, storage and metabolism of fat, and conservation of water.

Two broad categories of connective tissue cells exist. The first includes those found only in connective tissues—fibroblasts, adipocytes, macrophages, plasma cells, reticular cells, and, in the embryo, mesenchymal cells. The second category consists of hematogenous cells that move from the vascular system (i.e., marrow, spleen) into connective tissue. It includes lymphocytes, monocytes, granulocytes, and developing blood cells, which are discussed in later chapters.

Fibers found in the extracellular matrix include collagen and elastin. Collagen is the most abundant protein in the human body and the principal type of fiber found in connective tissue. Based on their morphology, amino acid composition, and physical properties, 10 to 12 different types of collagen have been identified, although the exact functions of many of them remain to be defined. Type I collagen has high tensile strength, comprises about 90% of the total body collagen, and is found in skin, tendon, bone, dentin, and loose connective tissue.

By LM, bundles of type I collagen are seen as eosinophilic fibers, while at the ТЕМ level the fibers are seen to be composed of individual fibrils with 64 nm periodicity. Type II collagen forms fibrils with 64 nm periodicity that can be detected by ТЕМ, but it does not form eosinophilic bundles by LM and is found in cartilage, the nucleus pulposus, the vitreous body, and cornea. Type III collagen, also known as reticular fibers, consists of fibrils with 64 nm periodicity that have a high content of carbohydrate. This explains their staining with silver and the periodic acid Schiff method.

Type III collagen does not form large bundles visible by LM and is distributed widely in the body, including skin, blood vessels, and the stroma of lymphoid, liver, and other organs. Type IV collagen does not form fibrils with 64 nm periodicity, but it comprises a fine filamentous meshwork known as the basal lamina of the basement membrane associated with epithelial cells and the basal lamina (external lamina) of muscle cells, Schwann cells, and adipocytes. Type V collagen is similar to type IV collagen in that it does not form fibrils. Although found in small amounts throughout the body, its structure and function still remain to be determined.

Recently type VII collagen has been identified in anchoring fibrils connecting the basement membrane of epithelia to underlying connective tissue, and type X collagen has been detected in the epiphyseal plate of bone. However, their functional roles and structure must still be determined. Besides the fibroblast, many types of collagen can be produced by a number of different connective tissue cell types (adipocytes, chondrocytes, osteocytes, and reticular cells). Type IV collagen found in the basal lamina is produced by epithelial cells, muscle cells, and Schwann cells of the peripheral nervous system.

Elastic fibers in the extracellular matrix have two structurally distinct components at the ТЕМ level. The first is an amorphous substance composed of elastin, a special protein rich in nonpolar amino acids, which accounts for its staining with the dye orcein (Verhoeff stain) at the LM level. The second component consists of microfibrils that incompletely surround strands of the amorphous component and may serve to orient the elastin. Like collagen, elastin can be produced by numerous cell types including fibroblasts, smooth muscle cells, and chondrocytes in elastic cartilage.

The ground substance of the extracellular matrix is composed of glycosaminoglycans (previously called mucopolysaccharides) and structural glycoproteins.

Glycosaminoglycans, or GAGs, are linear, unbranched polysaccharide chains composed of repeating disaccharide units. Because of their high content of acidic side groups (hydroxyl, carboxyl, and sulfate), they bind large amounts of water as a shell of hydration. GAGs in turn are covalently attached to a larger core protein and thus constitute a proteoglycan molecule. These proteoglycan molecules are usually connected via noncovalent bonds by a link protein to long chain-like molecules of hyaluronic acid (the only nonsulfated GAG molecule), which then form large molecular aggregates that gives connective tissue its semifluid gel-like properties.

Structural glycoproteins, such.as fiforonectin (abundant in all connective tissue; referred to as cold insoluble globulin in blood), chondronectin (cartilage), and osteonectin (bone), are composed chiefly of protein with attached branched carbohydrate side chains, They possess specialized regions for attachment to cellular surfaces and various GAGs in the extracellular matrix. Because of their bivalent symmetry, structural glycoproteins may play an important role in the adhesion of cells to one another and to components of the extracellular matrix, such as collagen, or GAGs bound to the surface of fibers or cells.

The general macroscopic classification of connective tissues is based, on the relative abundance of cells, extracellular fibers, and ground substance:

1. Mucous—contains large amounts of ground substance and few cells or fibers and found in the umbilical cord and embryonic tissues.

2. Reticular—composed of reticular fibers (type III collagen) and present in the stroma of liver, adipose tissue, and lymphoid organs.

3. Loose or areolar—made up of equivalent amounts of cells, fibers, and ground substance and found in subcutaneous regions and mesenteries and as an interface between adjacent tissues.

4. Adipose—composed of adipocytes arranged in loose clusters or dense masses of cells, as seen in subcutaneous fat, epiploic appendages of the large intestine, or the omenta.

5. Dense connective—comprised mainly of either collagen or elastin with few intervening cells. Regular dense connective tissue consists of fibers arranged in one plane (i.e., tendons), whereas irregular dense connective tissue has fibers traveling in multiple planes (i.e., dermis of the skin).

Specialised connective—includes those designed for support (bone and cartilage) or transport (blood), which are discussed in separate chapters.