Connective tissue fibers are of three principal types.
Connective tissue fibers are present in varying amounts, depending on the structural needs or function of the connective tissue. Each type of fiber is produced by fibroblasts and is composed of protein consisting of long peptide chains. The types of connective tissue fibers are
•Collagen fibers
•Reticular fibers
•Elastic fibers
Collagen Fibers and Fibrils
Collagen fibers are the most abundant type of connective tissue fiber.
Collagen fibers are the most abundant structural components of the connective tissue. They are flexible and have a remarkably high tensile strength. In the light microscope, collagen fibers typically appear as wavy structures of variable width and indeterminate length. They stain readily with eosin and other acidic dyes. They can also be colored with the dye aniline blue used in Mallory’s connective tissue stain or with the dye light green used in Masson’s stain.
Figure 6.5 Collagen fibrils in dense irregular connective tissue. Electron micrograph of dense…
When examined with the TEM, collagen fibers appear as bundles of fine, threadlike subunits. These subunits are collagen fibrils (Fig. 6.5). Within an individual fiber, the collagen fibrils are relatively uniform in diameter. In different locations and at different stages of development, however, the fibrils differ in size. In developing or immature tissues, the fibrils may be as small as 15 or 20 nm in diameter. In dense, regular connective tissue of tendons or other tissues that are subject to considerable stress, they may measure up to 300 nm in diameter.
Collagen fibrils have a 68-nm banding pattern.
When collagen fibrils stained with osmium or other heavy metals are examined with the TEM, they exhibit a sequence of closely spaced transverse bands that repeat every 68 nm along the length of the fibril (Fig. 6.5, inset). This regular banding pattern can also be observed on the surface of the collagen fibrils when they are examined with the atomic force microscope (AFM; Fig. 6.6). This banding pattern reflects the fibril’s subunit structure, specifically the size and shape of the collagen molecule and the arrangement of the molecules that form the fibril (Fig. 6.7). The collagen molecule (formerly called tropocollagen) measures about 300 nm long by 1.5 nm thick and has a head and a tail. Within each fibril, the collagen molecules align head to tail in overlapping rows with a gap between the molecules in each row and a one-quarter-molecule stagger between adjacent rows. These gaps are clearly visible with the AFM (see Fig. 6.6). The strength of the fibril is created by the covalent bonds between the collagen molecules of adjacent rows, not the head-to-tail attachment of the molecules in a row. The banding pattern observed with the TEM (see Fig. 6.5, inset) is caused largely by osmium deposition in the space between the heads and tails of the molecules in each row.
Figure 6.6 Collagen fibrils in dense irregular connective tissue. This atomic force microscopic image of…Figure 6.7 Diagram showing the molecular character of a type 1 collagen fibril in increasing order of…
Each collagen molecule is a triple helix composed of three intertwined polypeptide chains.
A single collagen molecule consists of three polypeptides known as α chains. The α chains intertwine, forming a right-handed triple helix (see Fig. 6.7d). Every third amino acid in the chain is a glycine molecule, except at the ends of the α chains. A hydroxyproline or hydroxylysine frequently precedes each glycine in the chain, and a proline frequently follows each glycine in the chain. Along with proline and hydroxyproline, the glycine is essential for the triple-helix conformation (see Fig. 6.7e). Associated with the helix are sugar groups that are joined to hydroxylysyl residues. Because of these sugar groups, collagen is properly described as a glycoprotein.
The α chains that constitute the helix are not all alike. They vary in size from 600 to 3,000 amino acids. To date, at least 42 types of α chains encoded by different genes have been identified and mapped to loci on several different chromosomes. As many as 28 different types of collagens have been categorized on the basis of the combinations of α chains they contain. These various collagens are classified by Roman numerals I to XXVIII according to the chronology of their discovery. A collagen molecule may be homotrimeric (consisting of three identical α chains) or heterotrimeric (consisting of two or even three genetically distinct α chains).
For example, type I collagen found in loose and dense connective tissue is heterotrimeric. Two of the α chains, identified as α1, are identical, and one, identified as α2, is different. Thus, in collagen nomenclature it is designated [α1(I)]2α2(I) (Table 6.2). Type II collagen is homotrimeric and present in hyaline and elastic cartilage, where it occurs as very fine fibrils. The collagen molecules of type II collagen are composed of three identical α chains. Because these α chains differ from those of other collagens, type II collagen is designated [α1(II)]3.