FIGURE 6.4 • Dense regular connective tissue—tendon. a. Electron micrograph of a tendon at low magnification, showing tendinocytes (fibroblasts) and their thin processes (arrows) lying between the collagen bundles. 1,600. b. A tendinocyte with prominent profiles of rough endoplasmic reticulum (rER) is shown at higher magnification. The collagen fibers (C) can be resolved as consisting of very tightly packed collagen fibrils. The arrows indicate processes of tendinocytes. 9,500. Inset. Photomicrograph of a tendon. Note the orderly and regular alignment of the bundles of collagen fibers. Tendinocytes are aligned in rows between the collagen fibers. 200. (Electron micrographs modified from Rhodin J. Histology. New York: Oxford University Press, 1974.) 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.
When examined with the TEM, collagen fibers appear as bundles of fine, threadlike subunits. These subunits are collagen fbrils (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 onequarter-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 6.5 • Collagen fbrils in dense irregular connective tissue. Electron micrograph of dense irregular connective tissue from the capsule of the testis of a young male. The threadlike collagen fibrils are aggregated in some areas (X ) to form relatively thick bundles; in other areas, the fibrils are more dispersed. 9,500. Inset. A longitudinal array of collagen fibrils from the same specimen seen at higher magnification. Note the banding pattern. The spacing of the arrows indicates the 68-nm repeat pattern. 75,000.
Each collagen molecule is a triple helix composed of three intertwined polypeptide chains.