Collagen fibrils in dense irregular connective tissue. Election micrograph of dense irregular connective tissue from the capsule of the testis of a young male. The thread-like collagen fibrils are aggregated in some areas (X) to form relatively thick bundles; in other areas, the fibrils are more dispersed. x9,500. 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, x75,000.

specifically, the size and shape of the collagen molecule and the arrangement of the molecules that form the fibril (Fig. 5.6). The collagen molecule, also called tropocolla-gen, measures about 300 nm long by 1.5 nm thick, with a head and a tail. In forming a fibril, the collagen molecules align head to tail in overlapping rows with a gap between the molecules within each row and a one-quarter-molecule stagger between adjacent rows. The strength of the fibril is due to covalent bonds between the collagen molecules of adjacent rows, not to the head-to-tail attachment of the molecules in a row. The banding pattern observed with the TEM is caused largely by osmium deposition in the space between the heads and tails of the molecules in each row.

a. Fibril a. Fibril b. Packing of molecules b. Packing of molecules hole zone_' //+- overlap zone

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