collagen fibrils

delicate fibrils of collagen in connective tissue, composed of molecules of tropocollagen in linear arrays. In Type I collagen, the most common type, the tropocollagen molecules are associated in periodic, staggered arrays that give the appearance of cross-banding, with a period of approximately 65 nm in the unit fibril; these unit fibrils are aggregated in bundles to form larger fibrils, with longitudinal striations, which may themselves be aggregated into fibers. Some other types of collagen also associate into fibrils (e.g., Types II, III, VI) but may not aggregate to show cross-banding or to form fibers. The terms fiber and fibril are sometimes interchanged loosely in descriptions of the hierarchy of collagen aggregation. See also under fiber and see illustration.

Type I collagen fibril (A); it is composed of unit fibrils, here shown negatively stained (B). The unit fibrils themselves comprise tropocollagen molecules in regular staggered arrays (C, D).