Direct observation of a transverse periodicity in collagen fibrils

Collagen fibrils have been extensively studied by electron microscopy, but only scant evidence has yet been obtained about their transverse structure. A transverse pediodicity of about 40 or 80 A might be expected if the microfibril model (until recently the common preference) is correct, but little direct evidence for such a periodicity has been obtained so far. We have studied electron micrographs of longitudinal cryosections of rat tail tendon. These appear very fibrous when studied by eye, but they produce at best only diffuse equatorial peaks in optical diffraction patterns. Nevertheless, we show here that like other fibrous protein structures, they do possess a strong transverse periodicity as revealed by their self-convolution functions (or auto-correlation functions). The observed periodicity is consistent with the presence of an 80 A structural unit in collagen fibrils in vivo.