Growth and development of collagen fibrils in immature tissues from rat and sheep.

The collagen fibril diameter distribution of four immature tissues from both rat and sheep have been determined from the transverse sections observed in the transmission electron microscope. In many instances before birth, the form of the distribution for the tissues is both unimodal and sharp and the mean diameters of the distributions lie close to a multiple of 80 A. For some tissues, the collagen fibril diameter distributions may be resolved into a number of components, each of which represents a population of fibrils with a diameter close to a multiple of 80 A (8 nm). These data confirm and extend previous observations by the authors that small collagen fibrils growth occurs by the accretion of 80 A units. The form of the collagen fibrils diameter distribution at birth is broad for the sheep tissues but narrow for the rat tissues, thus confirming that the range of fibril diameters at this stage of life reflects the differing degree of development of precocious and altricious animals.