The fine structure of developing bone in the embryonic fowl

Abstract The fine structure of the tarso-metatarsus of avian embryos has been investigated by means of thin sections in the electron microscope. The morphological features of osteoblasts are similar to those of fibroblasts, and where the organic matrix is just being deposited, fibrogenesis is found to occur in intimate association with the surface of the osteogenic cells. The newly formed collagen fibrils show an axial periodicity of about 640Å with a considerable degree of fine structure; densitometer traces demonstrate that five bands and five interbands within a period are normally apparent. As transverse sections of fibrils show that they increase in diameter during development and that each fibril is invested by a less dense interfibrillar material, it must be assumed that this substance contains collagen molecules or their precursors. In the early stages of mineralization of the bone matrix small particles of less than 100Å across are localized within the main indentation of each period of the collagen fibrils, namely, between the d and ab bands. Electron diffraction of these sections demonstrates that apatite is present; this indicates that the particles are crystalline in nature. As the particles have been found in sectioned collagen fibrils they must penetrate into the fibrils to some extent; their precise site in the main interband may have a bearing on the mechanism by means of which the apetite crystals ultimately become preferentially orienated with their long axis parrallel to the collagen fibre axis.