The formation of connective and skeletal tissues

Abstract By the use of the electron microscope and thin sections of connective and skeletal tissues of the fowl embryo, observations have been made on the fine structural detail and the interrelationship between the cells and the developing collagen fibres and ground substance. Submicroscopical filaments have been recorded in the cytoplasm of four types of fibrogenic cells in seven different sites where the rapid formation of collagenous tissues is known to occur. It is clear that the chondrogenic cell is closely associated with the formation of cartilage matrix, which is composed of interwoven fibrils and an amorphous material which may act as a cementing substance. In the development of bone the organic matrix is laid down by the adjacent osteoblasts; intracytoplasmic filaments of these cells seem to be involved with this process, and it appears that the filaments may become extracellular and form part of the bone matrix. In tissue that is growing under tension (cf. osteoblastic layer of the periosteum) the cytoplasmic filaments are predominantly alined in layers within the cells; by sectioning in known planes it is apparent that the structures are of a filamentous nature and are not at present consistent with the form of membranes or tubules. The evidence presented strongly suggests that the intracytoplasmic filaments are concerned with fibrogenesis; various possibilities of the relationship between these filaments and the fully developed tissues are discussed.