Osteogenesis was studied in the distal subchondral area of the femur of older but still immature rabbits (four to six months old, weighing approximately two kilograms), using routine histology and autoradiography with tritiated thymidine and tritiated glycine. At this age, the subchondral area in the rabbit is a zone of diverse histological appearance. The basal layer of cartilage is diffusely calcified and shows little proliferative activity. Bone formation appears to be occurring around a profusion of capillaries arising from the underlying nucleus of the epiphysis. Each capillary is ringed by osteoblasts and a delicate layer of bone.

Serial autoradiographs after intra-articular injection of tritiated thymidine demonstrated a progression of the "permanent" label from a cell of vascular origin (one hour), to osteoblasts (twenty-four hours), to the osteocytes in their lacunae (five to seven days). In similar studies with tritiated glycine the perivascular osteoblasts were labeled at one-half hour and the adjacent bone matrix at one hour. At later times, the label was found deeper in the bone mass.

One can conclude that osteogenesis in the subchondral bone is mediated by the capillary and that the osteoblasts arise from a cell lying in the wall or within the capillary.