Peculiar structures of the blood vessels forming the secondary ossification center in the rat femoral heads.

To investigate the development of the blood vessels forming the secondary ossification center in the femoral heads of growing rats, we observed the specimens of the proximal femurs having vascular casts using both an optical and a scanning electron microscopy (SEM). Histologically, the vascular invasion occurred from the lateral part of the cartilaginous epiphysis, prior to the appearance of the ossification center. The vascular casts were prominent within the cellular spaces consisting of a few hypertrophic chondrocytes. The SEM study suggested that the vascular buds had peculiar structures and invaded the cartilaginous epiphysis by breaking through the cellular spaces. The blood vessels budded out from the arch of the lateral epiphyseal vessels toward the femoral heads. The sizes of the large-caliber vascular buds of about 50 microns in diameter were similar to those of the cellular spaces, and they penetrated into the epiphysis by repeated merging and branching. Many deep impressions were noted on the surface of the vascular buds, and the tips showed the peculiar shape similar to a suction cup.