Microvascular features and ossification process in the femoral head of growing rats

In the epiphysis of long bones, different patterns of development of ossification processes have been described in different species. The development of the vascularisation of the femoral head has not yet been fully clarified, although its role in the ossification process is obvious. Our aim was to investigate ossification and vascular proliferation and their relationship, in growing rat femoral heads. Male Wistar rats aged approximately 1, 5 and 8 wk and 4, 8 and 12 mo were used. Light microscopy frontal sections and vascular corrosion casts observed by scanning electron microscopy were employed. In the rat proximal femoral epiphysis, ossification develops from the medullary circulation of the diaphysis, quickly extending to the neck and the base of the head. Hypertrophic chondrocytes occupy the epiphyseal cartilage, and a physeal plate with regular cell columns is present. Starting from about the end of the third month one or more points of fibrovascular outgrowth, above the physeal line, can be observed in each sample. They are often placed centrally or, sometimes, peripherally. The fibrovascular outgrowths penetrate deeply into the cartilage and extend laterally. At age 8 mo, large fibro-osseous peduncles connect the epiphysis to the diaphyseal tissue. At 12 mo, the entire epiphysis appears calcified with an almost total absence of residual cartilage islands. This situation differs in man and in other mammals due both to differing thickness of the cartilage and to the presence of more extensive sources of blood vessels other than the diaphyseal microcirculation, as supplied by the teres ligament and Hunter's circle. In young rats, subchondral vessels and the synovial fluid could play a role in feeding the ossifying cartilage. Later, a loss of resistance of the physis due to marked degeneration of the cell columns, and extensive chondrocyte hypertrophy permit fibrovascular penetration starting from diaphyseal vessels rather than neighbouring vascular territories, such as those of the periosteum and capsule.