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 ∼ 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.
