Although the formation of a secondary center of ossification is often
compared with that of the primary center, there are striking differences
between these processes. In the formation of the primary center, vascular
invasion is always associated with the maturation of chondrocytes, whereas
vascularization of the epiphysis can proceed in two different ways. In some
species, the epiphysis is vascularized by cartilage canals before the
appearance of the secondary center. However, in the mouse, the distal
femoral epiphysis is vascularized by peripheral vascular invasion without
pre-existing cartilage canals. Histological study of serial sections and
studies of vascularization by injection with India ink demonstrated the
relationship between hypertrophic chondrocyte formation, vascular invasion,
and the formation of the secondary center of ossification in the murine
distal femoral epiphysis.