Histological observation on the initial stage of vascular invasion into the secondary ossification of murine femoral epiphyseal cartilage.

It remains unknown whether the histology of vascular invasion during secondary ossification of epiphyseal cartilage is the same as that seen in primary ossification; we examined the initial processes of vascular invasion of secondary ossification in the murine femora. Many endomucin-immunoreactive blood vessels gathered at the central region of the articular surface, and buds of soft tissue, including glomerular loops of endomucin-immunoreactive blood vessels and TNALPase- immunopositive osteoblastic cells accompanied by TRAP-positive osteoclasts, had begun to invade the epiphyseal cartilage. The invading soft tissues formed cartilage canals displaying MMP9 immunoreactivity in the tip region, and cartilaginous collagen fibrils were not visible in the vicinity of the vascular wall of the blood vessels. Thus, the histological profile marked by invading glomerular vasculature and the erosion of the cartilage matrix near the vascular walls during secondary ossification differs from that seen during primary ossification.