Stages in bone formation.
A-B. First, mesenchymal cells form condensed regions, where they differentiate into chondrocytes and form the bone anlagen.
C-F. Centrally located chondrocytes progress through the prehypertrophic stage, stop proliferating, become hypertrophic and die by apoptosis, allowing blood vessels invasion. Cells of the outer perichondrium layer differentiate into osteoblasts, invade the hypertrophic zone and produce bone-specific matrix.
G-I. Proliferating chondrocytes are constantly present in the developing bone and form the growth plate. Through the repeated cycles of proliferation, hypertrophy and mineralization, the chondrocytes direct growth of longitudinal bone. Blood vessel invasion into the epiphyseal area brings to formation of the secondary ossification centers.