English: Existing paradigm for long bone endochondral ossification. a Primary endochondral ossification begins with the formation of a chondrocyte template during embryogenesis. Chondrocytes undergo hypertrophy beginning from the mid-diaphysis, eventually extending to the epiphyseal poles. Vasculature invades the forming bone, transporting marrow, mesenchymal stromal cells, and osteoclasts. Hypertrophic cells undergo apoptosis, aided by the removal of matrix by osteoclasts. Mesenchymal stromal cells differentiate into osteoblasts and then osteocytes. b Secondary ossification occurs at the epiphysis post-natally in rodents. Immature chondrocytes at the center of the epiphysis become hypertrophic to produce mineralized collagen and eventually undergo apoptosis. Vasculature invades and transports marrow, mesenchymal stromal cells, and osteoclast precursors. Bone formation initiates at the center and extends peripherally.