The second phase of skeletogenesis (the growth or formation of the skeleton) during embryogenesis, it is the process by which dense mesenchyme tissue is converted into cartilage and bone. Chondrification of the vertebrae and limb bones begins in the sixth embryonic week in the human. As growth continues, bone is continuously remodelled through the action of osteoclasts, which secrete a strong acid that dissolves the mineral components of bone and enzymes that digest the collagen. In reptiles, birds, and mammals, these osteoclasts tunnel into bone, making new paths for vascularization, and which line up on the inner wall of the tunnel. This leads to the concentric deposit of bone to form columns or osteons that run vertically through the bone. At the center of the bone is a marrow cavity, which also expands as the bone grows. The dense, load-bearing bone tends to lie around the periphery and is known as compact bone.
See Bone Morphogenetic Proteins (BMPs), Diaphysis, Embryogenesis, Endochondral ossification, Epiphysis, Mesenchyme, Musculoskeletal system, Ossification, Osteoclasts