Bone Development
Bone Cells
Ossification
Ossification is the process by which bones forms. It begins six weeks after embryonic fertilization and can occur in two ways:
Intramembranous Ossification
Intramembranous ossification gives rise to the flat bones in the skull as well as the clavicles.
Ossification begins when mesenchymal cells set the template of the future bone. They differentiate into osteoblasts that secrete an extracellular matrix and deposit calcium to harden it. The non-mineralized portion of the bone (osteoid) forms around blood vessels, and forms spongy bones. Spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.
Endochondral Ossification
Endochondral ossification gives rise to the rest of the bones of the body
Bone develops from hyaline cartilage and in long bones, chondrocytes form a template of the hyaline cartilage at the diaphysis. The matrix begins to calcify and this calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone. Osteoclasts then break down some of the spongy bone to create a marrow, or medullary, cavity in the center of the diaphysis. Dense, irregular connective tissue forms a sheath (periosteum) around the bones. The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide.