Modulation of vitamin D action by local factors in bone

Bone provides rigid mechanical support, protects vital organs and serves as a reservoir of ions, such as calcium and phosphate, that can be mobilized from bone. There are two types of bone: cortical bone, which is composed of densely packed, mineralized collagen laid down in layers and provides rigidity, and trabecular bone which is spongy in appearance, and also consists of mineralized collagen and provides strength and elasticity. There are also two types of bone formation: intramembranous ossification, which primarily takes place in the calvarium and starts within the connective tissue, and endochondral ossification, which takes place in long bones and starts with the formation of cartilage followed by bone formation and mineralization. The mineralized matrix is composed of collagenous and noncollagenous proteins and contains a variety of locally produced growth factors.