Bone

The structural material of the skeleton that provides a rigid framework for the muscles and protects certain body organs. Bone consists of several layers: a thin outer covering (the periosteum), which contains blood vessels and nerves; an inner shell of hard (compact or cortical) bone composed of columns of bone cells (osteoclasts and osteoblasts), each with a central hollow (haversian canal) that is important for the nutrition, growth, and repair of the bone; and a central, mesh-like structure (known as spongy, cancellous, or trabecular bone). The cavity in the centre of some bones, and the spaces in spongy bone, contain bone marrow. Bone is continuously reabsorbed by osteoclasts and replaced by osteoblasts. Osteoblasts encourage deposition of calcium phosphate on the protein framework of the bone, and osteoclasts remove it. The actions of these cells are controlled by hormones, which also maintain the calcium level in the blood. At birth, many bones consist mainly of cartilage, which ossifies later (see ossification). The epiphyses (growing ends of the long bones) are separated from the bone shaft ( diaphysis) by the epiphyseal plate. Some bones, such as certain skull bones, do not develop from cartilage and are known as membranous bones.