The skeleton is a highly dynamic organ that constantly undergoes changes and regeneration. It consists of specialized bone cells, mineralized and unmineralized connective tissue matrix, and spaces that include the bone marrow cavity, vascular canals, canaliculi, and lacunae containing osteocytes. Bone also contains water, which represents at least 25 percent of its wet weight and provides much of its unique strength and resilience.
The skeleton has both structural and metabolic functions:
- Its structural function is critical for locomotion, respiration, and protection of internal organs. The structural connection between the skeleton and the hematopoietic system is particularly intimate; these two systems share both cells and local regulatory factors.
- Its metabolic function is largely as a storehouse for calcium, phosphorus, and carbonate, and it can contribute to buffering changes in hydrogen ion concentration.
This topic will review how the skeleton develops and how the processes of bone formation and resorption are regulated. The pathogenesis of osteoporosis is reviewed separately. (See "Pathogenesis of osteoporosis".)
The vertebrate skeleton is subdivided into an axial and an appendicular component. The axial component comprises the skull, spine, sternum, and the ribs. The appendicular component comprises long bones and their appendices. There are two major types of bone in the adult skeleton (figure 1):