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The skeleton is composed of flat bones and long tubular bones. Flat bones such as the skull, sternum, and pelvic bones develop from fibrous tissue (through intramembranous ossification), whereas long tubular bones increase in length at a line of cartilage present near the growing bone ends known as the epiphysial plate or growth plate. Anatomic regions of long bones relate to the growth plate and include the epiphysis, which is the region between the growth plate and the nearest joint; the diaphysis, which is the shaft region of the bone between the two growth plates; and the metaphysis, which is the region of bone adjacent to the growth plate on the diaphysial side. The metaphysis is the area where new bone is laid down during growth, and in children it represents the most vascular and most metabolically active region of bone. For this reason, the metaphysial region is the area most susceptible to infections and neoplasm formation in childhood.

All bones are composed of an outer (cortical) shell of compact bone and an inner meshwork of cancellous bone composed of bony trabeculae separated by vascular connective tissue, which contains fat and bone marrow. Both cortical and cancellous bone are composed of bone cells embedded in a mineralizedmatrix. The main bone cells are osteoblasts, which secrete matrix protein, and osteoclasts, which resorb bone. The matrix of bone (called osteoid) is composed mainly of type I collagen. Other collagen types, other proteins such as osteocalcin and osteonectin (which may play a role in mineralization of the matrix), and glycoproteins complete the structure of bone matrix. The mineral phase of bone is composed predominantly of calcium hydroxyapatite with smaller amounts of calcium phosphate.

The main functions of bone are to act as a hard protective shell for vital structures (eg, skull, rib cage, pelvis), to provide support for the trunk and limbs, and to permit movement by the action of muscles attached to the bones. The long tubular bones of the limbs are ideal for their function because they are light and have high tensile strength. Bone mineral also acts as a massive reservoir for calcium and phosphorus.


Achondroplasia is transmitted as an autosomal dominant trait resulting from a genetic defect that probably leads to abnormal synthesis of cartilage matrix protein. It is characterized by failure of cartilage cell proliferation at the epiphysial plates of the long bones, resulting in failure of longitudinal bone growth and subsequent short limbs. Membranous ossification is not affected, so that the skull, facial bones, and axial skeleton develop normally. The result in adulthood is a normal-sized head and trunk but limbs that are much shorter than normal. Achondroplastic dwarfism is quite common. General health is not affected, and life expectancy is normal.

Osteogenesis Imperfecta (Brittle Bone Disease)

Osteogenesis imperfecta is a group of inherited diseases characterized by brittle bones. There ...

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