Bone structure and function

Bone is a complex, living, constantly changing tissue. The architecture and composition of cancellous and cortical bone allow the skeleton to perform its essential mechanical functions. The stiffer cortical bone responds more slowly to changes in loads while cancellous bone has a much larger surface area per unit volume and a greater rate of metabolic activity. Periosteum covers the external surface of bone and consists of two layers: an outer fibrous layer and an inner more cellular and vascular layer. The inner osteogenic layer or cambium layer can form new bone while the outer layer forms part of the insertions of tendons, ligaments and muscles. The cortical bone of diaphyses and metaphyses has a dual blood supply that allows loss of one source of circulation without adversely affecting the viability of the tissue. Many epiphyses, even in adults, depend only on a single source of blood supply, the penetrating epiphyseal vessels. For this reason epiphyseal bone may infarct more easily than metaphyseal or diaphyseal bone. The bone matrix has an organic component, primarily type I collagen, which gives it tensile strength and an inorganic component, primarily hydroxyapatite, which gives it stiffness to compression. Specialized populations of bone cells form, maintain and remodel this matrix. We recognize four types of bone cells based on their locations, morphology and functions: osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts. Osteoblasts develop from undifferentiated cells while osteocytes form from osteoblasts. Osteoclasts have a separate stem cell line, blood-borne monocytes. Bone matrix apparently attracts these monocytes and stimulates their differentiation into osteoclasts. The processes of bone modeling and remodeling require osteoclastic resorption of bone matrix and deposition of a new matrix by osteoblasts. Modeling shapes and reshapes bones during growth and stops at skeletal maturity. Physiologic remodeling does not change bone shape and consists of bone resorption followed by bone deposition in approximately the same location. Since it continues throughout life it appears to be important for maintenance of the skeleton, but its exact function remains obscure. Adaptive remodeling is the response of the bone to altered loads and may alter the strength, density and shape of bone. In recent years understanding of the control of bone cell function has increased significantly. The study of electrical effects on bone formation has lead to new treatments of nonunions and delayed unions. Physicians have applied understanding of matrix-induced bone formation to reconstruction of skeletal defects.(ABSTRACT TRUNCATED AT 400 WORDS)