Histology and Physiology of Bone

Living bone is continuously renewed by production and resorption that are mediated through the bioactivities of the osteoblasts and osteoclasts, respectively. The bone turnover is well balanced and in a state of equilibrium unless disturbed by disease and/or disuse. When bone production is out-balanced by bone resorption or destruction, as in acute osteomyelitis, tumor, or immobilization, osteolysis or osteopenia may ensue. In a reverse condition, osteoblastic reaction predominates, resulting in osteosclerosis or increased bone density.

Histologically, five different types of bone cells are known to exist. They are osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts, and bone-lining cells. Osteoprogenitor cells, also known as preosteoblasts, proliferate into osteo-blasts at the osseous surface. Osteoblasts are the main bone-forming cells both in membranous and endochondral ossification. The osteoblast, a mononuclear cell, produces collagen and muco-polysaccharide that form osteoid. It is also closely associated with osteoid mineralization. The osteocytes are the posterity cells of osteoblasts entrapped within bone lacunae. Their main functions are the nutritional maintenance of the bone matrix and osteocytic osteolysis. Being multinucleated, osteoclasts are involved in bone resorption by osteoclasia. Formerly, the osteoclast and osteoblast were considered to stem from the same or at least related sources. New evidence, however, has indicated that the cell lines for these two cells are histogenetically different (Owen 1985). At present, it is widely held that osteoclasts originate from stromal cells of mesenchymal tissue via osteoprogenitor cells, while osteoblasts originate from the mono-cyte-phagocyte line of the hematopoietic system. Bone-lining cells are probably the inactivated form of osteoblasts. Like osteoblasts, these cells line the osseous surface. The cells are flat and elongated in shape with spindle-shaped nuclei. Although not established yet, their function is probably related to the maintenance of mineral homeostasis and the growth of bone crystals.

Osteogenesis is accomplished by mineralization of organic matrix or osteoid tissue, which is composed mainly of collagen (90%) and surrounding mucopolysaccharide. Mineralization starts with the deposition of inorganic calcium and phosphate along the longitudinal axis of collagen fibrils, a process referred to as nucleation. Nucleation is precipitated by a chemical milieu in which the local phosphate concentration is increased or conversely calcium salt solubility is decreased. After nucleati-on, salt exists in a crystalline form and grows in size as more calcium and phosphate precipitate. Crystallized salt has resemblance to hy-droxyapatite [Cai0 (PO4)-6OH2j.

Bone formation is stimulated by various factors including physical stress and strain and calcium regulatory hormones (parathormone, calcitonin), growth hormone, vitamins A and C, and calcium and phosphate ions. On the other hand, bone resorption occurs as bone matrix is denatured by the proteolytic action of collagenase secreted by osteoclasts. Factors that stimulate osteoclastic activity include bodily immobilization, hyperemia, parathormone, biochemically active metabolites of vitamin D, thyroid hormone, heparin, interleukin-1, and prostaglandin E.

The skeletal muscles are rich in actin and myosin, the interactions of which cause contraction. They are composed of a large number of muscle fibers (cells). Muscle fibers, individually invested by the endomysium, are grouped in fascicles enveloped in successive connective tissue sheaths. Variable numbers of fascicles compose a skeletal muscle that is ensheathed by the epimysium. Tendon is a specialized connective tissue that unites with muscle belly forming the musculotendinous unit on one side and attaches to the periosteum, fibrous capsule of the joint, or directly to bone on the other side.

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