The skeletal system acts as levers to convert muscle contraction into movement, a storage depot for calcium and other physiologically important ions, blood cell formation sites and support and protection of fleshy structures. Bone tissue, as the chief component of the skeletal system, is specialized to aid in these functions. This type of connective tissue is composed of calcified bone matrix surrounding osteocytes, osteoblasts, and osteoclasts. Osteoclasts are multinucleate cells that secrete enzymes to breakdown bown tissue. Osteoblasts are cells that secrete the matrix and become embedded in it as they become osteocytes and begin to add calcium to the matrix.
Osteoclasts have receptors for calcitonin and osteoblasts have receptors for parathyroid hormone.
Parathyroid hormone stimulates osteoclast activity, presumably through a stimulating factor released by osteoblasts, while calcitonin diectly inhibits osteoclast activity. Interaction between these bone forming and bone resorbing cells helps maintain calcium homeostasis in the body. Calcium, phosphorus, magnesium, citrate, potassium, and sodium are found there but the substance that gives bone its hardness is hydroxyapatite made from calcium and phosphate. Hydoxyapatitecrystals are associated with collagen in bone giving it its strength and resistance. Bone, on gross observation, has two forms.