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TYPES OF BONE TISSUE
Bone was described as a tissue. Recall that bone cells are called osteocytes, and the matrix of bone is made of calcium salts and collagen. The calcium salts are calcium carbonate (CaCO3) and cal-cium phosphate (Ca3(PO4)2), which give bone the strength required to perform its supportive and pro-tective functions. Bone matrix is non-living, but it changes constantly, with calcium that is taken from bone into the blood replaced by calcium from the diet. In normal circumstances, the amount of calcium that is removed is replaced by an equal amount of calcium deposited. This is the function of osteocytes, to regu- late the amount of calcium that is deposited in, or removed from, the bone matrix.
In bone as an organ, two types of bone tissue are present (Fig. 6–1). Compact bone looks solid but is very precisely structured. Compact bone is made of osteons or haversian systems, microscopic cylinders of bone matrix with osteocytes in concentric rings around central haversian canals. In the haversian canals are blood vessels; the osteocytes are in contact with these blood vessels and with one another through microscopic channels (canaliculi) in the matrix.
The second type of bone tissue is spongy bone, which does look rather like a sponge with its visible holes or cavities. Osteocytes, matrix, and blood vessels are present but are not arranged in haversian systems. The cavities in spongy bone often contain red bonemarrow, which produces red blood cells, platelets, and the five kinds of white blood cells.
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