Ossification and Bone Metabolism

Calcium metabolism in bone cannot be properly appreciated without reviewing the histology of growing and adult bones. The morphological unit of the bone, the various types of bone, bone development, and bone involution will be described. 

Bone is composed of two major structural elements the fundamental substance and the cell. The fundamental substance includes collagen fibrils, an amorphous mucoid substance—the ossein, and calcium salts. At least three types of cells are found in bone tissues osteoblasts, osteocytes, and the osteoclasts. 

The osteoblast is found in young bones and is associated with bone development. The origin of the osteoblast is still debated. It is assumed to be derived from the differentiation of fibroblasts. The quasitotal absence of mitosis in osteoblasts is in agreement with such a hypothesis. The osteoblast looks like a meta-bolically active cell. It contains a large nucleus with a well-developed nucleolus. Its endoplasmic reticulum is made of densely packed lamellae studded with ribosomes. Occasionally, the endoplasmic lamellae are dilated to form cisternae. The Golgi apparatus can usually be readily recognized, and the osteoblast usually contains only a few mitochondria. 

Filamentous processes emerge from the cell surface to extend through fine canaliculi within the newly formed bone substance where they anastomose with similar processes emerging from the surrounding osteoblasts. The plasma membrane of the osteoblast is separated from the calcium-impregnated matrix by a wide intercellular space. 

Two histochemical findings are significant with respect to the alleged role of osteoblasts in bone formation the presence of glycogen vacuoles and alkaline phosphatase. (The properties of the osteoblasts will be further discussed when the mechanism of calcification is considered.) When the osteoblast is completely surrounded by the calcified matrix, it becomes an osteocyte, which is a smaller cell resembling a fibro- 

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Importance of Calcium 333 Sources of Calcium 333 Calcium Absorption 333 Serum Calcium 333 Ossification and Bone Metabolism 334 The Apatite Crystal 336 Fluoroapatite Bone Apatite... 

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