Osteocytes alkaline phosphatase

The alkaline phosphatases are found in bacteria, fungi, and higher animals but not in higher plants. In E. coli alkaline phosphatase is concentrated in the peri-plasmic space. In animals it is found in the brush border of kidney cells, in cells of the intestinal mucosa, and in the osteocytes and osteoblasts of bone. It is almost absent from red blood cells, muscle, and other tissues which are not involved extensively in transport of nutrients. 

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-... 

It is far from clear whether the incorporation of the citrate ion into calcified tissues is merely a consequence of the stability and solubility properties of calcium citrates or whether it plays an essential part in the mineralization process. It increases the solubility of apatite and inhibits alkaline phosphatase. Citrate accumulation in bone is influenced by both parathormone and vitamin D. High levels of vitamin D appear to stimulate osteocytes to produce citrate and this may play some part in the subsequent increase in resorption. Despite these observations no clear picture of the role of citrate in mineralization has emerged. 

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