Electron microscopic study of the effects of calcitonin on bone cells and their extracellular milieu

This is a scanning and transmission electron microscopic study of the changes produced by small doses of calcitonin (0.15 mU/g body weight) in the lining cells and their microenvironment at the endosteal surfaces of the tibia of neonate rats. The techniques used included "freeze substitution" preparation, staining with lead acetate, and with lanthanum added to the fixative. Rats were sacrificed 5, 15, and 30 minutes after subcutaneous injection of salmon calcitonin. The following observations were made; within 5 minutes following calcitonin injection, the response of the endosteal lining cells included increased numbers of microvilli and surface blebs. Cell contraction was apparent, including an irregular appearance of the plasma membrane and enlarged intercellular channel size, though cell-to-cell contact still occurred. By 15 minutes, following hormone injection, the cells were returning to normal morphology and were in close contact with each other. Calcitonin caused a marked accumulation of lanthanum around osteocytes and in bone fluid adjacent to lining cells. The lanthanum was found in large aggregates and appeared to "clump." Following "freeze substiuttion" preparation, the edge of the osteoid was bordered by what appeared to be mineral aggregates. We conclude that bone lining cells and osteocytes respond rapidly to low doses of calcitonin, thereby suggesting that they play a role in the physiological action of the hormone. This function includes a modification of the fluid microenvironment of these cells, possibly providing a site for temporary storage of calcium.