The ultrastructure of the osteoclast and its functional implications.

Recent findings on the ultrastructure of the osteoclast indicate that special attention should be given to the ruffled border, clear zone, and the vacuoles and vesicles of the cell and their significance for the mechanism of breakdown of bone matrix. The ruffled border is seen as an extensive area of cell surface where secretion of enzymes as well as uptake of matrix components takes place. The clear zone encircles the ruffled border completely and thus forms an integral part of the resorbing apparatus. Vacuoles and vesicles are thought to secret enzymes as well as take up extracellular material and possible digest or transport these products in the cell. The changes that occur in the ultrastructure of the osteoclast after exposure to parathyroid hormone and calcitonin indicate an important role of the ostioclast in bone metabolism. The cell can increase its activity very rapidly in response to parathyroid hormone, and decrease its activity in response to calcitonin.