Effect of PTH on osteocyte ultrastructure.

Osteocyte ultrastructure was studied in the cortical bone of the tibia of rats after acute or chronic administration of supraphysiological doses of PTH. Confirming previous reports, an increase in the width of the cytoplasm with the appearance of numerous thin cytoplasmic processes, an increase in rough ergastoplasmic reticulum and Golgi apparatus, an increase in lacunar width and lysis of the lacunar wall ("brush border" after Bonucci) were observed. Particularly striking was the appearance of numerous microfilaments and microtubules in the cytoplasm of activated osteocytes. The appearance of microfilaments, often densely packed in cytoplasmic processes or running parallel to the plasma membrane, points to a role of the cytoskeleton in mediating the effects of PTH on conformational changes of the plasma membrane (and possible on cell motility); microtubules were particularly prominent in the Golgi field and are presumably involved in the exocytosis of lysosomes. Another striking feature was the non-random distribution of periosteocytic osteolysis along the lacunar perimeter. Osteolysis was particularly pronounced at the cell pole opposite to the cell nucleus. After chronic administration of PTH, autolysis of osteocytes, associated with signs of excessive periosteocytic osteolysis, was frequently encountered.