The occurrence of specific cytoplasmic granules in the osteoclast

Abstract

The structure of the osteoclast in the tibiofemoral epiphyses of fetal rats has been studied by electron microscopy. The findings demonstrate the existence of dense specific granules which are apparently isomorphic with the primary lysosomes of soft tissue cell types. Points of morphological analogy include: an agranular unit membrane, a finely granular matrix, and a variable range of size and density. In further analogy, the specific granules also appear to be derived directly from the Golgi apparatus. The formed specific granules are localized in the cytoplasm subjacent to the ruffled border. At this site they undergo an apparent cyclic variation in numbers during which individual granules may undergo alternate transformations into cytoplasmic vacuoles or complex multivesicular and lamellated bodies identifiable with lysosomal residual bodies. Consideration of these morphological findings, along with recent cytochemical studies on the distribution of enzymatic activity in bone cell fractions, would seem to provide some further evidence that the underlying basis of the function of the osteoclast in bone resorption may involve an enzymatic mechanism.