The osteoclasts of hen medullary bone under hypocalcaemic conditions.

Osteoclasts of medullary bone after several days of hypocalcaemic diet are substituted on the trabecular surface by active osteoblasts. The fate and the ultrastructure of the osteoclasts withdrawn from medullary bone surfaces in the course of a low calcium diet has been studied in serial semithin and ultrathin sections. The cytoplasmic surface of osteoclasts located in marrow compartments presents blebs and protrusions and the whole cell is often irregularly branched in several directions. A large amount of granular endoplasmic reticulum is accumulated at the cell periphery; often the cisternae are distended to form vesicles with an inner core of dense material. Osteoclasts seem to divide into mono or polynucleated smaller units.