Box 5.33Generalized structure of bone and the process of osteocytic osteolysis

A schematic diagram of cortical and trabecular bone is shown in the upper figure together with the osteons where bone remodeling occurs. Surface osteoblasts become osteocytes as they become surrounded by mineralized bone but they remain in contact with the bone surface through long synctial (cytoplasmic) processes which run in the fluid filled lacunae. They are connected to the surface osteoblasts through gap junctions. Ca²⁺ in the lacunae is in equilibrium with plasma Ca²⁺ but, under the action of parathyroid hormone (PTH), Ca²⁺ is taken up by the osteocytes, transported to the surface osteoblasts which pump the Ca²⁺ into the extracellular fluid, thus raising plasma Ca²⁺ concentrations.