Extracellular Ca2+ in Bone Marrow.

Our blood serum Ca2+ levels are maintained within a narrow range (Ca2+ homeostasis) through a complex feedback system. However, local bone marrow Ca2+ levels can reach high concentrations, at least transiently, due to bone resorption, which is one of the notable features of the bone marrow stroma. Bone homeostasis is maintained by both the balance between osteoblastic bone formation and osteoclastic bone resorption and the balance of mesenchymal stem cell differentiation into osteoblasts and adipocytes. It has been reported that under culture conditions of infrequent adipocyte differentiation (no treatment with insulin or dexamethasone), high extracellular Ca2+ enhances osteoblast but not adipocyte accumulation in bone marrow stromal cells. In contrast, under culture conditions of predominant adipocyte differentiation (treatment with insulin and dexamethasone), high extracellular Ca2+ enhances adipocyte but not osteoblast accumulation in bone marrow stromal cells. Thus, the increased extracellular Ca2+ caused by bone resorption might enhance osteoblast development to reform missing bone under conditions of infrequent adipocyte differentiation (such as the normal physiological state) and might accelerate adipocyte accumulation instead of osteoblastic bone formation under conditions of predominant adipocyte differentiation (such as aging, obesity, use of glucocorticoids, and postmenopause). Moreover, increased adipocyte accumulation in bone marrow suppresses lymphohematopoiesis and contributes to a dysfunction of osteogenesis.