Abstract
Bone marrow is an environment rich in its diversity of cell types and niches. Both hematopoietic and osteogenic stromal cells are present and have been studied extensively. Less is known about the function of one of the most abundant cell types in the bone marrow: adipocytes. There are several hypotheses that have been proposed including: passive role as a space filler; active role in the body's general lipid metabolism; role in providing a localized energy reservoir for emergency situations affecting the bone or hematopoiesis; support of differentiation or function of other cell types (such as bone, endothelial, and other stromal cells).
There are several human pathologies associated with increases in adipocyte hypertrophy or proliferation including changes associated with aging, osteoporosis, and osteonecrosis. The reasons for these changes are poorly understood. One etiology associated with both osteoporosis and osteonecrosis, corticosteroid therapy, has been shown to increase the lipid content of osteoblasts and adipocytes.
With osteonecrosis, several pathogenetic mechanisms involving adipocytes have been proposed:
Mechanical - increased size and number cause increased intraosseous pressure and decreased venous outflow
Direct precursor cells away from osteoblastogenesis towards adipogenesis
Liquid fat causing a hypercoagulable state
Osteocyte dysfunction or apoptosis
Adipocyte and bone marrow necrosis
Release adipokines and other factors that have an effect on the cells within the bone marrow (inhibiting angiogenesis, e.g.)
The possibility that adipocytes may actually play an active role in propagating specific pathologic features has only recently been discussed. This is in part due to our increasing understanding that adipocytes have an endocrine role in metabolism.
Only recently have scientists tried to identify specific cellular mechanisms that may be involved in the pathogenesis of osteonecrosis. Results from these studies will not only contribute to our understanding of the disease of osteonecrosis (and other diseases such as osteoporosis) but will also help us to appreciate the multiple functionalities of the heretofore unappreciated adipocyte.
