Ultrastructural development of bone marrow adipose cell.

The postnatal development of bone marrow adipose cells was studied by sequential examination of marrow tissue obtained from the tibia and femur of 1- to 8-week-old rabbits. The bone marrow adipocyte begins to develop 2 weeks after birth. Its cellular origin differs from the "fibroblast-like" cell that is the progenitor of the extramedullary adipocyte. The marrow pre-adipocyte is associated with the sinus endothelium and adventitial cells, but is not associated with collagen and does not contain abundant organized rough endoplasmic reticulum. Unlike the extramedullary adipocyte, the course of differentiation of the medullary adipocyte is not characterized by the appearance of glycogen; furthermore, during the coalescence of lipid droplets to form larger fat globules, cytoplasmic organelles, such as mitochondria and ribosomes, are trapped within the lipid substance to form intralipid vacuoles containing degraded cytoplasmic organelles. The possible significance of these morphologic differences between the developing bone marrow adipocyte and that of its extramedullary counterpart is discussed.