Electron Microscopic Studies of the Differentiation of Fat Cells in Human Fetal Skin**From the Department of Dermatology, Yamaguchi University, School of Medicine, Ube, Japan.

ABSTRACT1)Electron microscopic studies of primitive fat organs showing various degrees of fat storage from human fetuses 20 to 26 weeks of age were undertaken, with special reference to the origin of fat cells and the lipid formations in them. Based on this study, three types of cells, namely an undifferentiated-type cell, a young-type fat cell and a mature-type fat cell were discerned ultrastructurally, but they are considered to be identical concerning the morphology of mitochondria, smooth endoplasmic reticula and glycogen granules.2)Concerning the origin of white fat cells, it was revealed that young-type and mature-type fat cells are not derived from reticuloendothelial cells or fibroblasts but from a certain definite type of mesenchymal cell which we have referred to as the undifferentiated-type cell.3)The finding that cytoplasmic microvesicles are more prominent in young-type fat cells than in mature-type ones might be interpreted that the young-type cells may be actively releasing lipids or free fatty acids according to Williamson's concept of these organelles. But, contrary to Williamson's postulation, the young-type fat cells seem to be accumulating lipids or free fatty acids