White adipocyte differentiation was studied ultrastructurally in the mouse mammary gland following stimulation with 17β-estradiol and relaxin. These hormones have been previously demonstrated by us to induce hyperplasia and hypertrophy in the adipose cells of the mouse mammary gland. Following hormone treatment new fat cells are formed around the growing ducts. In these sites there is a close relationship between blood vessel growth and adipocyte development, and stromal areas featuring embryonic fat organs can frequently be found. In the sites of adipocyte differentiation the most numerous cell types are undifferentiated mesenchymal cells and preadipocytes, while fibroblasts and macrophages are much less common. No endothelial cells or pericytes were found detaching from the blood capillary walls. There were no fibroblasts or macrophages containing intracellular lipid deposits. Actively degranulating mast cells were frequent. The above findings strongly suggest the direct origin of adipocytes from perivascular mesenchymal cells, without the intermediate stages of well-differentiated fibroblasts, macrophages, endothelial cells or pericytes. The earliest morphologically recognizable stage in adipocyte differentiation is a ‘pale preadipocyte’, characterized by its irregular shape due to cytoplasmic processes, clear cytoplasmic matrix, well-developed organelles (especially ribosomes and Golgi apparatus), few and small lipid droplets, numerous pinocytotic vesicles and a very thin basal lamina. The next stage in adipocyte differentiation is a ‘dark pfreadipocyte’ showing a denser cytoplasmic matrix, reduced organelles and more abundant lipid accumulations. The pinocytotic vesicles are very numerous and the enveloping basal lamina is still thin. The subsequent maturation stages are the well-known globular multivacuolated adipocyte and finally the mature univacuolated, signet-ring, white adipocyte.

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