Fat droplet formation in rat lactating mammary gland and mammary carcinomas viewed by freeze-fracture.

The freeze-fracture morphology of the endoplasmic reticulum and Golgi apparatus membranes was analyzed in lactating rat mammary glands and in mammary carcinomas induced by 7,12-dimethylbenzanthracene or N-nitrosomethylurea. Membranes in close proximity with fat droplets present a spectrum of transformations, from the normal, wavy, particle-rich appearance of cytoplasmic membranes to rigid, particle-free bilayers. The following sequence of events is proposed for the biogenesis of fat droplets: (1) formation of particle-free areas in the endoplasmic reticulum and Golgi membranes; (2) apposition of the bilayered lipid membranes to the growing fat droplet; and (3) progressive conversion of the membrane lipids (or amphypathic lipid precursors) into triglycerides at the periphery of the fat droplet. Our results suggest that membranes are not only involved in the synthesis and secretion processes, but that some of their components are incorporated in the fat droplets and contribute to the secretory product itself.