Fluorescence detection of mitochondrial clusters in mammalian white fat cells in vivo

Rhodamine 123 and epifluorescence microscopy revealed a significant portion of the fat cell's mitochondria existed in the form of clusters or aggregates, whereas the remainder were scattered about the cytoplasm. The aggregates were variable in size and number and apparently bore no fixed relationship to the nucleus or to each other. Mitochondrial clusters were seen in vivo in rat and mouse adipocytes of the mesenteric and epididymal depots, in excised tissue pieces of other depots, and in isolated fat cells. Physiological factors investigated such as species type (rat, mouse, rabbit, dog), sex, age, depot location (superficial vs. deep), fat cell size, hypercholesterolemia, and 24-h fasting had no apparent effect on cluster prevalence or size. Similar aggregates were not visible in several cultured cell lines studied nor in various non-fat cells, capillary endothelial cells, or nerve fibers contained within adipose depots examined. These results indicate that mitochondrial clusters exist naturally in mammalian white fat cells and conclude that they represent a form of cytoplasmic organization whose purposes are not well understood.