FINE STRUCTURE OF BROWN ADIPOSE TISSUE .1. COLD-INDUCED CHANGES IN RAT

During adaptation of rats to cold, the cellular morphology of their brown adipose tissue changes markedly. (1) Hyperemia and expansion of extracellular space develop immediately. Both are transitory. (2) Lipid droplets diminish in size. Simultaneously, small lipid droplets are formed in increasing numbers. After prolonged cold exposure, larger lipid droplets reappear; these, however, do not attain the dimensions of the original droplets. (3) Brown adipocyte precursor cells first become more frequent and then differentiate into mature brown fat cells. (4) Mitochondrial hypertrophy develops early, leading to crowding of mitochondria. Later, the ratio of mitochondria to extramitochondrial cytoplasm is restored. (5) Mitochondrial cristae are remodeled first into a more irregular pattern composed of semicircles and loops. Subsequently, the cristae become straight and also more numerous. © 1969 Academic Press, Inc.