Transmission electron micrograph of interscapular brown adipose cell from the bat. The mitochondria herein are exceedingly numerous and have an extensive internal membrane surface. Nearing the end of the donor animal's hibernation, the cell is depleted of lipid and most of the cytoplasm is occupied by large spherical mitochondria. The large number of mitochondria is believed to be due to high energy required for synthesizing fat from carbohydrate.