Ultrastructural changes in the mitochondria of brown adipose cells during the hibernation cycle of Citellus lateralis.

The mitochondrial structure in the brown adipose cells of the golden mantled squirrel, Citellus lateralis, was examined throughout the year in biopsy samples. The mitochondria showed remarkable and apparently reversible changes in size and internal structure related to the physiologic activity of the animal. In the active animal the size of the largest mitochondria was 2.4 micrometer X 1.5 micrometer; during hibernation it increased to 7 micrometer X 2.5 micrometer; and during arousal it reached 11.2 micrometer X 5.3 micrometer. The cristae of the mitochondria in the brown adipose cells of the animals in hibernation phase formed loops, whorls and mesh-like interconnections. During the arousal phase they underwent further configurational changes. The most remarkable structure was associated with mitochondria of most unusual proportions which by dissolution gave rise to a new generation. This was a common finding during arousal but did not occur in any other phase of the hibernation cycle. The new mitochondria were virtually indistinguishable from those of brown adipose cells of any active animal.