The effects of hibernation on cone visual cells in the ground squirrel.

The cone visual cells of active, hibernating, and aroused 13-line ground squirrels have been studied by microscopy and autoradiography. Major changes occur throughout the cells during hibernation. The outer segments are shortened, and the diameters of the membranous discs may be reduced. Mitochondria are diminished in size and number, ribosomes are depleted, and the Golgi complex is fragmented into vesicles. Calycal processes are thickened, and synaptic ribbons become aggregated ectopically within the synaptic body. When hibernation is terminated, the cells recover rapidly. First, the basic synthetic machinery (mitochondria, ribosomes, Golgi complex) is regenerated,, and then the outer segments are repaired. This process is completed within 1 week. Many of the structural changes observed during hibernation are interpreted as effects of a temporary metabolic imbalance in which degradative mechanisms, including autophagy, are emphasized. In contrast, recovery is achieved by a comparable imbalance in which there is a transient accentuation of formative mechanisms. The recovered cells thereafter maintain a steady state of continuous self-renewal, in which formation and degradation are in balance.