Ultrastructure of the Gunn rat substantia nigra. II. Mitochondrial changes.

The substantia nigra of hyperbilirubinemic (Gunn) rats was studied using light and electron microscopic techniques. PAS-positive inclusions were observed within the neuronal somata of animals 2 to 12 weeks old. Incubation of tissue sections in 0.5% diastase inhibited most of the positive PAS reaction, thus indicating the intraneuronal presence of glycogen. Electron microscopic examination of the tissue substantiated the histochemical data. Mitochondria in neuronal somata contained collections of osmiophilic granules, 200-300 A diameter, in their intracristal (intermembrane) space. These granules were typical of beta-glycogen particles. In increasingly older animals (from 2 to 12 weeks old) progressively fewer collections of intramitochondrial glycogen were observed. In these same animals there was, however, a concomitant increase of osmiophilic granules, 400-600 A diameter, located in the cisternae of granular endoplasmic reticulum. These granules corresponded to alpha-glycogen. After 12 weeks of age, only occasional glycogen granules were seen in either the mitochondria or endoplasmic reticulum. It is suggested that the intramitochondrial glycogen represents an alternate means by which the neuron is able to manufacture ATP following the disruption of oxydative phosphorylation caused by bilirubin. Furthermore it is suggested that with the recovery of oxydative phosphorylation the intramitochondrial collections of glycogen are transferred to the endoplasmic reticulum where glycogenolysis might possibly occur.