Mitochondrial distribution in visual and auditory cerebral cortex of the harbour porpoise

The distributions of mitochondria and synapses in two areas of harbour porpoise cerebral cortex were examined by quantitative electron microscopy of sections stained for cytochrome oxidase. The distribution of cytochrome oxidase-positive and total mitochondria in the visual cortex of the lateral gyrus and in the auditory cortex of the temporal operculum was related closely to that of total cytochrome oxidase staining seen by light microscopy in the relevant areas. There were two peaks of mitochondrial numerical density in visual cortex: in layer III and the upper part of layer I. Mitochondrial distribution was more uniform in temporal cortex, where the numbers of mitochondria in layers VI, V and lower I were similar to those in visual cortex, but fewer were present in layers III, II and upper I. The laminar distribution of axodendritic synapses in both cortices was relatively uniform, and there was not such a large difference between the two areas. As large numbers of mitochondria have been described in the layers of cat visual cortex showing dark staining for cytochrome oxidase and receiving thalamic afferent input, we regard our data as suggestive of the existence of two main thalamorecipient zones in cetacean cortex: one in layer III and the other in upper layer I.