Ultrastructure of synapses and golgi analysis of neurons in neocortex of the lateral gyrus (visual cortex) of the dolphin and pilot whale.

Qualitative and computerized quantitative analyses of ultrastructural features of synapses in different layers of the primary visual cortex in the dolphin (Stenella coeruleoalba) and the pilot whale (Globicephala melaena) were carried out. Also, Golgi and cytoarchitectonic analyses were performed in the same species of cetaceans and, additionally, in Tursiops truncatus and Phocaena phocaena. It was found that on a synaptic level, as well as in cytoarchitectonic and Golgi features, the neocortex of cetaceans combines evolutionary progressive features and conservative features with a marked prevalence of the latter. Thus, the total number of synapses in visual neocortex in cetaceans is closer to this value in higher Primates. On the other hand, the laminar density of synapses per mm3 is generally the same in all layers in cetacean visual cortex and numerically is close to values found in small lissencephalic brains. Also, the synapse/neuron ratio in the dolphin visual cortex is of the same order as in cortices of rodents and lagomorphs and much higher than in cortices of advanced terrestrial mammals. Layers I and II contain approximately 70% of the total synapses in the cortical slab through visual cortex. Layer I also contains the extraverted dendrites of neurons of layer II and thus these two layers resemble a paleoarchicortical type of organization superimposed on a more typical neocortical organization of the lower cortical layers. In this respect the convexity neocortex of cetaceans is generally similar to the neocortices of phylogenetically ancient extant mammals such as basal Insectivora and Chiroptera.