A quantitative study of neuronal and glial numerical density in the visual cortex of the bottlenose dolphin: evidence for a specialized subarea and changes with age.

Neuronal and glial numerical densities were measured in the lateral gyrus of the cerebral hemisphere of dolphins (Tursiops truncatus) from the neonatal period to adulthood. The cortex studied is the area known to be visually excitable in evoked potential studies. Two distinct parts of the adult lateral gyrus are identifiable, one relatively anterior, in which neuronal density is 23,000/mm3, the other more posterior, with almost double this density. In a neonate, the neuronal density in the anterior lateral gyrus was found to be more than double that of the adult. No samples from the immature posterior area were available. Glial density varies much less than neuronal density, both with age and between areas. Soon after birth the glia/neuron ratio is 1.6 in anterior lateral gyrus, rising to around 3 in the adult anterior lateral area, and rather less in the posterior region, where neuronal density is high. We speculate that the existence of a high numerical density of neurons in the posterior part of the dolphin visual cortex could perhaps indicate a specialized area corresponding to the primate primary visual cortex, also known to have high neuronal density.