Comparison of neuronal and glial numerical density in primary and secondary visual cortex of man.

The numerical density of neurons and glial cells was estimated in visual area 18 of the adult human cerebral cortex and compared with that of area 17. Blocks of areas 17 and 18 came from the same brains and this allowed the comparison of 1) neuronal and glial numerical densities through the whole cortical depth with calculation of the neuron/glia ratio, 2) neuronal and glial numbers under one square millimeter of cortical surface, and 3) neuronal numerical densities in three groups of identified layers. The mean neuronal density is approximately 40,000 neurons/mm3 in area 17 and 31,500/m3 in area 18. The mean glial density is around 27,000/mm3 in area 17 and 32,000/mm3 in area 18. This gives a neuron/glia ratio of approximately 1.5 in area 17 and of 1.0 in area 18, but the total cellular density is similar in both areas. There are about 90,000 neurons and 64,000 glial cells under one square millimeter of cortical surface in area 17, and some 73,000 neurons and 74,000 glial cells in area 18. The higher neuronal density in area 17 is found through the whole depth of cortex and does not seem to be more pronounced in layer IVc of area 17 compared to layer IV in area 18 than in the groups of layers II-III and V-VI.