Neuronal subtypes and anatomic asymmetry: changes in neuronal number and cell-packing density.

The combined volume of an asymmetric cytoarchitectonic area is smaller than that of symmetric homologs. Asymmetry reflects fewer numbers of neurons in the smaller of the two sides. In this study we examined two types of neurons to check whether lateral differences in neuronal numbers affect different types of neurons comparably in the neocortex of the rat. As with overall neuronal numbers, both parvalbumin-immunoreactive (mostly long projection) neurons, and vasoactive intestinal peptide-immunoreactive interneurons increase in number in the larger of the two sides. Moreover, the concentration of parvalbumin- but not vasoactive intestinal peptide-immunoreactive neurons also increases on the larger side. Thus, there may be qualitative as well as quantitative differences in the connectivity of the larger side of an asymmetric architectonic region.