Neuron numbers in sensory cortices of five delphinids compared to a physeterid, the pygmy sperm whale

The basic nonuniformity of the cerebral cortex

“…Our finding of a nonuniform number of neurons beneath a cortical surface unit across species, which is in line with observations from several other groups (12)(13)(14), challenges the notion of uniformity in the number of neurons contained in a cortical column, a notion that is widely used (15) or implied in models of cerebral cortical evolution, development, and connectivity. A variable number of neurons underneath 1 mm 2 of cerebral cortical surface does not invalidate the essence of models such as the radial unit hypothesis (8), which considers that cortical expansion in evolution happened through the increase in the number of neuronal precursors, which in turn increased ''the number of columnar units without changing the number of neurons within each unit significantly'' (20), despite expectations that small changes in cortical thickness depended on the number of cells in each radial unit.…”

“…We also examined predictions of the models of cortical expansion based on the addition of columns of constant numbers of neurons, such as the expected inverse proportionality between cortical thickness and neuronal density (10). Our results agree with stereological data from other mammalian orders that indicate the number of neurons beneath a surface unit of cerebral cortex is not constant but rather covaries with neuronal density (12)(13)(14).…”

“…Given that cortical thickness does not increase by a corresponding 10 times from mouse to capybara (S.H.-H., unpublished observation), this decrease in density is much faster than would be compatible with a constant number of neurons per unit area of cortical surface across species. Therefore, we predict that, in rodents and in primates, insectivores (13), and delphinids (12), the number of neurons per unit area of cortical surface decreases together with neuronal density. This correlation is expected whenever changes in neuronal density are not accompanied by an inversely proportional change in cortical thickness such that N/A ϭ D ϫ T. With similar thicknesses, the cortex with the larger density of neurons should also have a larger number of neurons beneath a unit area of cortical surface.…”

“…In that study, the authors used twodimensional counts of neurons in sections of the cerebral cortex from mouse, rat, cat, macaque, and man, and concluded that a uniform number of neurons beneath 1 mm 2 is present across species. The notion of numerical uniformity, although supported by data obtained through light microscopy analysis in the mouse (11), has since been disputed by separate research studies within (12,13) and across mammalian orders (14). Despite the evidence to the contrary, most models of cortical evolution and connectivity still assume that cortical columns hold a constant number of neurons across species (7,8,15,16).…”

A Neurological Comparative Study of the Harp Seal (Pagophilus groenlandicus) and Harbor Porpoise (Phocoena phocoena) Brain

“…To date, the Minke whale has the highest number of neocortical glial cells measured in any mammal with a ratio of 7.7/1 (Eriksen and Pakkenberg, 2007), whereas the fin whale (Balaenoptera physalus) has a ratio of 5/1 (OelschlŠ ger and OelschlŠ ger, 2009). This implies that a high number of glial cells could be an adaptation to maintaining and controlling a larger body, or a protective means when diving (Poth et al, 2005; OelschlŠ ger and OelschlŠ ger 2009). Also, it could be that large neurons require more glial cells (Eriksen and Pakkenberg, 2007).…”