Comparative neuroanatomy


The nervous system exhibits tremendous diversity in nature. Just among mammals, for instance, the size of the brain varies by about 100,000x from the smallest shrew to the largest whale. Amongst the diversity, however, lies some kind of order. For instance, all of the brains below have a readily recognizable cerebral cortex; a cerebellum in the back of the brain; olfactory bulbs up front, at its base; and a brainstem that is contiguous with the spinal cord. Comparative neuroanatomy is the branch of neuroscience that searches for the rules behind such organized diversity, which results from evolution.


Regarding the diversity in brain size, an unspoken assumption in the field has been that the same scaling rules apply to all mammalian brains - that is, that large brains were presumed to be larger versions of small brains, regardless of whether they were carnivore, insectivore, primate, cetacean or rodent brains. Given the methods available, comparative studies up until the 21st century analyzed neuronal density, the glia/neuron ratio, structure volume and surface area across species of all different mammalian orders as if they were all variations of the same model. According to these studies, larger brains were made of larger numbers of larger neurons, with ever increasing glia/neuron ratios (presumably because larger neurons need more metabolic support, provided by a larger number of glial cells).

Another common notion was that evolution often entailed an expansion of the cerebral cortex - which would have happened through the addition of columnar modules, each with a fixed number of neurons, to the cerebral cortex. Based on this notion, and in the absence of direct measurements of the number of neurons in the cortex, scaling models assumed, as a rule, that the number of cortical neurons could be estimated as the product of cortical surface area and the number of neurons underneath 1mm2 of the cortex - which was believed to be uniform across species.

With the development in our lab of a novel method for determining cell numbers in the brain, however, this scenario is changing. We have shown that all mammalian brains can no longer be considered equal, as different cellular scaling rules apply to rodents, primates and insectivore brains. For instance, while larger rodent brains are indeed composed of larger numbers of larger neurons, primate brains grow with the addition of neurons whose average size does not increase, and insectivore brains combine these two strategies. We have also shown that the number of neurons underneath a unit surface area of the cerebral cortex is not uniform across species, a finding that calls into question the validity of many scaling models. We are now examining how cortical folding, the most evident feature or cortical neuroanatomy, is related to changing numbers of neurons in the brain.

Our goal, therefore, is to use quantitative comparative studies of neuroanatomy to determine what different brains have in common, and also how it is that they differ, in order to unravel the mechanisms that, operating in evolution, give rise to the ordered diversity found in the nervous system.