The evolution of neuron classes in the neocortex of mammals

A number of cellular markers, such as neurotransmitters, structural proteins, and calcium-buffering proteins, display a highly specific distribution in distinct classes of neocortical neurons in a large number of mammalian species. The organization of the cerebral cortex in mammals has been traditionally investigated using Nissl, Golgi, or myelin stains in species commonly used for laboratory research such as the mouse, rat, cat, and macaque monkey as well as in postmortem human brain specimens. There are comparatively few studies on the relationships between cytoarchitecture, neuronal morphology, and neurochemical specialization in less common species. This chapter presents an overview of the morphologic characteristics of the principal types of neocortical neurons as identified by neurochemical markers such as calcium-binding proteins and cytoskeletal proteins in representative species of the major subdivisions of mammals. The distribution of these neurochemical markers defines several species- and order-specific patterns that permit assessment of the degree to which neuronal morphomolecular specialization, as well as the regional and laminar distribution of distinct cell types in the neocortex, represent derived or ancestral features. Despite the remarkable diversity in morphologic and cellular organization that occurred during mammalian neocortical evolution, such neurochemical patterns identify several associations among taxa that closely match their phylogenetic relationships.