Neurochemical and cellular specializations in the mammalian neocortex reflect phylogenetic relationships: evidence from primates, cetaceans, and artiodactyls

Most of the available data on the cytoarchitecture of the cerebral cortex in mammals rely on Nissl, Golgi, and myelin stains and few studies have explored the differential morphologic and neurochemical phenotypes of neuronal populations. In addition, the majority of studies addressing the distribution and morphology of identified neuronal subtypes have been performed in common laboratory animals such as the rat, mouse, cat, and macaque monkey, as well as in postmortem analyses in humans. Several neuronal markers, such as neurotransmitters or structural proteins, display a restricted cellular distribution in the mammalian brain, and recently, certain cytoskeletal proteins and calcium-binding proteins have emerged as reliable markers for morphologically distinct subpopulations of neurons in a large number of mammalian species. In this article, we review the morphologic characteristics and distribution of three calcium-binding proteins, parvalbumin, calbindin, and calretinin, and of the neurofilament protein triplet, a component of the neuronal cytoskeleton, to provide an overview of the presence and cellular typology of these proteins in the neocortex of various mammalian taxa. Considering the remarkable diversity in gross morphological patterns and neuronal organization that occurred during the evolution of mammalian neocortex, the distribution of these neurochemical markers may help define taxon-specific patterns. In turn, such patterns can be used as reliable phylogenetic traits to assess the degree to which neurochemical specialization of neurons, as well as their regional and laminar distribution in the neocortex, represent derived or ancestral features, and differ in certain taxa from the laboratory species that are most commonly studied.