An architectonic study of three mammals: grey squirrels (Sciurus carolinesis), tree shrews (Tupaia belangeri) and galagos (Otolemur garnetti)
Studies of cortical architecture have been used to subdivide cortex into areas of presumed functional significance and reveal features of areal differentiations across species. In the present studies, cortical architectures were compared across three key mammals in an effort to better understand how neocortex evolved in early primates. Thus, cortex of tree shrews, the closest available relative of primates, was compared with cortex of prosimian galagos, an extant primate that is thought to resemble early primates. As both primates and tree shrews are highly visual, a further comparison was made with grey squirrels, a highly visual rodent that, with tree shrews and primates, belongs to the larger clade of euachontoglires. Several different histochemical and immunohistochemical procedures were used to reveal the architectonic characteristics of the various cortical areas. The histochemical methods used included the traditional Nissl, cytochrome oxidase and myelin stains, as well as a zinc stain, which reveals free ionic zinc in the axon terminals of neurons. Immunohistochemical methods include parvalbumin (PV) and calbindin (CB), both calcium-binding proteins, and the vesicle glutamate transporter 2 (VGluT2). These procedures allowed a number of homologous cortical areas to be identified in all three species and cortical areas to be characterized in terms of levels of protein expression, cellular composition, laminar properties, types of differentiation, and whether the dominant input to the cortical area originates from the thalamus or not. Visual areas were especially well differentiated in all three species, reflecting in a level of parallel evolution in grey squirrels. Neocortex of galagos is the most highly differentiated and more cortical areas could be identified. The increase in the number of visual, somatosensory and motor areas in galagos appears to have occurred with the emergence of primates.