Abstract
Primary visual cortex (V1) is clearly distinguishable from other cortical areas by its distinctive pattern of neocortical lamination across mammalian species. In some mammals, primates in particular, the layers of V1 are further divided into a number of sublayers based on their anatomical and functional characteristics. While these sublayers are easily recognizable across a range of primates, the exact number of divisions in each layer and their relative position within the depth of V1 has been inconsistently reported, largely due to conflicting schemes of nomenclature for the V1 layers. This conflict centers on the definition of layer 4 in primate V1, and the subdivisions of layer 4 that can be consistently identified across primate species. Brodmann’s (1909) laminar scheme for V1 delineates three subdivisions of layer 4 in primates, based on cellular morphology and geniculate inputs in anthropoid monkeys. In contrast, Hässler’s (1967) laminar scheme delineates a single layer 4 and multiple subdivisions of layer 3, based on comparisons of V1 lamination across the primate lineage. In order to clarify laminar divisions in primate visual cortex, we performed NeuN and VGLUT2 immunohistochemistry in V1 of chimpanzees, Old World macaque monkeys, New World squirrel, owl, and marmoset monkeys, prosimian galagos and mouse lemurs, and non-primate, but highly visual, tree shrews. By comparing the laminar divisions identified by each method across species, we find that Hässler’s (1967) laminar scheme for V1 provides a more consistent representation of neocortical layers across all primates, including humans, and facilitates comparisons of V1 lamination with non-primate species. These findings, along with many others, support the consistent use of Hässler’s laminar scheme in V1 research.
Highlights
IntroductionV1, in most mammalian species is clearly distinguishable from other cortical areas by the crisp, stratified appearance of its cortical layers (Campbell, 1905; Brodmann, 1909; Hässler, 1967; Garey, 1971; Billings-Gagliardi et al, 1974; Braak, 1984; Jones, 1984; Le Brun Kemper and Galaburda, 1984)
Numerous classification systems have been proposed over the history of V1 research; some based on laminar variations in staining intensity for histological markers such as cytochrome oxidase (CO; Livingstone and Hubel, 1982; Horton, 1984), some based on changes in cell density and morphology through the cortical sheet, as seen with Nissl, myelin, or Golgi stains (Lewis, 1880; Cajal, 1899; Campbell, 1905; Brodmann, 1909; Clark, 1925; von Economo and Koskinas, 1925; von Bonin, 1942; Hässler, 1967; Garey, 1971; Valverde, 1977), and still others based on variations in layer-specific gene and protein expression patterns through the areal extent of V1 (Hevner et al, 2003; Watakabe et al, 2006; Yamamori and Rockland, 2006; Bernard et al, 2012; Bryant et al, 2012; Takahata et al, 2012)
neuronal nuclear antigen (NeuN) and VLGUT2 IR revealed the relative shifts of neocortical layers at the boundary of V1 with V2 (Figures 3 and 5), providing evidence for a single layer 4 with two subdivisions in V1 that continues as a single undifferentiated layer in V2
Summary
V1, in most mammalian species is clearly distinguishable from other cortical areas by the crisp, stratified appearance of its cortical layers (Campbell, 1905; Brodmann, 1909; Hässler, 1967; Garey, 1971; Billings-Gagliardi et al, 1974; Braak, 1984; Jones, 1984; Le Brun Kemper and Galaburda, 1984). In primates, including humans (Allman and Kaas, 1971; Lund, 1973; Braak, 1976; Fitzpatrick et al, 1983; Preuss et al, 1993; Casagrande and Kaas, 1994; de Sousa et al, 2010; Wong and Kaas, 2010), as well as some related non-primate species (Collins et al, 2005; Wong and Kaas, 2008; Wong and Kaas, 2009b), most of these layers have expanded and segregated into sublayers with distinct functional and connectional properties, making V1 even more conspicuous in comparison to other cortical areas. In less specialized primates such as prosimians or lemurs (Preuss et al, 1993; Preuss and Kaas, 1996; Wong and Kaas, 2009a; Wong et al, 2009), where cortical layers are less distinct, only six to eight layers are commonly identified in anatomical studies, making the comparison of Frontiers in Neuroanatomy www.frontiersin.org
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