Evidence for ape and human specializations in geniculostriate projections from VGLUT2 immunohistochemistry

Abstract

Vesicular glutamate transporters (VGLUTs) reuptake glutamate into synaptic vesicles at excitatory synapses. VGLUT2 is localized in the cortical terminals of neuronal somas located in the main sensory nuclei of the thalamus. Thus, immunolabeling of cortex with antibodies to VGLUT2 can reveal geniculostriate terminal distributions in species in which connectivity cannot be studied with tract-tracing techniques, permitting broader comparative studies of cortical specializations. Here, we used VGLUT2 immunohistochemistry to compare the organization of geniculostriate afferents in primary visual cortex in hominid primates (humans, chimpanzees, and an orangutan), Old World monkeys (rhesus macaques and vervets), and New World monkeys (squirrel monkeys). The New and Old World monkeys had a broad, dense band of terminal-like labeling in cortical layer 4C, a narrow band of labeling in layer 4A, and additional labeling in layers 2/3 and 6, consistent with results from conventional tract-tracing studies in these species. By contrast, although the hominid primates had a prominent layer 4C band, labeling of layer 4A was sparse or absent. Labeling was also present in layers 2/3 and 6, although labeling of layer 6 was weaker in hominids and possibly more individually variable than in Old and New World monkeys. These findings are consistent with previous observations from cytochrome oxidase histochemistry and a very small number of connectivity studies, suggesting that the projections from the parvocellular layers of the lateral geniculate nucleus to layer 4A were strongly reduced or eliminated in humans and apes following their evolutionary divergence from the other anthropoid primates.

Figure 1

Phylogenetic tree showing the evolutionary relationships among the species examined in this study, sample sizes, and the distribution of character-state changes in LGN projections inferred from previous research. The present study provides additional evidence to support state change 2, the reduction or loss of direct projections from the parvocellular LGN to layer 4A after the divergence of the Hominidae and Cercopithecidae.

Figure 2

The marked reduction of VGLUT2 immunolabeling at the transition between areas V1 and V2 is illustrated in (a) a squirrel monkey (Saimiri) and (b) a human (Homo). The arrowheads in (a) mark the location of the layer 4A labeling band. Scale bars = 250 μm.

Figure 3

Anti-VGLUT2 DAB labeling of a perfusion-fixed chimpanzee (a) and an immersion-fixed chimpanzee (b). Scale bars = 100 μm.

Figure 4

The laminar distribution of VGLUT2 immunolabeling in area V1 depicted in: (a) squirrel monkey (Saimiri), (b) rhesus macaque (Macaca), (c) chimpanzee (Pan), and (d) human (Homo). Each figure pair represents a VGLUT2 immonostained section that was counterstained for Nissl with thionin. After scanning, the color channels were separated as described in the text to produce separate images of the blue Nissl staining (the left figure in each pair) and the red-brown VGLUT2 immunolabeling (the right figure in each pair). Note the lack of VGLUT2 labeling in layer 4A of the hominids (c, d) compared to the monkeys (a, b), as well as the weak labeling of layer 6 in the hominids compared to the monkeys. Scale bars = 250 μm.

Figure 5

Detail of VGLUT2 labeling in layer 4 of (a) squirrel monkey (Saimiri sciureus), (b) vervet monkey (Cercopithecus aethiops), (c) rhesus macaque (Macaca mulatta), (d) orangutan (Pongo pygmaeus), (d) chimpanzee (Pan troglodytes), and (e) human (Homo sapiens). Higher magnification of 4A labeling shown in insets. Main scale bars = 100 μm, inset scale bars = 25 μm.

Figure 6

Detail of cortical layers with terminal-like labeling in (a) layer 4C of rhesus macaque (Macaca), (b) layer 4A of vervet monkey (Cercopithecus), and (c) layer 6 of orangutan (Pongo). Scale bars = 100 μm.

Figure 7

Detail of VGLUT2 immunolabeling in the superficial layers of area V1. (a) Arrows denote centers of blob-like labeling in layer 3 in a chimpanzee. (b) Higher magnification of the left blob-like territory shown in (a). (c) A labeled, vertically oriented fiber in layer 3 of a human showing apparent en passant terminal boutons. Scale bars = 250 μm (a); 125 μm (b); 10 μm (c).