Architectonic features and relative locations of primary sensory and related areas of neocortex in mouse lemurs

Abstract

Mouse lemurs are the smallest of the living primates, and are members of the understudied radiation of strepsirrhine lemurs of Madagascar. They are thought to closely resemble the ancestral primates that gave rise to present day primates. Here we have used multiple histological and immunochemical methods to identify and characterize sensory areas of neocortex in four brains of adult lemurs obtained from a licensed breeding colony. We describe the laminar features for the primary visual area (V1), the secondary visual area (V2), the middle temporal visual area (MT) and area prostriata, somatosensory areas S1(3b), 3a, and area 1, the primary motor cortex (M1), and the primary auditory cortex (A1). V1 has "blobs" with "nonblob" surrounds, providing further evidence that this type of modular organization might have evolved early in the primate lineage to be retained in all extant primates. The laminar organization of V1 further supports the view that sublayers of layer 3 of primates have been commonly misidentified as sublayers of layer 4. S1 (area 3b) is proportionately wider than the elongated area observed in anthropoid primates, and has disruptions that may distinguish representations of the hand, face, teeth, and tongue. Primary auditory cortex is located in the upper temporal cortex and may include a rostral area, R, in addition to A1. The resulting architectonic maps of cortical areas in mouse lemurs can usefully guide future studies of cortical connectivity and function.

The sensory thalamus and visual midbrain in mouse lemurs

“…We have previously described the sensory and motor areas of neocortex in mouse lemur brains (Saraf et al, ). The main findings are that primary sensory and motor areas can be identified by the histological criteria and that these cortical areas occupy much of neocortex, suggesting that mouse lemurs have fewer cortical areas than even small monkeys such as marmosets.…”

“…The results provide further evidence that the posterior nucleus of early primates rotated ventrally and anterior to subdivide and become the Pip and PIcm nuclei of the inferior pulvinar of haplorrhine primates, while the superior and inferior retinotopic pulvinar maps of early primates rotated to form the PL and PIcl nuclei of haplorrhine primates. While all primates have cortical visual area MT, including mouse lemurs (Saraf et al, ), MT may have inputs from several places in the pulvinar of early primates rather than from a specific nucleus such as PIm (Moore et al, ).…”

“…All the experimental procedures were performed in accordance with the animal core and use guidelines established by the European Communities Council and National Institutes of Health. The methods have been described previously in our study of cortical sensory and motor areas from the same material (Saraf et al, ).…”

“…All antibodies have been previously characterized in architectonic studies of primates (e.g., Wong & Kaas, ) and closely related nonprimate species (e.g., Wong & Kaas, ). For more detailed information, please refer to Saraf et al ().…”

“…A more extensive study of cortical cytoarchitecture was published somewhat later (Zilles, Zilles, & Schleicher, ). More recently, our study of cortical architecture in mouse lemur (Saraf et al, ) was based on brain sections processed for cell bodies (Nissl or NeuN), the vesicular glutamate transporter 2 (VGLUT2), parvalbumin (PV), nonphosphorylated neurofilament (SMI‐32), or cytochrome oxidase (CO). Thus, multiple histological procedures were used to define and characterize sensory and motor areas of cortex.…”