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Within afrotherians, sirenians are unusual due to their aquatic lifestyle, large body size and relatively large lissencephalic brain. However, little is known about the neuron type distributions of the cerebral cortex in sirenians within the context of other afrotherians and aquatic mammals. The present study investigated two cortical regions, dorsolateral cortex area 1 (DL1) and cluster cortex area 2 (CL2), in the presumptive primary somatosensory cortex (S1) in Florida manatees (Trichechus manatus latirostris) to characterize cyto- and chemoarchitecture. The mean neuron density for both cortical regions was 35,617 neurons/mm3 and fell within the 95% prediction intervals relative to brain mass based on a reference group of afrotherians and xenarthrans. Densities of inhibitory interneuron subtypes labeled against calcium-binding proteins and neuropeptide Y were relatively low compared to afrotherians and xenarthrans and also formed a small percentage of the overall population of inhibitory interneurons as revealed by GAD67 immunoreactivity. Nonphosphorylated neurofilament protein-immunoreactive (NPNFP-ir) neurons comprised a mean of 60% of neurons in layer V across DL1 and CL2. DL1 contained a higher percentage of NPNFP-ir neurons than CL2, although CL2 had a higher variety of morphological types. The mean percentage of NPNFP-ir neurons in the two regions of the presumptive S1 were low compared to other afrotherians and xenarthrans but were within the 95% prediction intervals relative to brain mass, and their morphologies were comparable to those found in other afrotherians and xenarthrans. Although this specific pattern of neuron types and densities sets the manatee apart from other afrotherians and xenarthrans, the manatee isocortex does not appear to be explicitly adapted for an aquatic habitat. Many of the features that are shared between manatees and cetaceans are also shared with a diverse array of terrestrial mammals and likely represent highly conserved neural features. A comparative study across manatees and dugongs is necessary to determine whether these traits are specific to one or more of the manatee species, or can be generalized to all sirenians. © 2015 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Inhibitory GABAergic interneurons are important for shaping patterns of activity in neocortical networks. We examined the distributions of inhibitory interneuron subtypes in layer II/III of areas V1 and V2 in 18 genera of anthropoid primates including New World monkeys, Old World monkeys, and hominoids (apes and humans). Interneuron subtypes were identified by immunohistochemical staining for calbindin, calretinin, and parvalbumin and densities were quantified using the optical disector method. In both V1 and V2, calbindin-immunoreactive neuron density decreased disproportionately with decreasing total neuronal density. Thus, V1 and V2 of hominoids were occupied by a smaller percentage of calbindin-immunoreactive interneurons compared to monkeys who have greater overall neuronal densities. At the transition from V1 to V2 across all individuals, we found a tendency for increased percentages of calbindin-immunoreactive multipolar cells and calretinin-immunoreactive interneurons. In addition, parvalbumin-immunoreactive cell soma volumes increased from V1 to V2. These findings suggest that modifications of specific aspects of inhibition might be critical to establishing the receptive field properties that distinguish visual areas. Furthermore, these results show that phylogenetic variation exists in the microcircuitry of visual cortex that could have general implications for sensory processing. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]