Interspecies comparison of cholinergic axon densities in the lateral geniculate nucleus of primates
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however, marmosets differ from the other primates in the sample as they exhibit significantly lower axon length density per neuron. Given the widespread use of marmosets as neuroscience models for the human brain, these findings highlight the importance of species-specific variation and caution against assuming uniformity in brain regions across primate species.
Primates rely extensively on visual input, particularly for guiding forelimb grasping and reaching, likely due to their early ancestry in an arboreal and fine-branch niche where they fed on insects and fruit. Unlike many other mammals, primate vision also plays a central role in mediating social interactions through facial gestures and body postures, with high acuity required to maintain social bonds. The lateral geniculate nucleus (LGN) is the first-order thalamic relay for visual information, integrating retinal input and modulatory afferents prior to processing in the primary visual cortex (V1). Cholinergic innervation of the LGN regulates visual signal transmission and refines acuity, but the distribution of these axons remains poorly understood across primate species. This study compares the quantitative density of neurons and cholinergic afferents in the LGN of common marmosets, rhesus macaques, chimpanzees, and humans using stereological approaches. Neuronal density layer profiles were found to be constant across species, but overall densities of both magnocellular and parvocellular layers scale systematically with brain size. In contrast, cholinergic innervation density appears to be conserved among catarrhine primates
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