The challenge of defining “cell types” was the genesis of modern neuroscience.57 More recently, we have come to understand cell types as complex distributions of molecular processes.58,59 The dynamics of such processes rarely fit simple boundaries, but nonetheless we continue to use the idea of cell types to abstract molecular, neurophysiological, and morphological patterns that we observe in our cells of study. We foresee at least three critical reasons to avidly continue doing so in NHP. First, Old-world monkeys, like Rhesus macaques, are more similar to humans than any other research animal that allows for invasive neurophysiological experiments. Accordingly, Rhesus macaque cell types, including highly specialized neurons like Betz cells, von Economo neurons, and even striatal interneuron types, recapitulate homologous human cell types better than cells from rodents or even from marmosets.18,60,61 Second, single cell studies performed on post-mortem human tissue are subject to different ethical constraints that manifest as relatively long and highly variable postmortem intervals.62 In contrast, NHP experiments can be performed in a highly controlled and more timely fashion. Finally, NHPs have resisted the widespread application of
