Origin and Development of the Claustrum in Rhesus Macaque

Understanding the claustrum’s functions has recently progressed thanks to new anatomical and behavioral studies in rodents, which suggest that it plays an important role in attention, salience detection, slow-wave generation, and neocortical network synchronization. Nevertheless, knowledge about the origin and development of the claustrum, especially in primates, is still limited. Here, we show that neurons of rhesus macaque claustrum primordium are generated between embryonic day E48 and E55 and express some neocortical molecular markers, such as NR4A2, SATB2, and SOX5. However, in the early stages, it lacks TBR1 expression, which separates it from other surrounding telencephalic structures. We also found that two waves of neurogenesis (E48 and E55) in the claustrum, corresponding to the birthdates of layers 6 and 5 of the insular cortex, establish a “core” and “shell” cytoarchitecture, which is potentially a basis for differential circuit formation and could influence information processing underlying higher cognitive functions of the claustrum. In addition, parvalbumin-positive interneurons are the dominant interneuron type in the claustrum in fetal macaque, and their maturation is independent of that in the overlaying neocortex. Finally, our study reveals that the claustrum is likely not a continuance of subplate neurons of the insular cortex, but an independent pallial region, suggesting its potentially unique role in cognitive control. Significance Statement The claustrum is believed to have a role in many high cognitive functions. However, the origin and development of this mysterious structure remain unknown, and the understanding of its relationship with the neocortex is ambiguous. Here we examined neuron origin and development of claustrum in rhesus macaque during the prenatal and postnatal periods. We found that the claustrum is formed as an independent telencephalic area as early as E55, and it seems not related to the subplate of the insula, although it shares some molecular characteristics with the neocortex. The claustrum excitatory neurons are generated sequentially around E48 and E55 and build a “core and shell” structure that may be a basic computing neuronal circuit unit underlying higher cognitive functions. ### Competing Interest Statement The authors have declared no competing interest.