The primate cochlear nuclei: loss of lamination as a phylogenetic process.

In primates from prosimians to hominoids (lorisoid prosimians, marmoset and ceboid monkeys, cercopithecoid monkeys, and gibbon), there are differences in the location, depth, and extent of the granular layer of the cochlear nuclei. In the prosimians, the deep granular layer of the DCN is similar to that of other mammals, but there is, in addition, a superficial or subependymal layer of granule cells in the DCN. In ceboid and cercopithecoid monkeys, only a superficial or external granular layer is present in the DCN, and the granular layer over the VCN is much reduced. In the gibbon, there is no granular layer in either of the cochlear muclei. In conjunction with the progressive reduction of the cochlear granular layer in primates, fusiform cells lose their position as a radially oriented peripheral cell layer in the DCN and become located in the central region of the nucleus. These changes in primates are interpreted as resulting from failure of inward migration and increasing cell death in the ontogeny of the cochlear external granular layer, with concomitant changes occurring in the position and orientation of their target neurons, the fusiform cells.