Thalamic Neuron Origins

Humans differ from other primates in having thalamic neurons derived from the telencephalon as well as the diencephalon. In most mammals that have been examined, cells that reside in the thalamic nuclei are derived from the embryonic diencephalon. In human embryos, however, there is a large migratory stream passing between the ganglionic eminence (GE) of the ventral telencephalon and that thalamus in the diencephalon; this stream is called the corpus gangliothalamicum (CGT) (Rakic & Sidman, 1969). The migrating neurons express Dlx1/2 homeodomain-containing proteins, use homotypic-neurophilic guidance cues and eventually form GABAergic neurons in the dorsal thalamus, particularly the pulvinar and mediodorsal nucleus (Letinic & Rakic, 2001). It may be significant that these association nuclei are anatomically related to the association areas of the neocortex that are involved in the highest cognitive functions, including symbolic reasoning or language. This migratory stream has not been observed in carnivores, rodents, New World monkeys, nor Old World monkeys. It may therefore be a specialization of humans, although the possibility that it is a specialization of apes and humans cannot be ruled out without examination of embryonic tissue from apes.