The genetics of early telencephalon patterning: some assembly required

Abstract

The embryonic telencephalon starts off as a simple sheet of neuroepithelial cells. This neuroepithelium becomes patterned into distinct proliferative zones that ultimately generate the different neural cell types that make up the adult cerebral hemispheres. Four broad regions can be defined in the early telencephalic neuroepithelium: a cortical area that generates primarily glutamatergic neurons, a dorso-medial area that induces the hippocampus and produces the choroid plexus, a ventro-medial area that generates subtypes of GABA (γ-aminobutyric acid)-ergic interneurons, and a caudo-lateral area that generates different subtypes of interneurons and inhibitory projection neurons. The development of the ventral areas of the telencephalon is established by the attenuation of dorsalizing signals by sonic hedgehog and the activation of ventral patterning by fibroblast growth factors. Forkhead box G1 interacts with different factors and is required for both dorsal and ventral patterning. Establishment of the pallial–subpallial boundary involves antagonistic interactions between key transcription factors, including NKX2.1, paired box 6, GSH2 and TLX. A dorsal midline organizer helps to establish pallial subdivisions.