Spatiotemporal distribution of PAX6 and MEIS2 expression and total cell numbers in the ganglionic eminence in the early developing human forebrain.

The development of the human neocortex is a complex and highly regulated process involving a time-related expression of many transcription factors including the homeobox genes Pax6 and Meis2. During early development, Pax6 is expressed in nuclei of radial glia cells in the neocortical proliferative zones and controls the differentiation and neurogenetic fate of these cells in the dorsal telencephalon in rodents. Animal studies on the Meis2 gene have revealed expression in the developing telencephalon and Meis2 is known to regulate the expression of Pax6 in the eye and pancreas. Because of this functional relation between Pax6 and Meis2, we studied the spatial and temporal expression of PAX6, and MEIS2 using a developmental series of human fetal brains at 7-19 postconceptional weeks with emphasis on the forebrain to investigate whether the two genes are expressed in the same regions and zones in the same time window. We demonstrate by in situ hybridization and immunohistochemistry that the two homeobox genes are expressed during early fetal brain development in humans. PAX6 mRNA and protein were located in the proliferative zones of the neocortex and in single cells in the cortical preplate at 7 fetal weeks and in the developing cortical plate from 8 or 9 to 19 fetal weeks. The expression of PAX6 expanded into the ganglionic eminence just prior to the stage at which a stereological estimation showed an exponential rise in total cell number in this area. The MEIS2 gene was also present in the proliferative zones of the human fetal neocortex and a higher expression of MEIS2 than PAX6 was observed in these areas at 9 fetal weeks. Further, MEIS2 was expressed at a very high level in the developing ganglionic eminence and at a more moderate level in the cortical plate.