Expression of Foxb1 reveals two strategies for the formation of nuclei in the developing ventral diencephalon.

Fork head b1 (Foxb1; also called Fkh5, HFH-e5.1, Mf3) is a winged helix transcription factor gene whose widespread early expression in the developing neural tube is soon restricted to the ventral and caudal diencephalon. During diencephalic neurogenesis, Foxb1 is expressed in one patch of neuroepithelium comprising a large mammillary portion and a smaller tuberal portion. The labeled cells coming from this patch contribute to nuclear formation by means of two different strategies: (1) caudally, the young neurons aggregate and settle immediately, giving rise to the nuclei of the mammillary body; (2) rostrally, the young neurons separate from the neuroepithelium forming a trail of cells which spans the mantle layer mediolaterally and which will give rise to two separate cell groups (the dorsal premammillary and part of the lateral hypothalamic area). Our results show the elaborate, regionalized histogenetic mechanisms necessary for the differentiation of the caudal diencephalon; moreover, they suggest that specifically labeled populations, arising from specifically labeled neuroepithelial patches and giving place to specific brain nuclei could be a common mechanism to build complex, nonlaminar regions of the forebrain.