Formation of Rathke's pouch requires dual induction from the diencephalon

Targeted disruption of the homeobox gene T/ebp (Nkx2.1, Ttf1, TitfI) in mic e results in ablation of the pituitary, Paradoxically, while T/ebp is expre ssed in the ventral diencephalon during forebrain formation, it is not expr essed in Rathke's pouch or in the pituitary gland at any time of embryogene sis. Examination of pituitary development in the T/ebp homozygous null muta nt embryos revealed that a pouch rudiment is initially formed but is elimin ated by programmed cell death before formation of a definitive pouch. In th e diencephalon of the mutant, Bmp4 expression is maintained, whereas Fgf8 e xpression is not detectable. These data and additional genetic and molecula r observations suggest that Rathke's pouch develops in a two-step process t hat requires at least two sequential inductive signals from the diencephalo n. First, BMP4 is required for induction and formation of the pouch rudimen t, a role confirmed by analysis of Bmp4 homozygous null mutant embryos. Sec ond, FGF8 is necessary for activation of the key regulatory gene Lhx3 and s ubsequent development of the pouch rudiment into a definitive pouch. This s tudy provides firm molecular genetic evidence that morphogenesis of the pit uitary primordium is induced in vivo by signals from the adjacent diencepha lon.