Kathleen Mahon

During pituitary organogenesis, the progressive differentiation of distinct pituitary-specific cell lineages from a common primordium involves a series of developmental decisions and inductive interactions. Targeted gene disruption in mice showed that Lhx3, a LIM homeobox gene expressed in the pituitary throughout development, is essential for differentiation and proliferation of pituitary cell lineages. In mice homozygous for the Lhx3 mutation, Rathke's pouch formed but failed to grow and differentiate; such mice lacked both the anterior and intermediate lobes of the pituitary. The determination of all pituitary cell lineages, except the corticotrophs, was affected, suggesting that a distinct, Lhx3-independent ontogenetic pathway exists for the initial specification of this lineage.

Lhx3 and Lhx4 (Gsh4), two closely related LIM homeobox genes, determine formation of the pituitary gland in mice. Rathke's pouch is formed in two steps-first as a rudiment and later as a definitive pouch. Lhx3 and Lhx4 have redundant control over formation of the definitive pouch. Lhx3 controls a subsequent step of pituitary fate commitment. Thereafter, Lhx3 and Lhx4 together regulate proliferation and differentiation of pituitary-specific cell lineages. Thus, Lhx3 and Lhx4 dictate pituitary organ identity by controlling developmental decisions at multiple stages of organogenesis.

Targeted disruption of the homeobox gene T/ebp (Nkx2.1, Ttf1, Titf1) in mice results in ablation of the pituitary. Paradoxically, while T/ebp is expressed in the ventral diencephalon during forebrain formation, it is not expressed in Rathke's pouch or in the pituitary gland at any time of embryogenesis. Examination of pituitary development in the T/ebp homozygous null mutant embryos revealed that a pouch rudiment is initially formed but is eliminated by programmed cell death before formation of a definitive pouch. In the diencephalon of the mutant, Bmp4 expression is maintained, whereas Fgf8 expression is not detectable. These data and additional genetic and molecular observations suggest that Rathke's pouch develops in a two-step process that requires at least two sequential inductive signals from the diencephalon. First, BMP4 is required for induction and formation of the pouch rudiment, a role confirmed by analysis of Bmp4 homozygous null mutant embryos. Second, FGF8 is necessary for activation of the key regulatory gene Lhx3 and subsequent development of the pouch rudiment into a definitive pouch. This study provides firm molecular genetic evidence that morphogenesis of the pituitary primordium is induced in vivo by signals from the adjacent diencephalon.

We have isolated a new murine homeobox gene, Rpx (for Rathke's pouch homeobox), that is dynamically expressed in the prospective cephalic region of the embryo during gastrulation. Early expression is seen in the anterior midline endoderm and prechordal plate precursor. Expression is subsequently activated in the overlying ectoderm of the cephalic neural plate, suggesting that inductive contact with Rpx-expressing mesendoderm is required for this expression. Subsequently, Rpx expression is extinguished in the mesendoderm while remaining in the prospective prosencephalic region of the neural plate ectoderm. Ultimately, transcripts become restricted to Rathke's pouch, the primordium of the pituitary, which is known to be derived from the most anterior ectoderm of the early embryo. Down regulation of Rpx in the pouch coincides with the differentiation of pituitary-specific cell types. Rpx is the earliest known marker for the pituitary primordium, suggestive of a role in the early determination or differentiation of the pituitary. Since Rpx is expressed so dynamically and so early in the anterior region of the embryo, and since its early expression domain is much more extensive than the region fated to form the pituitary, it is likely that Rpx is involved in the initial determination of the anterior (prechordal) region of the embryo.