Pax6 is essential for establishing ventral-dorsal cell boundaries in pituitary gland development

Pax6, a highly conserved member of the paired homeodomain transcription fac tor family that plays essential roles in ocular, neural, and pancreatic dev elopment and effects asymmetric transient dorsal expression during pituitar y development, with its expression extinguished before the ventral --> dors al appearance of specific cell types. Analysis of pituitary development in the Small eye and Pax6 -/- mouse mutants reveals that the dorsoventral axis of the pituitary gland becomes ventralized, with dorsal extension of the t ranscriptional determinants of ventral cell types, particularly PFrk. This ventralization is followed by a marked decrease in terminally differentiate d dorsal somatotrope and lactotrope cell types and a marked increase in the expression of markers of the ventral thyrotrope cells and SF-l-expressing cells of gonadotrope lineage. We suggest that the transient dorsal expressi on of Pax6 is essential for establishing a sharp boundary between dorsal an d ventral cell types, based on the inhibition of Shh ventral signals.