Induction of proopiomelanocortin mRNA expression in animal caps of Xenopus laevis embryos.

To convert animal pole cells of a frog embryo from an ectodermal fate into a neural one, inductive signals are necessary. The alkalizing agent NH4Cl induces the expression of several anterior brain markers and the early pituitary marker XANF-2 in Xenopus animal caps. Here it is demonstrated that NH4Cl also induced proopiomelanocortin (POMC)-expressing cells (the first fully differentiated pituitary cell type) in stage 9 and 10 Xenopus animal caps, and that all-trans retinoic acid, a posteriorizing agent, was able to block this induction when it was administered within 2 h after the start of NH4Cl incubation. Thus, after 2 h, the fate of Xenopus animal cap cells was determined. Microinjection of ribonucleic acid (RNA) encoding noggin, an endogenous neural inducer, led to the induction of POMC gene expression in animal caps of stage 10 embryos, suggesting that noggin represents a candidate mesodermal signal leading to the POMC messenger (m) RNA producing cell type in uncommitted ectoderm. Hence, an alkalizing agent and a neural inducer can generate a fully differentiated POMC cell lineage from Xenopus animal caps.