Antero-posterior patterning of the vertebrate digestive tract: 40 years after Nicole Le Douarin's PhD thesis.

This review is dedicated to the work on chick digestive tract organogenesis that Nicole Le Douarin performed as a PhD student under the direction of Etienne Wolf. I discuss how she laid the grounds for future work by establishing fate maps at somitic stages, by describing morphogenetic movements between germ layers and by pointing to signaling events between endoderm and mesoderm. Her inspiring work was extended by others, in particular at the molecular level, leading to a better understanding of antero-posterior patterning in the digestive tract. Antero-posterior patterning of endoderm is initiated at gastrulation when future anterior and posterior endoderm ingress at different times and accordingly express different genes. Plasticity is however maintained at somite stages and even later, when organ primordia can be delineated. There is a cross-talk between endoderm and mesoderm and the two layers exchange instructive signals that induce specific antero-posterior identities as well as permissive signals required for organogenesis from previously patterned fields. Recent experiments suggest that several signaling molecules involved in neural tube antero-posterior patterning are also instrumental in the digestive tract including retinoic acid and FGF4.