Patterning the neural crest and the isthmus organizer within the ectoderm of Xenopus embryos

R. MAYOR; A. GLAVIC; S. VILLANUEVA; C. TRÍBULO; M. AYBAR; F. BASTIDAS; G. ACUÑA; E. RODRIGUEZ

Airlie Conference Center, Warreton, VA, USA.

Conferencia; Molecular Genetics of Development; 2002

The ectoderm is divided during development in epidermis, neural plate and neural crest. The neural plate will produced the central nervous system with all its divisions, such as the isthmic organizer, while form the neural crest the peripheral nervous system and several other cell types will be originated. We will discuss here the molecular model that explains how the neural crest are induce between the neural plate and the epidermis and how the isthmic organizer is specified between the midbrain and the hindbrain. The first induction of the neural crest begins at the open neural plate stage. We have made use of the expression of several molecular markers of the neural crest to analyze the cellular and molecular signals that controls this early induction. Our results show that the initial induction of the neural crest is dependent on a gradient of BMP activity established in the ectoderm. The neural plate border, induced at a precise location within the medio-lateral axis of the ectoderm, has an anterior character. At a later stage, but still during the early neural crest induction phase, posteriorizing signals originating from the posterior part of the embryo transform a region of the previously induced anterior neural plate border into prospective neural crest cells. We have evidenced that those signals correspond to Wnts, FGFs and retinoic acid. The anterior neural plate border either does not receive such signals or these are inhibited by other agents produced by the anterior regions of the embryo, such as cerberus and dikkopf, two known Wnts inhibitors, and as a consequence, this region does not develop into neural crest. The isthmic organizer, wich patterns the anterior hindbrain and mindbrain, is one of the most studied secondary organizers. We present here evidence that interaction between Otx2 and Gbx2 genes is necessary and sufficient for the induction of the isthmic organizer. Finally we have added a novel genetic element to this process, Xiro1, wich encode another homeoprotein. We show that this gene activate the expression of Gbx2 in the hindbrain. We discuss a model where Otx2 and Gbx2 that are necessary for the specification of the isthmic organizer.