The embryonic progenitors that give rise to the vertebrate retina acquire their cell fate identity through a series of transitions that ultimately determine their final, differentiated retinal cell fates. In Xenopus, these transitions have been broadly defined as competence, specification, and determination. The expression of several transcription factors within the anterior neural plate at the time when the presumptive eye field separates from other neural derivatives suggests that these genes function to specify competent embryonic progenitors toward a retinal fate. In support of this, we demonstrate that some transcription factors expressed in the anterior neural ectoderm and/or presumptive eye field (otx2, pax6, and rx1) change the fate of competent, ventral progenitors, which normally do not contribute to the retina, from an epidermal to a retinal fate. Furthermore, the expression of these factors changes the morphogenetic movements of progenitors during gastrulation, causing ventral cells to populate the native anterior neural plate. In addition, we experimentally demonstrate that the efficacy of pax6 to specify retinal cells depends on the position of the affected cell relative to the field of neural induction. Thereby, otx2, pax6, and rx1 mediate early steps of retinal specification, including the regulation of morphogenetic cell movements, that are dependent on the level of neural-inductive signaling.
(c) 2001 Elsevier Science.