Elucidating the framework for specification and determination of the embryonic retina

How cell determination is regulated remains a major unsolved problem in developmental biology. The early embryonic rudiments of many tissues and organs are difficult or impossible to identify, isolate and study at the time when determination occurs. We have examined the commitment process leading to retina formation in Xenopus laevis, where presumptive eye tissue can be identified and studied to assay its biological properties during the events leading up to determination. We find that for the retina, specification, the point at which a tissue placed in neutral culture medium can first properly differentiate, occurs during mid-gastrulation. By late gastrulation, determination, the final, irreversible step in commitment, has occurred. At this stage, the presumptive retina will differentiate and cannot be reprogrammed even if exposed to other active inducers, e.g. when challenged by transplantation to ectopic sites in the embryo. Key eye regulatory genes are initially expressed in the retinal field during specification and/or determination (e.g. rax, pax6, lhx2, and fzd5) potentially linking them, or genes that regulate them, to these processes. This study provides essential groundwork for defining the mechanisms for how these important developmental transitions occur.