Developmental programming for retinotectal patterns.

Programming events at stages 28-31 in Xenopus specify the cellular positional information that individual retinal ganglion cells will use to derive theri appropriate locus specificity for assembly of the retinotectal map. The 'programme' that emerges in the stage 31 retina affects the entire ganglion cell population (99 percent of which is generated later) and refers positional information to intraretinal axes (AP and DV) and zero-points. Its expression in intact retinae was not modified by repeated reintroduction into pre-stage 28 orbits, prolonged eye culture in vitro, or severe disruption of the timing and sequence of ganglion cell births or of optic fibre arrivals in the tectum. In contrase, intraretinal reorganizations did not produce major modifications (e.g. after transection, partial ablation, fragment fusion etc.) in the set of locus specificities arising in various retinal regions and, in some instances, in the reference axes themselves. The modified programmes were characteristic of the components undergoing reorganization, but were convergent (many sets of reorganizing components gave a few final patterns); they appeared to involve a stable and rapid reprogramming of certain components by others, in a hierarchical fashion. The remaining experiments focus on the problem of localizing the 'trigger' for the transition from the unspecified to the specified state in the retina at stages 28-31. Specification can occur in vitro, based on reversible AP and DV orientational markers which are present in the pre-stage 28 eye primordium; in heterochromically grafted eyes, specification was neither precipitated nor delayed by altering the stage of the host. Finally, chemical dissection of the differentiating eye primordium confirmed the inference (from Jacobson's [3-H] thymidine labelling kinetics) that a specific gangliogenic precursor cell type exists in the stage 28 retinal neuroepithelium, and suggested that differentiative events in these cells trigger the specification process.