Specification of positional information in retinal ganglion cells of Xenopus laevis: intra-ocular control of the time of specification.

Programming events in the stage 28-31 clawed frog embryo partly determine the organization of the future retinotectal map by specifying the permanent reference axes for cellular positional information in the retina. Thus, when transplanted in 180 degrees -rotated orientation into a stage 27/28 orbit, an unspecified (stage 28 or younger) eye can acquire new axes and develop a normally-oriented retinotectal map, whereas a specified (stage 31 or older) eye retains its original axes and develops an inverted map. We have used the retinotectal map to determine when (and under what conditions) specification with changes in orientation occurred in eye primordia serially transplanted between embryos of different stages. Specification was not precipitated when stage 22/23 eyes were grafted into stage 28-32 embryos or explanted in vitro, nor was specification delayed when stage 28 eyes were grafted into embryos younger than stage 28. Control experiments confirmed the general correlation between the time of specification and the stage of the eye primordium. We infer that intraocular regulatory mechanisms control the time of specification.