Transdifferentiation of ocular tissues in larval Xenopus laevis.

Transdifferentiation phenomena offer a useful opportunity to study experimentally the mechanisms on which cell phenotypic stability depends. The capacities of vertebrate eye tissues to reprogram cell differentiation are well known in avian and mammalian embryos, and in larval and adult newt. From research into the capacity of anuran eye tissues to reprogram differentiation into a new pathway, considerable data have accumulated concerning the transdifferentiative capacities of eye tissues in larval Xenopus laevis. This work reviews the data concerning the transdifferentiative phenomena of eye tissues in that species and, based on these, aims to establish the extent of our knowledge about the mechanism controlling these processes. In larval Xenopus laevis the outer cornea can regenerate a lens by a lens-transdifferentiation process triggered and substained by a factor(s), probably of a protein nature, produced by the neural retina. In a normal eye phenotypic stability of the outer cornea is guaranteed by the presence of the inner cornea and lens, which prevent the spread of retinal factor(s). The stimulus for lens transdifferentiation of the outer cornea can be supplied by other tissues as well, but this capacity is not widely distributed. The iris and retinal pigmented epithelium can transdifferentiate into neural retina if isolated from the surrounding tissues and implanted in the vitreous chamber. As for lens transdifferentiation of the outer cornea, retinal transdifferentiation of the iris can be stimulated by certain nonocular tissues as well.