LENS REGENERATION IN LARVAL XENOPUS-LAEVIS - EXPERIMENTAL-ANALYSIS OFTHE DECLINE IN THE REGENERATIVE CAPACITY DURING DEVELOPMENT

In Xenopus laevis, the capacity to regenerate a new lens from the oute r cornea gradually decreases between stages 50 and 58, is almost negli gible during the metamorphic climax, and disappears after metamorphosi s. The factors responsible for lens transdifferentiation of the outer cornea are produced by the neural retina and are located in the vitreo us chamber. This decrease in the regenerative capacity may be due to: (1) a reduction of the inductive power of the retina, (2) a reduction of lens-forming competence of the outer cornea, (3) an inhibition of t he lens transdifferentiation process, (4) a combination of these cause s. In order to test these hypotheses, fragments of outer cornea or of outer and inner corneas joined together were isolated from early larva e, late larvae and froglets, and implanted into the eye of host larvae during the premetamorphosis or the metamorphic climax. Results from i mplants of outer cornea into the vitreous chamber showed that the drop in lens regeneration capacity during the metamorphic climax is not du e to a decrease in the inductive power of the retinal factor and that the gradual decrease in the regenerative capacity observed between sta ges 50 and 58 is not related to a substantial diminution in the capaci ty of outer cornea cells to transdifferentiate into lens fibers. Resul ts from implants of outer and inner corneas joined together showed tha t in these implants the lens transdifferentiation of the outer cornea was partially inhibited. These findings indicate that the decrease in lens regeneration is mainly due to an inhibition of the lens transdiff erentiation process of the outer cornea by the inner cornea. However, even implants of cornea (multilayered epithelium and substantia propri a) excised from metamorphosed animals were able to form lens fibers, a lthough to a lesser percentage than that obtained after implantation o f fragments of larval outer and inner corneas. Thus, the lens-forming competence in the corneal epithelium is still present to a certain deg ree even when lens regeneration capacity is lost. Several observations suggest that in the lentectomized eye of late larvae and froglets the mechanical inhibition of lens trans differentiation process exerted b y the inner cornea (or the substantia propria), due to the rapid forma tion of a connective barrier against the spreading of the retinal fact or toward the outer cornea, has a decisive role in maintaining the phe notypic stability of the outer cornea. (C) 1997 Academic Press.