Lens regeneration in Xenopus is not a mere repeat of lens development, with respect to crystallin gene expression

The spatio-temporal expression of three crystallin genes (alpha A, beta B1 and gamma) in lenses of Xenopus laevis was studied by in situ hybridization to compare the process of lens formation in embryonic development with tha t of lens regeneration from cornea that occurs in the tadpole. During embry onic lens development, all three crystallin transcripts were initially dete cted at the same stage of lens placode formation, and subsequently their si gnals became restricted to the presumptive lens fiber region. At later stag es, the three crystallin genes were expressed in primary and secondary lens fibers, but not in lens epithelium. During lens regeneration, alpha A- and beta B1-crystallin signals were first detected in the presumptive lens fib er region of the lens vesicle. The expression of gamma-crystallin, however, appeared later than the other two crystallin genes and was detected only i n morphologically discernible lens fibers. In the later stages of lens rege neration, expression of these crystallins was observed only in the lens fib er region, similar to embryonic lens development. These results reveal that lens regeneration from the inner layer of the outer cornea is not simply a repetition of embryonic lens development, when examined at the level of cr ystallin gene transcription.