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 that of lens regeneration from cornea that occurs in the tadpole. During embryonic lens development, all three crystallin transcripts were initially detected at the same stage of lens placode formation, and subsequently their signals became restricted to the presumptive lens fiber region. At later stages, 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 fiber region of the lens vesicle. The expression of gamma-crystallin, however, appeared later than the other two crystallin genes and was detected only in morphologically discernible lens fibers. In the later stages of lens regeneration, expression of these crystallins was observed only in the lens fiber 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 crystallin gene transcription.