A set of novel tadpole specific genes expressed only in the epidermis are down-regulated by thyroid hormone during Xenopus laevis metamorphosis.

Four genes were identified in a screen for thyroid hormone-induced down-regulation of gene expression in Xenopus laevis tadpole tails. All four encode extracellular glycoproteins that are expressed exclusively in the apical cell layer of the entire tadpole epidermis, which is the equivalent of the mammalian fetal periderm. The onset of the four novel genes' expression late in embryogenesis, their activity throughout the life of the tadpole, their repression by exogenously added thyroid hormone, and the spontaneous cessation of their expression at the end of tadpole life are closely coordinated. These facts suggest that the protein products of these genes form a novel albeit temporary barrier or other structure in the tadpole epidermis that functions in lieu of the cornified, stratified epithelium of the adult epidermis. We have exploited the cloning of these genes for use as cell-specific markers to follow the appearance and loss of apical cells during development. We were able to demonstrate directly that the apical cells are derived from a stratification of the embryonic ectoderm at the onset of the formation of a true epidermis. The apical cells uniformly cover the surface of the tadpole until metamorphosis, when the expression of the four larval epidermis-specific genes is lost coordinately over the entire tadpole. In contrast, the adult epidermis develops with a distinct regional specificity: adult keratin is first expressed up to a line separating the body and tail epidermis and finally appears in the tail only at metamorphic climax. Finally, our analysis reveals that the TH-induced down-regulated gene expression program during metamorphosis is very different from the previously described up-regulated program which involves multiple cell types and several waves of gene expression changes. The down-regulated program only consists of the repression of a small number of genes which are expressed in larval cells preprogrammed to die during the larval to adult transition at metamorphosis.