Cell-type-specific expression of epidermal cytokeratin genes during gastrulation of Xenopus laevis.

Analysis of the spatial pattern of expression of embryo-specific epidermal cytokeratin genes in Xenopus laevis shows earliest activity in the animal pole cells of stage-9 blastulae. These genes are transcribed predominantly in the epithelial or outer ectoderm, to a lesser extent in the sensorial or inner ectoderm, and at low levels if at all in other regions of the embryo. In the early gastrula the entire ectoderm, including preneural and preepidermal regions, expresses cytokeratin mRNAs; accumulation of these mRNAs in preneural cells is terminated after contact is made with involuting chordamesoderm. On the basis of this and earlier work (Sargent et al. 1986) we suggest that the pattern of expression of cytokeratin genes in frog embryogenesis is based on prelocalized components modulated by the inductive influence of involuting chordamesoderm. The cytokeratin proteins are deposited in the form of filamentous networks in both layers of the epidermis. In the epithelial layer, a much denser mesh of filaments is facing the outside of the embryo. This polarity is established at the onset of the polymerization of these filaments. Thus, intraembryonic and intracellular localization of keratin gene expression and protein deposition is established at the onset of activation of these genes.