Tight junction biogenesis in the early <i>Xenopus</i> embryo

Tight junctions (TJs) perform a critical role in the transport functions and morphogenetic activity of the primary epithelium formed during Xenopus cleavage. Biogenesis of these junctions was studied by immunolocalization of TJ-associated proteins (cingulin, ZO-1 and occludin) and by an in vivo biotin diffusion assay. Using fertilized eggs synchronized during the first division cycle, we found that membrane assembly of the TJ initiated at the animal pole towards the end of zygote cytokinesis and involved sequential incorporation of components in the order cingulin, ZO-1 and occludin. The three constituents appeared to be recruited from maternal stores and were targeted to the nascent TJ site by different pathways. TJ protein assembly was focused precisely to the border between the oolemma-derived apical membrane and newly-inserted basolateral membrane generated during cytokinesis and culminated in the formation of functional TJs in the two-cell embryo, which maintained a diffusion barrier. New membrane formation and the generation of cell surface polarity therefore precede initiation of TJ formation. Moreover, assembly of TJ marker protein precisely at the apical-basolateral membrane boundary was preserved in the complete absence of intercellular contacts and adhesion. Thus, the mechanism of TJ biogenesis in the Xenopus early embryo relies on intrinsic cues of a cell autonomous mechanism. These data reveal a distinction between Xenopus and mammalian early embryos in the origin and mechanisms of epithelial cell polarization and TJ formation during cleavage of the egg