Tight junction biogenesis in the early Xenopus embryo

Tight junctions (TJs) perform a critical role in the transport functions an d morphogenetic activity of the primary epithelium formed during Xenopus cl eavage. Biogenesis of these junctions was studied by immunolocalization of TJ-associated proteins (cingulin, ZO-1 and occludin) and by an in vivo biot in diffusion assay. Using fertilized eggs synchronized during the first div ision cycle, we found that membrane assembly of the TJ initiated at the ani mal polo towards the end of zygote cytokinesis and involved sequential inco rporation of components in the order cingulin, ZO-1 and occludin. The three constituents appeared to be recruited from maternal stores and were target ed to the nascent TJ site by different pathways. TJ protein assembly was fo cused precisely to the border between the oolemma-derived apical membrane a nd newly-inserted basolateral membrane generated during cytokinesis and cul minated in the formation of functional TJs in the two-cell embryo, which ma intained a diffusion barrier. New membrane formation and the generation of cell surface polarity therefore precede initiation of TJ formation. Moreove r, assembly of TJ marker protein precisely at the apical-basolateral membra ne boundary was preserved in the complete absence of intercellular contacts and adhesion. Thus, the mechanism of TJ biogenesis in the Xenopus early em bryo relies on intrinsic cues of a cell autonomous mechanism. These data re veal a distinction between Xenopus and mammalian early embryos in the origi n and mechanisms of epithelial cell polarization and TJ formation during cl eavage of the egg. (C) 2000 Elsevier Science Ireland Ltd. All rights reserv ed.