Biogenesis of structural intercellular junctions during cleavage in the mouse embryo

The preimplantation embryo differentiates the trophectoderm epithelium which, from the 32-cell stage, generates the blastocoel of the blastocyst and, after implantation, gives rise to most extraembryonic lineages of the conceptus. Trophectoderm differentiation begins at compaction (8-cell stage) when cell-cell adhesion, mediated by uvomorulin, and epithelial cell polarisation first occur. Here, we review our work on the biogenesis of tight junctions and desmosomes during epithelial differentiation. Tight junction construction begins at compaction and appears to be a gradual process, both at morphological and molecular levels. This maturation pattern may be due in part to sequential expression of tight junction constituents from the embryonic genome. Tight junction formation is dependent upon uvomorulin adhesion but can be inhibited by different means without apparently disturbing cell adhesion or polarisation. Cell interactions appear to regulate tight junction tissue specificity, in part by controlling the level of synthesis of constituents. Desmosome formation begins at the 32-cell stage, particularly as the embryo initiates blastocoel accumulation, and, in contrast with tight junction formation, does not appear to be a gradual process. Thus, nascent desmosomes appear mature in terms of their molecular composition. Desmosomal proteins are synthesised well in advance of desmosome formation but the synthesis of the principal glycoprotein components begins at the blastocyst stage and may regulate the timing of junction assembly. Implications of these differing patterns of biogenesis for the embryo are discussed.