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 blasto cyst and, after implantation, gives rise to most extraembryonic lineag es of the conceptus. Trophectoderm differentiation begins at compactio n (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 dif ferentiation. Tight junction construction begins at compaction and app ears to be a gradual process, both at morphological and molecular leve ls. This maturation pattern may be due in part to sequential expressio n of tight junction constituents from the embryonic genome. Tight junc tion formation is dependent upon uvomorulin adhesion but can be inhibi ted by different means without apparently disturbing cell adhesion or polarisation. Cell interactions appear to regulate tight junction tiss ue specificity, in part by controlling;the level of synthesis of const ituents. Desmosome formation begins at the 32-cell stage, particularly as the embryo initiates blastocoel accumulation, and, in contrast wit h tight junction formation, does not appear to be a gradual process. T hus, nascent desmosomes appear mature in terms of their molecular comp osition. Desmosomal proteins are synthesised well in advance of desmos ome formation but the synthesis of the principal glycoprotein componen ts begins at the blastocyst stage and may regulate the timing of junct ion assembly. Implications of these differing patterns of biogenesis f or the embryo are discussed.