Epithelial differentiation and intercellular junction formation in the mouse early embryo

Trophectoderm differentiation during blastocyst formation provides a model for investigating how an epithelium develops in vivo. This paper briefly reviews our current understanding of the stages of differentiation and possible control mechanisms. The maturation of structural intercellular junctions is considered in more detail. Tight junction formation, essential for blastocoele cavitation and vectorial transport activity, begins at compaction (8-cell stage) and appears complete before fluid accumulation begins a day later (approx 32-cell stage). During this period, initial focal junction sites gradually extend laterally to become zonular and acquire the peripheral tight junction proteins ZO-1 and cingulin. Our studies indicate that junction components assemble in a temporal sequence with ZO-1 assembly preceding that of cingulin, suggesting that the junction forms progressively and in the ‘membrane to cytoplasm’ direction. The protein expression characteristics of ZO-1 and cingulin support this model. In contrast to ZO-1, cingulin expression is also detectable during oogenesis where the protein is localised in the cytocortex and in adjacent cumulus cells. However, maternal cingulin is metabolically unstable and does not appear to contribute to later tight junction formation in trophectoderm. Cell-cell interactions are important regulators of the level of synthesis and state of assembly of tight junction proteins, and also control the tissue-specificity of expression. In contrast to the progressive nature of tight junction formation, nascent desmosomes (formed from cavitation) appear mature in terms of their substructure and composition. The rapidity of desmosome assembly appears to be controlled by the time of expression of their transmembrane glycoprotein constitutents; this occurs later than the expression of more cytoplasmic desmosome components and intermediate filaments which would therefore be available for assembly to occur to completion.