Junctional barrier complexes undergo major alterations during the plasma membrane transformation of uterine epithelial cells.

Junctions in the plasma membrane of uterine epithelial cells as well as between these cells and their extracellular environment are examined in this review to see if a synthetic appreciation of their role can be gained from the disparate evidence presently available. Major changes in most junctional components are noted during early pregnancy and the role of progesterone and oestrogen in promoting these changes is examined. In particular it is noted that while tight junctions become deeper and morphologically 'tighter' towards the time of implantation, other basolateral junctional structures as well as their cytoskeletal associations are absent. These junctional alterations are part of the 'plasma membrane transformation' of early pregnancy and allow the conclusion that while paracellular permeability is reduced by the time of blastocyst attachment, the epithelial cells are paradoxically less firmly attached to each other, and to their extracellular environment.