Involvement of gap junctions in placental functions and development.

Connexin (Cx) expression and gap junctional intercellular communication (GJIC) are involved in development and differentiation processes. Mediating exchanges between mother and fetus, the placenta is formed when fetal membranes are apposed or even fusing or destroying the uterine mucosa. Therefore, an extraordinary variability of placental structures is observed throughout the mammalian species. This variability affect mainly, the maternofetal blood flow interrelationships, the kind and number of tissue layers separating maternal and fetal bloods, the trophoblast invasiveness and the formation of a syncytium (syncytiotrophoblast). Here, the expression, the localisation and the possible role of Cx and GJIC in placental functions and development are discussed. In rodents, gene knock out in mice have vastly improved our understanding of the role of Cx genes in mouse placental development: Cx26 in transplacental uptake of glucose, Cx31 in the proliferative process of trophoblastic cells and Cx45 in placental vascularisation. In human, it appears that Cx43 allows a GJIC required for the fusion process of cytotrophoblastic cells leading to the formation of the syncytiotrophoblast, the site of the numerous placental functions. On other hands, Cx40 plays a critical role in the switch from a proliferative to an invasive phenotype of the trophoblastic cells invading the endometrium. Owing to the striking diversity of Cx expression in placental structures, we must be careful when extrapolating findings from one species to another.