“…Data suggest that in the mouse and rat placenta, as in other epithelia, Ca 2+ transport is likely to involve three main steps (Atkinson et al, 2006;Belkacemi et al, 2005): firstly, diffusion into the trophoblast from maternal plasma down an electrochemical gradient through epithelial Ca 2+ channels of the transient receptor potential (TRP) gene family; secondly, transfer across the trophoblast cytoplasm bound to the calcium binding protein calbindin-D 9K (Glazier et al, 1992); and, thirdly, active extrusion into the fetal compartment via plasma membrane Ca 2+ -ATPase (PMCA) localized to the BM (Borke et al, 1989;Fisher et al, 1987). Over the last third of gestation, gene and protein expression for calbindin-D 9K increases markedly in both mouse and rat placenta Glazier et al, 1992;Hamilton et al, 2000;Mathieu et al, 1989) and correlates with the increase in unidirectional maternofetal 45 Ca clearance measured over the same period (Glazier et al, 1992), suggesting that transcytosolic transfer on this protein and the dynamic equilibrium between bound and free Ca 2+ in the syncytiotrophoblast might be the rate limiting step of transfer in these species. The situation may well be quite different in the human placenta where it has proved difficult to show the expression of calbindin-D 9K (Belkacemi et al, 2004), or to determine if another protein is involved instead.…”