Transport of polar non-electrolytes across the intact and perfused guinea-pig placenta.

The permeability of the mature placenta of the guinea-pig has been measured to several polar non-electrolytes of differing molecular weights and to two cations. Two methods were used: on the one hand, direct analysis of the fetus after it has taken up labelled solutes from maternal blood, and, on the other, perfusion of the umbilical circulation in situ. In the intact condition, fluorescein dextran of 3000 molecular weight and 51Cr-EDTA permeated from mother to fetus at rates proportional to their free diffusion coefficients in water, indicating the presence of large pores. Perfusion of the umbilical circulation with a physiological salt solution containing dextran increased permeability to these two solutes and hence total pore area three to four times. The movement of 14C-erythritol, 24Na and 42K was such as to suggest that they also cross the placenta by an additional route (or routes), which is not enchanced by perfusion. This route presumably traverses the plasma membranes of the syncytiotrophoblast.