In vitro permeability of the pig placenta in the last third of gestation.

The pig placenta is membranous throughout gestation and is suitable for investigation as a membrane system. The site of maternal-fetal exchange in vivo is used for permeability measurements in vitro and its surface area can be measured morphometrically. Fetal weight, placental weight, placental DNA content, placental macroscopic surface area, epithelio-chorial membrane area and thickness, and permeability in vitro to urea, tritiated water and sodium were measured in eight conceptuses aged between 79 and 111 days. These solutes were selected in order to facilitate comparison with data for sheep. Urea permeability per unit DNA increased throughout the last third of gestation although placental weight and DNA content ceased to increase before term.