Filtration and diffusion across the immature placenta of the anaesthetized rat embryo

Transplacental clearances of Cl-, Na+, SO4(2-), mannitol, raffinose and inulin were measured in anaesthetized rats of 14-19 days gestational age. Water flow across the placenta was calculated from embryonic (74 per cent/day) and extra-embryonic fluid (94 mg/day) growth. Before 16 days, more than half of the tracer contents of the conceptuses were located in the extra-embryonic fluids. Clearances were not demonstrably affected by the presence or polarity of an electrical charge on the tracer. Clearances fell off with increasing molecular weights but faster than the corresponding decreases in the coefficients of free diffusion. The dimensions of the paracellular transplacental passages were calculated with the Patlak equation and equivalent pore theory. Pore radii were not significantly different from 3.3 nm at any gestational age but in the last 2 days, the geometric constant (number of pores per gram x pore area/pore length) increased threefold, to 31 cm/gram placental weight. The calculated parameters were insensitive to deletion of Na+ and Cl- from the data set. In haemochorial placentae, steric restriction does not prevent the entry into the embryo of solutes circulating in maternal blood, except for macromolecules.