DIFFUSION PERMEABILITY OF THE IMMATURE PLACENTA OF THE RABBIT EMBRYO TO INERT HYDROPHILIC MOLECULES

The clearances of a series of hydrophilic probes (radii of 2-10 angstr om) were determined in rabbit placentas of 13-15 days of gestation (te rm, 31 days). Maternal placental blood flows were measured by means of radiolabeled microspheres. None of the transfers of the tracers were limited by maternal or embryonic arteriovenous differences in the plac enta, and the clearances decreased with increasing molecular radius. T he transplacental water filtration rate calculated from conceptual gro wth was comparable in magnitude to the clearances of the largest trace rs. Application of Patlak's modification of the Hertzian equation and pore theory suggested an equivalent pore radius of 17 angstrom. Placen tal permeability surface area products computed from this value decrea sed much more steeply with increasing molecular dimension than the mea sured clearances, suggesting a highly significant contribution by filt ration despite the very large diffusional gradients that existed under the experimental conditions. The results indicate that the size selec tivity of the hemochorial embryo placenta in vivo is not very signific ant, and that under normal conditions filtrate of maternal plasma cons titutes a major contributor to embryonic supply.