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Diffusion permeability of the immature placenta of the rabbit embryo to inert hydrophilic molecules
1Department of Physiology, School of Medicine, Oregon Health Sciences University, Portland 97201.
The clearances of a series of hydrophilic probes (radii of 2-10 A) were determined in rabbit placentas of 13-15 days of gestation (term, 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 placenta, and the clearances decreased with increasing molecular radius. The transplacental water filtration rate calculated from conceptual growth was comparable in magnitude to the clearances of the largest tracers. Application of Patlak's modification of the Hertzian equation and pore theory suggested an equivalent pore radius of 17 A. Placental permeability surface area products computed from this value decreased much more steeply with increasing molecular dimension than the measured clearances, suggesting a highly significant contribution by filtration despite the very large diffusional gradients that existed under the experimental conditions. The results indicate that the size selectivity of the hemochorial embryo placenta in vivo is not very significant, and that under normal conditions filtrate of maternal plasma constitutes a major contributor to embryonic supply.
The clearances of a series of hydrophilic probes (radii of 2-10 A) were determined in rabbit placentas of 13-15 days of gestation (term, 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 placenta, and the clearances decreased with increasing molecular radius. The transplacental water filtration rate calculated from conceptual growth was comparable in magnitude to the clearances of the largest tracers. Application of Patlak's modification of the Hertzian equation and pore theory suggested an equivalent pore radius of 17 A. Placental permeability surface area products computed from this value decreased much more steeply with increasing molecular dimension than the measured clearances, suggesting a highly significant contribution by filtration despite the very large diffusional gradients that existed under the experimental conditions. The results indicate that the size selectivity of the hemochorial embryo placenta in vivo is not very significant, and that under normal conditions filtrate of maternal plasma constitutes a major contributor to embryonic supply.