LOCALIZATION OF RESISTANCE TO DIFFUSION IN 3-LAYERED PLACENTA OF RABBIT

The tracer-dilution method was used to determine the resistance to diffusion offered by the first barrier confronting the maternal microcirculation (syncytiotrophoblast) in the placenta of the rabbit. Gestational ages were 27-29 days. Diffusional resistances were obtained for the following lipid-insoluble materials urea, 6; chloride ion, 2.5; sodium ion, 4; and inulin, 22 (minutes per milliliter in one gram placental tissue). The first three of these values are probably overestimates. The resistance offered to the diffusion of water was too low to be measurable. Data previously published from our laboratory show that in rabbits of the same gestational age the diffusional resistances of these substances in the whole barrier are very much greater than those found in the endothelial layer, and also than those now found in the syncytiotrophoblast. When the diffusional resistances of the two outer layers of the rabbit placenta (endothelium and syncytiotrophoblast) are subtracted from the total resistance of the barrier, the middle layer (cytotrophoblast) is found to contribute about 80% of the total resistance to diffusion. The disproportionate resistance of the middle layer may be due to the kind and number of cell junctions found in the cytotrophoblast, or to the long diffusional pathways in the interlaminar fluid layers that join the pores in the outer and the pores in the middle layers of the placenta. © 1969 Springer-Verlag.