Developmental changes in epithelial transport characteristics of preimplantation rabbit blastocysts.

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Abstract
1. Transepithelial sucrose and urea permeability coefficients were measured in different‐aged preimplantation rabbit blastocysts. Sucrose permeability remained constant from 4 to 6 days after fertilization (approximately 1 X 10(‐8) cm s‐1), but thereafter increased. Urea permeability was very low throughout this period (5 X 10(‐7) cm s‐1) but a phloretin‐sensitive component to urea influx appeared between day 6 and day 7 post coitum (p.c.). 2. Glucose uptake was small (0.2‐0.5 nmol cm‐2 s‐1 and independent of external sodium throughout the preimplantation period. 3. Methionine uptake was 10 times greater than glucose uptake, and was strongly dependent upon the presence of external sodium in 5 and 6 day p.c. blastocysts. The sodium dependence of methionine uptake was lost by the seventh day after fertilization. 4. The transepithelial electrical resistance was always less than 35 omega cm2 in 4 day p.c. blastocysts, and averaged 1758 omega cm2 in 6 day p.c. blastocysts. 5. The transepithelial influx of 140 La averaged 0.46 +/‐ 0.11 nmol cm‐2 h‐1 (n = 6) in 4 day p.c. blastocysts. The 5 day p.c. embryos, on the other hand were impermeable to lanthanum (n = 10). 6. These results indicate that the epithelium of the preimplantation rabbit blastocyst undergoes dramatic changes in transport characteristics with development. The blastocyst can be characterized as a ‘leaky’ transporting epithelium during the early blastocyst period, and as a ‘tight’ epithelium during the mid to late stages of the preimplantation period.