The effect of the oviduct, uterine, and in vitro environments on zona thinning in the mouse embryo.

Objective: To evaluate the impact of the oviduct, uterine, and in vitro environments on zona pellucida thinning in the mouse embryo.

Design: Female mice were stimulated with pregnant mare serum gonadotropin and mated and hCG injection. Unilateral oviduct ligation was performed on day 2 of gestation using the dorsal approach. The mice were divided into equal groups and killed on days 2, 3, 4, 5, and 10 of gestation. In vitro incubated embryos served as controls. Average daily zona thickness measurements were subjected to analysis of variance and paired Student's t-test.

Setting: The laboratory of the assisted reproductive program of Rush University Medical Center.

Main Outcome Measure(s): Progressive daily decrease in average zona thickness.

Result(s): Zona measurements of embryos flushed out of uterine horns, ligated oviducts, and in vitro incubation demonstrated statistically significant decreases in zona thickness, from 9.6 +/- 1.6 microns (day 3) to 6.0 +/- 0.8 microns (day 5), from 11.6 +/- 2.2 microns (day 2) to 6.0 +/- 1.6 microns (day 5), and from 11.1 +/- 2.0 microns (day 2) to 6.0 +/- 1.6 microns (day 5), respectively. There were no differences in average zona thickness for embryos in the same cell stage and same protocol day in all three locations.

Conclusion(s): Zona thinning seems to be induced primarily by the dividing embryo before implantation. A substantial tubal and uterine contribution to zona thinning was not detected in this mouse embryo model.