Effect of postcoital oestradiol treatment upon transport, growth, differentiation and viability of preimplantation mouse embryos.

This study was designed to assess the relative contribution of ovum transport anomalies and alterations of various developmental parameters upon the contraceptive effect of postcoital estrogen treatment in mice. Females received a single s.c. injection from 0.001 to 100 micrograms of oestradiol or vehicle on day 1 of pregnancy and were sacrificed 28, 52 or 76 hours after treatment to determine the location and development of embryos in the genital tract or on day 12 of pregnancy to assess the number of viable implanted embryos. Preimplantation embryos were classified according to developmental parameters into normal, retarded or arrested. Pregnancy was completely suppressed by 10 and 100 micrograms of oestradiol. A partial and dose related block of pregnancy was observed with 0.01 to 1 microgram of oestradiol. Treatment with these doses was associated with delayed oviductal transport, decreased cleavage rate, inhibition of blastulation and arrested development. The number of implanted embryos recorded on day 12 coincided with the number of normal embryos observed at 76 hours, regardless of their location and number of cells. Statistically significant but minor decreases in the cleavage rate and transitory oviductal retention caused by oestradiol neither prevented implantation nor affected subsequent viability of mouse embryos. It is concluded that the inhibition of pregnancy that follows postcoital administration of oestradiol is associated with a variety of effects upon embryo transport and preimplantation development, which appear in different combinations with each dose and contribute unequally to its interceptive effect.