Effect of estrogen deprivation on follicle/oocyte maturation and embryo development in mice

It is believed that estrogen plays pivotal roles in the regulation of follicle/oocyte maturation and oocyte fertilizability. It is also involved in the functional preparation of the fallopian tubes for subsequent gamete interaction, in early embryonic development occurring in the tubal microenvironment, and in the preparation of the uterus for implantation. This study was designed to determine whether estrogen is required for follicular and embryonic development.

The biosynthesis of estrogen was blocked by a daily injection of the aromatase inhibitor, Arimidex, at a dose of 100 micro g/d, using 3 - 4 week old C57B6 F1 female mice. Injections were continued for 3 days in experiment 1 (n = 10) and for 5 days in experiment 2 (n = 23). Mice in the control group (n = 27) were given the same amount of saline. Exogenous gonadotrophin [7.5 IU pregnant mare serum gonadotrophin (PMSG)] was administered to induce follicular growth and development on the second day. In experiment 1, we tested estrogen and progesterone levels and examined ovary morphology two days later. In experiment 2, 47 hours after PMSG injection, 5 IU human chorionic gonadotropin (hCG) was given and two female mice were then caged with a male mouse overnight. Two days later, we measured estrogen and progesterone levels. We then removed the embryos, cultured them, and examined embryonic development every 24 hours for 3 days.

Before hCG injection, estrogen levels in mice from the Arimidex group were suppressed by 94%, and progesterone levels were suppressed by 75%. There was no difference between the two groups in mean number of total follicles found per animal (30.4 follicles/animal in the control group and 27 follicles/animal in the Arimidex group). Two days after hCG injection, estrogen levels in the Arimidex group were significantly lower than that in the control group (P < 0.01), while progesterone levels were not significantly lower (P > 0.05). The rate of development of embryos, morulae, blastocysts, and hatching blastocysts was not significantly different between the two groups (P = 0.20, 0.10, 0.44, and 0.38, respectively).

In the present study, by depriving mice of normal estrogen support, we have been able to rule out the absolute need for rising levels of estrogen for the completion of the follicular maturation process and the development of embryos in vitro.