Bisphenol A exposure in utero disrupts early oogenesis in the mouse

Abstract

Estrogen plays an essential role in the growth and maturation of the mammalian oocyte, and recent studies suggest that it also influences follicle formation in the neonatal ovary. In the course of studies designed to assess the effect of the estrogenic chemical bisphenol A (BPA) on mammalian oogenesis, we uncovered an estrogenic effect at an even earlier stage of oocyte development--at the onset of meiosis in the fetal ovary. Pregnant mice were treated with low, environmentally relevant doses of BPA during mid-gestation to assess the effect of BPA on the developing ovary. Oocytes from exposed female fetuses displayed gross aberrations in meiotic prophase, including synaptic defects and increased levels of recombination. In the mature female, these aberrations were translated into an increase in aneuploid eggs and embryos. Surprisingly, we observed the same constellation of meiotic defects in fetal ovaries of mice homozygous for a targeted disruption of ERbeta, one of the two known estrogen receptors. This, coupled with the finding that BPA exposure elicited no additional effects in ERbeta null females, suggests that BPA exerts its effect on the early oocyte by interfering with the actions of ERbeta. Together, our results show that BPA can influence early meiotic events and, importantly, indicate that the oocyte itself may be directly responsive to estrogen during early oogenesis. This raises concern that brief exposures during fetal development to substances that mimic or antagonize the effects of estrogen may adversely influence oocyte development in the exposed female fetus.

Figure 1. Pachytene Analysis

(A) Frequency of synaptic abnormalities in 402 pachytene cells from ten placebo mice and 648 cells from ten BPA-exposed females. (B) Pachytene oocyte from placebo-exposed female immunolabeled with SCP3 (red) and MLH1 (green) and showing normal synapsis. (C and D) Pachytene oocytes from BPA-exposed females showing incomplete synapsis of a single pair of chromosomes (arrow) (C) and abnormal end-to-end associations involving multiple SCs (arrows) (D).

Figure 2. Disturbances in Exchange Frequency and Placement Influence Meiotic Chromosome Segregation

(A) Distribution of MLH1 foci. Proportion of SCs with zero, one, two, or three MLH1 foci (exchanges) in pachytene cells from placebo and BPA-exposed females. (B–E) Air-dried chromosome preparations from BPA-exposed females. (B) Diakinesis cell containing a bivalent with three chiasmata (arrow). (C–D) Hyperploid metaphase II eggs with (C) 21 chromosomes and (D) 20 chromosomes plus one prematurely separated sister chromatid (arrow). (E) Hyperploid blastomere with 41 chromosomes from two-cell embryo.

Figure 3. Analysis of Exchanges in Pachytene Oocytes from Unexposed and BPA-Exposed βERKO Females

(A) For unexposed animals, there was no difference in mean number of MLH1 foci/cell between wild-type (26.3 ± 3.0) and heterozygous (25.8 ± 2.8) females, but unexposed mutants (28.7 ± 3.2) were highly significantly increased over wild type (t = 6.0; p < 0.001). These data represent the results from five unexposed pregnant females. For +/+ animals, 124 cells were analyzed from four females; for +/−, 44 cells from three females; and for −/−, 114 cells from four females. Data are provided as mean ± standard deviation. (B) Among exposed animals, the mean values for the three genotypes were virtually identical, but all had highly significantly elevated means over that of unexposed wild-type animals (28.6 ± 3.5, t = 3.5, p < 0.001; 28.1 ± 3.5, t = 4.5, p < 0.001; 28.1 ± 3.5, t = 3.9, p < 0.001 for +/+, +/−, and −/−, respectively). These data represent the results from six pregnant females implanted with BPA pellets. For +/+ animals, 35 cells were analyzed from two females; for +/−, 136 cells from six females; and for −/−, 89 cells from five females. Data are provided as mean ± standard deviation.