Last week, I discussed the effects of the estrogenic polymer, bisphenol A, on comb development and spermatogenesis in male chickens. Over the next few weeks, I’ll cover several recent studies implicating BPA in the disruption of gamete formation (oogenesis) in female mammals. In both male and female mammals, the germ cell precursors for sperm and eggs proliferate by mitotic divisions during fetal development. While germ cells in the testes undergo mitotic arrest and remain quiescent until after birth, germ cells in the developing ovaries initiate their first meiotic divisions. During fetal development, these egg cell precursors will complete the initial prophase events of meiosis, including pairing and recombination between homologous chromosomes, but the first meiotic division is arrested until just prior to ovulation, in the adult ovary. The second meiotic division of the oocyte is not completed until fertilization occurs. Thus, to understand the effects of endocrine disruptors, such as BPA, on the process of oogenesis, it is important to examine the consequences of exposure during fetal, neonatal, and adult stages.
As part of their ongoing studies on the effects of BPA on oogenesis in the mouse, researchers in the laboratory of Dr. Patricia Hunt identified a very early effect of this estrogenic chemical on meiosis. Unlike the potent estrogenic drug diethylstilbestrol (DES), BPA was never used clinically; however, this compound is present in polycarbonate plastics (such as those used in cages and water bottles for laboratory mice) and in the linings of food and drink containers. In 2007, Susiarjo and colleagues reported that exposure to environmentally relevant levels of BPA during fetal development disrupted oocyte development in ways that could be detected much later, in mature female mice. In these studies, fetal mice were exposed to BPA at low doses (20 micrograms per kilogram body weight per day, in the form of an implanted time-release pellet) for a one-week period, beginning on day 11 of gestation. At day 18, ovaries were isolated from female mouse fetuses, and the oocytes were prepared so that meiotic chromosomes could be examined for abnormalities. Although the rate of progression through prophase stages (pachytene, diplotene) was not affected by BPA, the movements and interactions of meiotic chromosomes were disrupted by the estrogenic chemical. The figure below summarizes the abnormalities in the exchanges between homologous chromosomes (synapsis), and shows an example of an incomplete synapsis in a BPA-exposed pachytene oocyte.
A. Abnormalities in synapsis in pachytene cells from placebo- and BPA-treated mice C. Pachytene chromosomes from BPA-treated female mice; arrow points to an incomplete synapsis (from Susiarjo et al., 2007)
The researchers used antibodies to visualize the chromosome associations (synaptonemal complexes) and the recombination exchanges between chromosomes, and found that 52% of the oocytes from BPA-exposed females had abnormal synapses. Only 16% of control (placebo-treated) oocytes had synaptic abnormalities, and none were of the novel “end-to-end association” type found in some BPA-exposed oocytes. BPA also appeared to disrupt the placement and spacing of the recombinant exchanges between homologous chromosomes. This result was confirmed by counting “crosses” (chiasmata) in metaphase chromosomes from BPA-treated females allowed to go to term and to develop postnatally for a month.
Abnormal chromosome movements and exchanges during meiosis may lead to nondisjunction, which can ultimately cause human trisomy disorders such as Down Syndrome. By allowing some of the in utero BPA-treated female mice to develop to reproductive age, Susiarjo and colleagues were able to show that the early meiotic abnormalities caused by the estrogenic compound resulted in abnormalities of chromosome number (aneuploidy) in the unfertilized eggs and two-cell embryos produced later in life. To determine whether BPA was acting through estrogen receptors to induce these chromosome abnormalities, the researchers researchers exposed mice that lacked either the alpha or beta estrogen receptor (ERα or ERβ) to the compound. Surprisingly, the ERβ-/- mice exhibit meiotic abnormalities in the absence of BPA treatment, and exposure to the compound did not increase the number of chromosome abnormalities. The investigators speculate that BPA may act as an estrogen antagonist, in the context of early meiotic events in oogenesis. Previous results from this laboratory indicated that BPA disturbed spindle formation and chromosome segregation during meiosis, and the present study adds another effect of this compound: to disturb synapsis and recombination between homologous chromosomes. The impact of BPA spans three generations, from the pregnant dams exposed to the compound, through disrupted fetal oogenesis, to the resulting aneuploid embryos.
Susiarjo, M., Hassold, T.J., Freeman, E., Hunt, P.A. (2007). Bisphenol A Exposure In Utero Disrupts Early Oogenesis in the Mouse. PLoS Genetics, 3(1), e5. DOI: 10.1371/journal.pgen.0030005