In vitro exposure to benzo[a]pyrene damages the developing mouse ovary

Abstract

Females are born with a finite number of oocytes, collectively termed the ovarian reserve, established within the developing fetal ovary. Consequently, maternal exposure to reproductive toxicants can have harmful effects on the future fertility of her unborn female fetus. The chemical benzo[a]pyrene (B[a]P) is a prominent component of cigarette smoke. Despite it being a known ovotoxicant, around 8% of women in Europe smoke during pregnancy. The purpose of this research was to examine the effect of B[a]P on the developing ovary, using the mouse as a model and with experiments carried out in vitro. B[a]P-exposure to the fetal ovary prior to follicle formation reduced the number of germ cells and subsequently, the number of healthy primordial follicles, by up to 76%; however, while proliferation of germ cells was not affected, the germ cells contained higher levels of DNA double-strand breaks. Exposure to B[a]P also affected the proportion of oocytes progressing through prophase I of meiosis. B[a]P exposure to neonatal mouse ovaries, after follicle formation, resulted in an 85% reduction in the number of healthy follicles, with a corresponding increase in apoptotic cell death and reduction in somatic cell proliferation. Although there was a trend towards a higher level of oxidative stress in B[a]P-exposed ovaries, this was not statistically significant; likewise, the antioxidant melatonin failed to protect against the B[a]P-induced ovarian damage. Together, the results here demonstrate that B[a]P-exposure damages the developing ovary, both before and shortly after follicle formation, an effect that could lead to a subsequent decrease in fertility. Lay summary Cigarette smoking during pregnancy can affect the fertility of the offspring, yet in Europe around 1 in 12 children born have been exposed to cigarette smoke before birth due to their mother smoking. Benzo[a]pyrene (B[a]P), one of the main chemicals found in cigarette smoke, can have damaging effects on the ovary as it develops in the fetus during the time that the population of future eggs, known as ovarian germ cells also develop. In this research, ovaries from mouse fetuses at stages of development, equivalent to the second and third trimesters of a human pregnancy, were cultured with or without B[a]P. Fetal mouse ovaries exposed to B[a]P had fewer germ cells and larger numbers of cells did not survive. Overall, the results suggest that development of the ovary of a fetus could be affected if the mother is exposed to B[a]P, whether that is through cigarette smoke, or other types of exposure.