Sex-bias in utero alters ovarian reserve but not uterine capacity in female offspring

Abstract

Environmental stressors to which a foetus is exposed, affect a range of physiological functions in post-natal offspring. Such stressors include disproportionate steroid hormone concentrations in the uterine environment. We aimed to determine the in-utero effect of steroid hormones on reproductive potential of female offspring using a porcine model. Hypothesising that an in-utero sex bias will influence ovarian reserve and endometrial morphology in the breeding gilt. Reproductive tracts of pigs from female-biased litters (>65% female, n=15), non-biased litters (45-54.9% female, n=15), and male-biased litters (<35% females, n=9) were collected at slaughter (95-115 kg). Ovaries and uterine horns were processed for histological approaches and stained using H&E or IHC techniques. All measurements were conducted in QuPath (Bankhead et al, 2017). Variability of data within groups was analysed with a Levenes test, whilst data was analysed using linear models in R. In the ovarian reserve, there was a significant interaction between the birth weight and the sex ratio of a litter from which a pig originated (p=.015), with low-birth-weight pigs from male-biased litters having a higher number of primordial follicles and the opposite trend seen in pigs from female-biased litters. This was not reflected in recruited, nor atretic follicles. In the uterine horn sex bias held no effect on development as seen in this study. Birth weight held more effects on the gilts. A lower BW decreased the proportion of glands found in the endometrium (p=.045). BW was found to be far more variable in both male-biased and female-biased litters (p=.026). The variability of primordial follicles from male-biased litters was greater than non-and female-biased litters (p=.014). Similarly, endometrial stromal nuclei had a greater range in male- and female-biased litters than non-biased litters (p=.028). There was a greater effect on both ovarian reserve and uterine development of piglet BW than the litter bias. There seems a benefit of being androgenised on ovarian reserve whilst no effects were found for the morphology or endometrial gland proliferation of the uterine horns. However, a crucial finding was in the variability of the data. Both primordial follicles in the male-biased ovary, and stromal nuclei in the male- and female-biased uterine horns had a wider spread in numbers than non-biased litters. This could be inflating the variability of reproductive success seen in animals form male-biased litters by two means. Firstly, by a higher likelihood of insufficient primordial pools. Secondly, through a potential impact on stromal-derived growth factors or insufficient support of the underlying implantation structures, leading to an increased variability in uterine implantation capabilities, and thus survival of the embryo.