Systemic Compensatory Response to Neonatal Estradiol Exposure Does Not Prevent Depletion of the Oocyte Pool in the Rat

Abstract

The formation of ovarian follicles is a finely tuned process that takes place within a narrow time-window in rodents. Multiple factors and pathways have been proposed to contribute to the mechanisms triggering this process but the role of endocrine factors, especially estrogens, remains elusive. It is currently hypothesized that removal from the maternal hormonal environment permits follicle formation at birth. However, experimentally-induced maintenance of high 17β-estradiol (E2) levels leads to subtle, distinct, immediate effects on follicle formation and oocyte survival depending on the species and dose. In this study, we examined the immediate effects of neonatal E2 exposure from post-natal day (PND) 0 to PND2 on the whole organism and on ovarian follicle formation in rats. Measurements of plasma E2, estrone and their sulfate conjugates after E2 exposure showed that neonatal female rats rapidly acquire the capability to metabolize and clear excessive E2 levels. Concomitant modifications to the mRNA content of genes encoding selected E2 metabolism enzymes in the liver and the ovary in response to E2 exposure indicate that E2 may modify the neonatal maturation of these organs. In the liver, E2 treatment was associated with lower acquisition of the capability to metabolize E2. In the ovary, E2 depleted the oocyte pool in a dose dependent manner by PND3. In 10 µg/day E2-treated ovaries, apoptotic oocytes were observed in newly formed follicles in addition to areas of ovarian cord remodeling. At PND6, follicles without any visible oocyte were present and multi-oocyte follicles were not observed. Our study reveals a major species-difference. Indeed, neonatal exposure to E2 depletes the oocyte pool in the rat ovary, whereas in the mouse it is well known to increase oocyte survival.

Environmental influences on ovarian dysgenesis — developmental windows sensitive to chemical exposures

“…An opposite effect, reduction in primordial follicles at 8 dpp, was seen in a study by Rodríguez et al 23 where neonatal rat pups had been exposed to DES (0.2 and 20µg/kg) and BPA (20mg/kg) at 1, 3, 5, and dpp. This suggests that mice and rats are affected differentley, supported by a study showing a reduced oocyte pool at 3 and 6 dpp in rat pups treated with estrogen between 0-2 dpp 82 . Finally, in the hamster it seems that estrogen stimulates follicle assembly rather than oppose it as is the case in mice 83 .…”

Embryonic exposure to the widely-used herbicide atrazine disrupts meiosis and normal follicle formation in female mice

“…To examine how in utero exposure to ATZ affects the oocyte pool, we counted the number of primordial follicles in 6-day-old (d.o.) control and ATZ-exposed ovaries on sections immunostained for the MSY2 protein, a marker of the oocyte 37 . The numbers of primordial, primary and secondary follicles were not altered in ATZ-exposed ovaries; in contrast, the number of MOF per ovary was increased more than 4 times (Fig.…”

“…Progesterone (P), pregnenolone (Preg) and estradiol (E2) were assayed in two steps using mass spectrometry coupled with gas chromatography 37 . Briefly, sera were overloaded with deuterated steroid internal standards (CDN isotopes Inc., Canada) and extracted with 1-chlorobutane.…”

“…The serum was collected from ketamine/xylazine-anesthetized females at the day of the opening of the vaginal cavity (vaginal opening) by terminal cardiac exsanguination, and aliquots were stored at −80 °C. Progesterone (P), pregnenolone (Preg) and estradiol (E2) were assayed in two steps using mass spectrometry coupled with gas chromatography 37 . Briefly, sera were overloaded with deuterated steroid internal standards (CDN isotopes Inc., Canada) and extracted with 1-chlorobutane.…”

Analgesic exposure in pregnant rats affects fetal germ cell development with inter-generational reproductive consequences

“…F2 ovaries (Pnd25) were serial sectioned from 3–6 animals from each treatment group. Three sections per ovary, representing ~10, 30 and 50% points through the tissue block, were immunostained for the oocyte marker YBX2 58 and the number of primordial, transitional, primary, secondary and antral follicles quantified using Image-Pro software (Media Cybernetics Inc, USA). Only follicles containing a YBX2-positive oocyte nucleus were counted.…”