Action of estradiol on müllerian duct regression induced by treatment with norethindrone of female chick embryos

Molecular and histological endpoints for developmental reproductive toxicity in Xenopus tropicalis: Levonorgestrel perturbs anti-Müllerian hormone and progesterone receptor expression

There is an increasing concern regarding the risks associated with developmental exposure to endocrine disrupting chemicals and the consequences for reproductive capability. The present study aimed to refine the Xenopus (Silurana) tropicalis test system for developmental reproductive toxicity by characterising molecular and histological features of sexual development, and to explore effects of exposure to the progestagen levonorgestrel (LNG). Larvae were exposed to LNG (0, 3, 30, 300 ng/L) over the first three weeks of development, encompassing the beginning of gonadal differentiation. mRNA levels of amh (anti-Müllerian hormone), amhr2 (amh receptor 2), ipgr (intracellular progesterone receptor), mpgr beta (membrane progesterone receptor beta), and cyp19a1 (cytochrome p450 19a1) were quantified in larvae and juveniles (4 weeks post-metamorphosis). Relative cyp19a1 and amh expression was used as a molecular marker for phenotypic sex of larvae. Gonadal and Müllerian duct development were characterised histologically in juveniles. Compared to controls, LNG exposure increased the expression of amh and ipgr in male larvae. In juveniles, mpgr beta expression was increased in both sexes and amhr2 expression was decreased in males, implying persistent effects of developmental progestagen exposure on amh and pgr expression signalling. No effects of LNG on the gonadal or Müllerian duct development were found, implying that the exposure window was not critical with regard to these endpoints. In juveniles, folliculogenesis had initiated and the Müllerian ducts were larger in females than in males. This new knowledge on sexual development in X. tropicalis is useful in the development of early life-stage endpoints for developmental reproductive toxicity.

Early life progestin exposure causes arrested oocyte development, oviductal agenesis and sterility in adult Xenopus tropicalis frogs

Levonorgestrel (LNG) is a commonly used pharmaceutical progestin found in the environment. Information on the long-term toxicity of progestins following early life exposure is scant. We investigated the effects of developmental LNG exposure on sex differentiation, reproductive organ development and fertility in the model frog Xenopus tropicalis. Tadpoles were exposed to 0, 0.06 or 0.5nM LNG via the water from hatching until metamorphosis. At metamorphosis effects on gonadal differentiation were evaluated using a subsample of frogs. Remaining animals were held unexposed for nine months, at which time reproductive organ structure, function and fertility were determined. LNG exposure severely impaired oviduct and ovary development and fertility. All adult females in the 0.5nM group (n=10) completely lacked oviducts. They also displayed a significantly larger fraction of immature oocytes, arrested in meiotic prophase, than control females. Upon mating with unexposed males, only one of 11 LNG-exposed females laid eggs, whereas all control females did. No effects on testicular development, sperm count or male fertility were observed. At metamorphosis, no effects on sex ratio or gonadal histology were evident. The effects on ovarian and oviductal development were detected at adult age but not at metamorphosis, emphasising the importance of investigating the long-term consequences of developmental exposure. This is the first developmental reproductive toxicity study of a progestin in an aquatic vertebrate. Considering that several progestins are present in contaminated surface waters, further investigation into the sensitivity of frogs to progestins is warranted to understand the risk such compounds may pose to wild frog populations.

Effects of embryonic estrogen on differentiation of the gonads and secondary sexual characteristics of male zebra finches

Male zebra finches sing to court females, whereas females do not normally sing. In parallel, the telencephalic brain regions that control song are larger in volume and contain larger cells in males than in females. The vocal control organ (syrinx) is also larger in males. Some evidence suggests that the sexual differentiation of both anatomy and behavior is under the regulation of gonadal hormones during early development, yet recent data conflict with the idea that the sole source of masculinization of the neural song system is the testes. In the present experiment, we treated genetic males with estradiol benzoate on embryonic day 5 and measured the volume of and neuron soma size in robust nucleus of the archistriatum (RA) and the high vocal center (HVC), two telencephalic song control nuclei. We also weighed the syrinx, the muscles of which are the target of the motor pathway containing the two brain regions. The estrogen treatment disrupted testicular morphology, and induced an oviduct in six of seven animals, but it had no effect on any of four measures of masculinization of the neural song system. These results suggest that normal testicular tissue is not required for masculine development of the neural song system.

The effects of norethindrone on the mullerian ducts of the American alligator

Norethindrone, a reported aromatase inhibitor, has been used to examine the role of estrogens in the unilateral regression of the mullerian ducts that occurs in female chick embryos. The mullerian ducts are embryonic oviducts that regress in most male vertebrates under the influence of the testicular hormone, mullerian inhibiting substance (MIS). The ovaries of the chick also produce MIS during early development, but only the right duct regresses. Based on the finding that the left duct contains significantly more estradiol binding sites than the right duct, it has been proposed that the left duct is protected from the effects of MIS by preferentially binding estradiol from the ovaries. In support of this theory, norethindrone (0.1 and 0.5 mg) injected into the airsac of chick eggs results in regression of the left mullerian duct of female embryos, presumably by blocking the synthesis of estradiol. In the present study, it was hypothesized that crocodilians, because of the common ancestry they share with birds, would respond in a similar manner by exhibiting regression of both mullerian ducts in response to norethindrone. However, the application of norethindrone (0.5 mg) to the chorioallantoic membrane of female alligator embryos in ovo resulted in significant hypertrophy of the ducts, indicating that norethindrone had an estrogenic effect in the alligator rather than acting as an aromatase inhibitor. There was no effect of norethindrone on the gonads.