Effect of estrogen on chick primordial follicle development and activation

Steroid hormones and their receptors play pivotal roles in avian reproduction and development. 17β-estradiol (E₂ ) is an essential mediator in ovarian development. In this study, Day 3 (D3) chicks were injected with E₂ (0.025, 0.25 or 2.5 mg/kg body weight) for 4 days to assess the effects of estrogen on avian follicle development. Morphological analysis showed that treatment of E₂ at 2.5 mg/kg increased the number of growing follicles by 28. 5% and activated follicles by 8.9%, compared with the group. Treatment of E₂ also led to increased diameter and area of three kinds of follicles and oocytes, respectively. However, this stimulating effect of E₂ was inhibited by combined treatment of the estrogen receptor antagonist tamoxifen, resulting in decreased number of the growing follicles by 14.3% and the activated follicles by 5.8%, respectively. Immunohistochemical staining of the proliferating cell nuclear antigen (PCNA) revealed that the ratios of the PCNA-positive granulosa and interstitial cells were increased by 14.0% and 21.5%, respectively. Western blot analysis of PCNA confirmed this stimulating effect of E₂ . Expression of ERα mRNAs was elevated by administration of E₂ in vivo and in vitro. Furthermore, E- and N-cadherin displayed increased expression after E₂ treatment in vivo and in vitro. In conclusion, E₂ can promote chicken primordial follicle development and activation involving its increased receptor and cadherin production at the early stage of ovarian development.