LAUSR

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.