Subdominant hierarchical ovarian follicles are needed for steroidogenesis and ovulation in laying hens (Gallus domesticus)

Ovarian follicle development in avian species is characterized by a strict hierarchical arrangement. The hierarchical follicles secrete progesterone, which induces the LH surge, but the capacity to produce other steroids decreases with development. Our aim was to evaluate the complementary action of subdominant follicles (F4–F6) on ovulation and steroidogenesis of the preovulatory follicles (F1–F3) in domestic laying hens. The first study included four groups: control (C); sham-operated (SO); large hierarchical follicles (LHF) from which F4–F6 follicles were extracted; and subdominant hierarchical follicles (SHF) from which F1–F3 follicles were extracted. Blood samples were collected every 2 h from 12 h before estimated ovoposition until 2 h after ovoposition. Egg laying continued at the same rates in C and SO hens, with normal preovulatory surges of oestradiol, testosterone, progesterone and LH. In contrast, in LHF and SHF groups, ovoposition was blocked; oestradiol concentrations were not affected; but no preovulatory surges of testosterone, progesterone or LH were seen. Further, the testosterone surge was required for the occurrence of progesterone and LH surges. In the second study StAR and steroidogenic enzyme mRNA expression was evaluated within F1–F3 follicles from a LHF group and C-14 and C-8 controls groups, in which follicles were collected 14 h and 8 h before expected ovoposition, respectively. Extraction of F4–F6 follicles caused a significant reduction in StAR and 3β-HSD expressions within theca, but not in granulosa cells. In conclusion, subdominant hierarchical follicles (F4–F6) are required for the preovulatory release of testosterone, progesterone and LH, which are highly inter-correlated.