Vol. 9, Issue 1, Part E (2025)

In hens, the reproductive cycle is governed by a complex interplay of circadian rhythms, hormonal regulation, and follicular development. The ovary exhibits a hierarchical structure, with daily ovulation and oviposition displaying well-characterized rhythms. Studies in chickens and quails reveal that gonadotropins, cell signaling, and the ovarian clock regulate preovulatory follicle development. Clock genes, such as Per2, Cry1, and Bmal1, play crucial roles in this process. The decentralized circadian timing system in birds, involving multiple pacemakers, ensures coordinated rhythmicity. Follicular development and maturation are controlled by follicle-stimulating hormone (FSH) and other factors, with granulosa cells exhibiting daily rhythmicity regulated by the suprachiasmatic nucleus (SCN). Progesterone induction of the pre-ovulatory luteinizing hormone (LH) surge requires estrogen presence. The oviposition-ovulation interval is sequence-length dependent, with shorter sequences exhibiting prolonged intervals. Melatonin, thyroid hormones, and gonadotropin inhibitory hormone (GnIH) also play significant roles in avian reproductive physiology. Melatonin receptors are expressed in ovarian follicles, and their administration increases receptor expression. Thyroid hormones influence egg laying, with decreases in triiodothyronine (T₃) and thyroxine (T₄) before ovulation. GnIH depresses LH and FSH release and expression. Adrenal cortical hormones, such as glucocorticoids, can influence ovulation, with dexamethasone blocking ovulation and premature ovulation induced by deoxycorticosterone, progesterone, and corticosterone. Understanding the intricate relationships between circadian rhythms, hormonal regulation, and follicular development is essential for elucidating the mechanisms governing avian reproductive physiology. Further research is needed to uncover the specific roles of individual pacemakers and hormones in regulating the reproductive cycle.