Abstract
The hypothalamic-pituitary-ovarian (HPO) axis is the key regulator of female reproductive functions. HPO axis integrates internal and external signals to coordinate reproductive functions through the neuroendocrine network.
Gonadotropin-releasing hormone (GnRH) plays a central role in the HPO axis to control pituitary gonadotropin secretion and ovarian steroidogenesis. The ovarian steroids, particularly estradiol regulate GnRH secretion through the negative feedback mechanism. At the initiation of puberty, the negative feedback of estradiol decreases, leading to activation of the GnRH surge centre and commencement of the estrous cycle, growth of the follicles, and ovulation. The estrous cycle divides into four distinct stages: estrus, proestrus, metestrus, and diestrus. The follicles’ development started during the foetal life and continued through a dynamic process during various phases of post-natal periods called folliculogenesis. Rhythmic alterations of follicular dynamics occur during each estrous cycle after puberty. The maturation of the female gamete, the ovum, is completed after puberty which is subsequently released from the matured or Graafian follicles at the end of estrus, followed by the formation of corpus luteum (CL). The metestrus and dies- trus stage is under the control of progesterone secreted from CL. The CL undergo lysis if the animals fail to conceive by prostaglandin F2α (PGF2α) secreted from the endometrium. Successful conception prevents the release of PGF2α, and the CL is sustained throughout the pregnancy. The reproductive cycles of females are species specific and are controlled by several factors like environmental cues, photoperiod, nutritional state, stress, and diseases. This chapter encompasses the integrated neuroendocrine and molecular mechanisms involved in different aspects of female reproduction, such as initiation of puberty, ovarian steroidogenesis, oogenesis, and folliculogenesis, including ovulation in domestic, wild animals, and birds.P. K.
Das (X) ∙ J. Mukherjee ∙ D. BanerjeeDepartment of Veterinary Physiology, West Bengal University of
Animal & Fishery Sciences, Kolkata, West Bengal, India
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023
P. K. Das et al. (eds.), Textbook of Veterinary Physiology, https://doi.org/10.1007/978-981-19-9410-4_22
Graphical Abstract
Description of the graphic: All the events of female reproduction control by the Hypothalamic-Pituitary-Ovarian (HPO) axis (1). The oogenesis and folliculogenesis (2) initiate during foetal life and undergo rhythmic alterations during each estrus cycle to become dominant follicles through follicular waves (3). The matured follicle undergoes ovulation and the formation of the corpus luteum (4). The activation of the HPO axis leads to the attainment of puberty (5), controls ovarian steroidogenesis (6), rhythmic sexual behavioural patterns, the estrous cycle (7), and altered endocrine milieu (8)
Keywords
HPO axis ∙ Puberty and estrous cycle ∙ Oogenesis and folliculogenesis ∙ Ovulation in mammals and birds ∙ Corpus luteum
Learning Objectives
• Role of various hormones, growth factors, and cytokines in female reproduction.
• The neuroendocrine control of puberty and the estrous cycle.
• Ovarian gametogenesis and steroidogenesis, including its endocrine and molecular mechanisms.
• Follicular dynamics, ovulation, and corpus luteum formation and regression.
• Ovulation and egg formation in birds.
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