Reproductive Processes and the HPG Axis Notes

Overview of the HPG Axis

Function: Regulates the intricate processes of gamete production and the secretion of sex hormones, which are crucial for reproductive health and function.

Hormones Involved:

• GnRH (Gonadotropin-releasing hormone): Secreted by the hypothalamus in a pulsatile manner, playing a pivotal role in controlling the release of gonadotropins.

• Gonadotropins:

  • FSH (Follicle-stimulating hormone): Released by the anterior pituitary and critical for the stimulation of follicle growth in females and spermatogenesis in males.

  • LH (Luteinizing hormone): Also released by the anterior pituitary; it triggers ovulation in females and stimulates testosterone production in males, essential for maintaining reproductive function.

Feedback Mechanism: The gonads (testes in males, ovaries in females) produce sex hormones (testosterone in males and estrogen/progesterone in females) that exert negative feedback on both the hypothalamus and pituitary gland, modulating the levels of GnRH, FSH, and LH based on the body’s hormonal status.

Hormonal Regulation in Males

• FSH (Follicle-stimulating hormone):

  • Stimulates nurse cells (sustentocytes) to:

  • Secrete androgen-binding protein (ABP), which is vital for maintaining optimal levels of testosterone in the seminiferous tubules, thus supporting spermatogenesis.

  • Release inhibin, which plays a significant role in suppressing spermatogenesis by providing feedback inhibition on FSH release from the pituitary gland.

• LH (Luteinizing hormone):

  • Stimulates interstitial endocrine cells (Leydig cells) in the testes to produce and release testosterone, a key hormone influencing male reproductive health.

• Testosterone:

  • Essential for the regulation of spermatogenesis and the development of male secondary sexual characteristics.

  • High levels of testosterone exert a negative feedback effect by inhibiting GnRH secretion, thereby regulating the secretion of FSH and LH.

Hormonal Regulation in Females

• GnRH:

  • Secretion begins to increase during late childhood and adolescence, driving the release of FSH and LH, which are critical for ovarian function.

• Ovarian Follicles:

  • These structures release estrogen and progesterone, which are essential for maintaining the menstrual cycle and overall reproductive health.

  • Activation of follicles requires sufficient body fat and is indicated by leptin levels, which signals energy availability for reproductive function.

Puberty and Development

• Puberty:

  • Initiated by a surge of GnRH, resulting in the production of viable sperm in males and the onset of the first menstrual cycle in females.

• Adolescence:

  • The period extending from the onset of puberty until the attainment of adult height and sexual maturity.

• Males:

  • Development of secondary sexual characteristics, including increased muscle mass, deepening of voice, and facial hair growth.

• Females:

  • Development includes:

  • Breast growth (thelarche): Induced by estrogen, signaling the onset of female maturation.

  • Pubic hair development (pubarche): Associated with the effects of adrenal androgens.

  • Menarche: The first occurrence of menstruation, indicating reproductive capability.

The Male Sexual Cycle

• Erection:

  • Involves the engorgement of erectile tissues with blood, facilitated by the release of nitric oxide, which leads to vascular relaxation and the constriction of drainage veins, allowing for the buildup necessary for an erection to occur.

• Ejaculation: Consists of two phases:

  • Emission: The movement of semen from the reproductive tract towards the urethra in preparation for ejaculation.

  • Expulsion: Muscular contractions of the pelvic floor lead to the expulsion of semen, while preventing urinary backflow to ensure proper ejaculation.

• Resolution:

  • The final phase where the body returns to its pre-excitement state, involving the constriction of blood vessels and drainage of blood away from erectile tissue, leading to detumescence.

The Female Sexual Cycle

• Monthly Cycle: Typically lasts around 28 days and is regulated by the HPG axis, encompassing both the ovarian and menstrual cycles:

1. Ovarian Cycle (2 phases):

   • Follicular Phase (Day 1-14):

     • Involves the development of ovarian follicles, culminating in ovulation; characterized by a peak in LH and estrogen levels.

   • Luteal Phase (Day 15-28):

     • After ovulation, the corpus luteum forms and secretes hormones (estrogen, progesterone) which inhibit further cycle initiation until regression into corpus albicans occurs.

2. Menstrual Cycle (4 phases):

   • Proliferative Phase (Day 6-14):

     • The endometrium thickens in response to rising estrogen levels, preparing for potential implantation.

   • Secretory Phase (Day 15-26):

     • The endometrium undergoes further changes in preparation for a fertilized egg, stimulated primarily by progesterone.

   • Premenstrual Phase (Day 27-28):

     • A decline in hormone levels leads to the degeneration of the endometrial lining, resulting in menstruation.

   • Menstrual Phase (Day 1-5):

     • Shedding of the functional layer of the endometrium accompanied by bleeding, marking the beginning of a new cycle.

Endocrine-related Health Issues

• Hormonal Birth Control:

  • Types:

  • Progesterone-only: (e.g., minipill) and Combined oral contraceptives: containing both estrogen and progesterone (e.g., Yaz).

  • Mechanisms of Action:

  • Prevent ovulation by maintaining consistent hormone levels, thicken cervical mucus to impede sperm passage, and thin the endometrium to prevent implantation should fertilization occur.

• Endometriosis:

  • Characterized by endometrial tissue growth outside the uterine cavity, leading to symptoms such as chronic pain, heavy menstrual bleeding, and fertility problems.

  • Management options include pain relief medications, hormonal therapies to suppress endometrial growth, and surgical interventions (e.g., hysterectomy) in severe cases.