Reproductive Processes and The HPG Axis

Reproductive Processes

Overview of the HPG Axis

• Function: Regulates production of gametes and sex hormones.
• Hormones Involved:
  • GnRH (Gonadotropin-releasing hormone): Secreted by the hypothalamus.
  • Gonadotropins (FSH & LH): Released by the anterior pituitary; essential for gonad activity.
  • Feedback Mechanism: Gonads produce hormones (testosterone in males, estrogen/progesterone in females).

Hormonal Regulation in Males

• FSH (Follicle-stimulating hormone): Stimulates nurse cells (sustentocytes) to (1) secrete androgen-binding protein (ABP) for maintaining testosterone levels and (2) release inhibin to suppress spermatogenesis.
• LH (Luteinizing hormone): Stimulates testosterone production in interstitial endocrine cells.
• Testosterone: Regulates spermatogenesis and suppresses GnRH secretion with high levels.

Hormonal Regulation in Females

• GnRH secretion increases during late childhood, stimulating FSH and LH.
• Ovarian follicles release estrogen, progesterone, and inhibin, requiring sufficient body fat (indicated by leptin levels) for activation.

Puberty and Development

• Puberty: Initiated by a surge in GnRH, leading to the first viable sperm production or first menstrual cycle.
• Adolescence: The period from puberty to the attainment of adult height.
  • Males: Development of secondary sexual characteristics (e.g., hair growth, increased muscle mass).
  • Females: Development includes breast growth (thelarche), pubic hair (pubarche), and menarche (first menstruation).

The Male Sexual Cycle

• Erection: Involves the filling of erectile tissues with blood, facilitated by nitric oxide, leading to vascular relaxation and constriction of drainage veins.
• Ejaculation: Consists of two phases:
  • Emission: Movement of semen from reproductive tract.
  • Expulsion: Muscles contract to expel semen, preventing urinary and semen backflow.
• Resolution: End of sexual excitement, involving constriction of blood vessels and drainage of blood from erectile tissue.

The Female Sexual Cycle

• Monthly Cycle: Typically lasts around 28 days, regulated by the HPG axis, includes:
  • Ovarian Cycle (2 phases):
  • Follicular Phase (Day 1-14): Development of follicles, leading to ovulation; peak in LH and estrogen.
  • Luteal Phase (Day 16-28): After ovulation, corpus luteum forms, releasing hormones and inhibiting further cycle initiation until regression into corpus albicans.
  • Menstrual Cycle (4 phases):
  • Proliferative Phase (Day 6-14): Endometrial thickening due to estrogen.
  • Secretory Phase (Day 15-26): Endometrial preparation for potential implantation, stimulated by progesterone.
  • Premenstrual Phase (Day 27-28): Decline in hormones leads to degeneration of the endometric lining.
  • Menstrual Phase (Day 1-5): Shedding of the functional layer of the endometrium, with bleeding.

Endocrine-related Health Issues

• Hormonal Birth Control:
  • Types: Progesterone-only (e.g., minipill) and combined estrogen + progesterone (e.g., Yaz).
  • Mechanisms: Prevent ovulation, thicken cervical mucus, and thin endometrium to prevent implantation.
• Endometriosis: Characterized by endometrial growth outside the uterus, causing pain and bleeding.
  • Management options include medication, hormone therapy, or surgery (e.g., hysterectomy in severe cases).