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.