In-Depth Notes on Reproductive Endocrinology and the Menstrual Cycle

Reproductive Endocrinology and the Menstrual Cycle

Overview of Reproductive Endocrinology

• Reproductive endocrinology studies the hormones involved in reproduction and reproductive functions.

• Hormones play a crucial role from birth through maturation until conception and beyond.

Types of Hormones

• Steroid Hormones:

  • Examples: testosterone, estrogen, progesterone.

  • Derived from cholesterol, lipid-soluble; can cross cell membranes.

• Glycoprotein Hormones:

  • Examples: LH, FSH, hCG, inhibin, activin, relaxin.

  • Typically larger molecules with sugar chain modifications, often influence gonadal function.

• Peptide Hormones:

  • Examples: GnRH, oxytocin, prolactin.

  • Smaller hormones regulating various reproductive functions.

The Hypothalamus-Pituitary-Gonad (HPG) Axis

• The HPG axis is essential for controlling the reproductive system through hormonal signaling.

• Begins with GnRH release from the hypothalamus, regulating LH and FSH release from the anterior pituitary.

• LH and FSH act on gonads (testes in males and ovaries in females) to stimulate hormone production and gametogenesis.

Menstrual Cycle

• The menstrual cycle involves cyclical changes occurring approximately every 28 days, consisting of three main stages:

  1. Menstrual Phase (Days 1-5):

     • Shedding of the endometrium.

     • Hormonal levels decrease (estrogen, progesterone).

  2. Proliferative Phase (Days 6-14):

     • Estrogen levels rise, stimulating endometrial thickening and follicle maturation.

     • Estrogen induces the release of LH, leading to ovulation.

  3. Secretory Phase (Days 15-28):

     • Corpus luteum produces progesterone and estradiol, maintaining the endometrium for potential implantation.

     • If no implantation occurs, the corpus luteum degenerates, leading to the menstrual phase.

Functions of Steroid Hormones

• Estrogens:

  • Involved in bone growth, regulating metabolism, and female secondary sexual characteristics.

• Progesterone:

  • Helps prepare the endometrium during the menstrual cycle, modulates immune responses during pregnancy.

• Androgens (Testosterone, DHT):

  • Essential for male reproductive function, influence muscle growth, red blood cell production, and sexual desire.

Hormonal Regulation in Males

• GnRH stimulates the anterior pituitary to release LH and FSH.

• LH prompts testosterone production in Leydig cells; FSH aids spermatogenesis in Sertoli cells via androgen-binding protein.

• Negative feedback mechanisms via testosterone and inhibin regulate further GnRH, LH, and FSH secretion.

Hormonal Regulation in Females

• The HPG axis orchestrates the menstrual and reproductive cycles.

• Increased estrogen during puberty leads to secondary sexual characteristics and regulates the menstrual cycle's feedback mechanisms.

• Positive feedback from high estrogen leads to LH surges causing ovulation, while progesterone maintains endometrial preparation for implantation.

Conception and Hormonal Control

• If fertilization occurs, the blastocyst secretes hCG, maintaining the corpus luteum and preserving high hormone levels to support early pregnancy.

Summary of the Menstrual Cycle

• Stages:

  • Menstrual Phase (Days 1-5): Low hormones.

  • Proliferative Phase (Days 6-14): Estrogen peaks before ovulation.

  • Secretory Phase (Days 15-28): Corpus luteum produces hormones for implantation; if not fertilized, the cycle restarts.