Endocrine System III: Reproductive Hormones

Endocrine System and Reproductive Hormones

Sex Determination

• Females are XX, and males are XY.
• For about six weeks after fertilization, the gonads are the same in both males and females.
• These undifferentiated gonads consist of:
  • Germ cells
  • Supporting cells
  • Steroidogenic cells

SRY Gene

• The SRY gene is a sex-determining region on the Y chromosome.
• Around week 6, the SRY gene in males causes:
  • Supporting cells to differentiate into Sertoli cells.
  • Steroidogenic cells to differentiate into Leydig cells, which form testes that become microscopically identifiable and begin producing hormones by week 8.
  • Germ cells to become spermatogonia.
• Without the SRY gene, ovaries form during months 2-6, and germ cells become ovarian follicles.

Gonadal Differentiation

• The presence of the SRY gene stimulates the production of proteins that cause indifferent gonads to develop into testes.
• In females, the absence of the SRY region results in indifferent gonads developing into ovaries.

Phenotypic Sex Determination

• After gonads develop, the hormones they produce regulate the formation of sex organs.
• In the testes:
  • Sertoli cells secrete Mullerian Inhibiting Substance (MIS).
  • Leydig cells secrete testosterone (T).

Hormone Effects on Development

• Testes:
  • MIS causes the Mullerian duct to degenerate.
  • Testosterone stimulates the Wolffian ducts to develop into the epididymis and vas deferens.
  • Testosterone stimulates the urogenital sinus to develop into the prostate, penis, and scrotum.
• Ovaries (no testosterone or MIS):
  • The Mullerian duct develops into the uterus and fallopian tubes.
  • The Wolffian ducts degenerate.
  • The urogenital sinus develops into the vagina, labia, and clitoris.

Sex Hormones and Sexual Characteristics

• Sex hormones are responsible for primary and secondary sexual characteristics.
• Primary characteristics: Growth and development of the gonads.
• Secondary characteristics:
  • Male: facial and body hair, deepening of voice, maturation of testes and penis, strengthening of muscles.
  • Female: body hair, breasts, widening of pelvis, fat deposits.

Common Sex Hormones in Males and Females

• Gonadotropin-releasing hormone (GnRH):
  • Secreted by the hypothalamus.
  • Stimulates the pituitary to release FSH and LH.
• Follicle-stimulating hormone (FSH):
  • Secreted by the anterior pituitary.
  • Stimulates production of sperm (\male) and stimulates the ovaries to secrete estrogen (\female).
• Luteinizing hormone (LH):
  • Secreted by the anterior pituitary.
  • Stimulates sex organs to secrete testosterone (\male) and progesterone and some estrogen (\female).

Male Reproductive Anatomy

• Testes:
  • Paired organ suspended in the scrotum.
  • Produces hormones and sperm.
  • Optimal temperature for spermatogenesis is a few degrees lower than body temperature.
• Penis
• Glands: Produce semen, which nourishes the sperm and buffers to neutralize vaginal acidity.

Male Sex Hormones

• Testosterone:
  • Stimulates secondary sexual characteristics in males and the production of sperm.
  • Regulates the production of sperm and itself via a negative feedback loop.

Testosterone/LH Feedback Loop

• When sperm production decreases, the brain signals the hypothalamus to secrete GnRH.
• GnRH stimulates the secretion of LH from the anterior pituitary, which triggers an increase in testosterone production in the testes, increasing the sperm count.
• When testosterone levels are very high, testosterone stimulates the hypothalamus to stop producing GnRH (self-regulating).

Male FSH Feedback Loop

• FSH stimulates the production of sperm in males by acting on the Sertoli cells (in seminiferous tubules).
• When FSH levels are high, Sertoli cells secrete inhibin.
• Inhibin stimulates the anterior pituitary to stop secreting FSH and the hypothalamus to stop secreting GnRH.

Male Feedback Systems

• Hypothalamus releases GnRH, stimulating the anterior pituitary.
• Anterior pituitary releases FSH and LH.
• FSH acts on Sertoli cells to promote spermatogenesis; inhibin provides negative feedback.
• LH acts on Leydig cells to produce testosterone, promoting secondary sex characteristics; testosterone provides negative feedback.

Key Differences Between LH and FSH in Males

• LH (Luteinizing Hormone):
  • Primary Role: Stimulates the production of testosterone.
  • Target: Leydig cells in the testes.
  • Function: LH signals Leydig cells to produce testosterone, which is crucial for sperm production, the development of secondary sexual characteristics, and maintaining libido.
• FSH (Follicle-Stimulating Hormone):
  • Primary Role: Supports sperm production (spermatogenesis).
  • Target: Sertoli cells in the testes.
  • Function: FSH stimulates Sertoli cells to nourish and support developing sperm cells. It also enhances the binding of testosterone in the testes, which is necessary for spermatogenesis.
• Both hormones work together: FSH initiates and supports the environment for sperm development, while LH ensures the production of sufficient testosterone, which is essential for the process.

Female Reproductive System

• Ovaries contain connective tissue and small groups of cells called follicles.
• Ovulation: Release of the egg from the follicle held within the ovary.
• Corpus luteum: Mass of follicle cells that forms within the ovary after ovulation.

Female Sex Hormones

• Estrogen:
  • Secreted by the ovary.
  • Activates female secondary sex characteristics and increases the thickness of the endometrium (lining of the uterus where the embryo implants).
• Progesterone:
  • Produced by the ovary.
  • Inhibits ovulation, inhibits uterine contractions, stimulates the endometrium to thicken.

Female Menstrual Cycle

• A 28-day cycle divided into 4 phases:
  • Menstrual Phase
  • Preovulatory Phase
  • Ovulation Phase
  • Postovulatory Phase
• Each phase is characterized by different hormone levels.

Menstrual Phase (Days 1-5)

• Also called menstruation or menses.
• Events in the Ovaries: ~20 follicles in the ovaries begin to enlarge.
• Events in the Uterus: 50-150 mL of blood, tissue fluid, mucus, and epithelial cells (endometrium) are discharged due to a decline in progesterone levels, causing arterioles in the uterus to contract, leading to oxygen deprivation and cell death in the endometrium.

Preovulatory Phase (Days 6-13)

• The time between menstruation and ovulation; the most variable phase in terms of length.
• Also called the follicular phase.
• Events in the Uterus: Estrogen causes the uterine lining (endometrium) to thicken.
• Events in the Ovaries: FSH continues to stimulate the growth of follicles, which begin to secrete estrogen. One follicle outgrows the rest, and the estrogen from it inhibits FSH secretion, stopping the development of the other follicles. LH stimulates the mature follicle to continue secreting estrogen.

Ovulation (Day 14)

• The mature follicle ruptures, releasing the egg (oocyte).
• High levels of estrogen at the end of the preovulatory phase exert a positive feedback effect on both LH and GnRH.
• After ovulation, the mature follicle collapses to form the corpus luteum.
• LH causes the corpus luteum to secrete estrogen and progesterone.

Postovulatory Phase (Days 15-28)

• Also called the luteal phase.
• Events in the Ovary: LH stimulates the corpus luteum to secrete increasing levels of progesterone and estrogen.
• Events in the Uterus: Progesterone and estrogen stimulate the thickening of the endometrium. If fertilization does not occur, the level of progesterone declines due to degeneration of the corpus luteum, and menstruation occurs. The corpus luteum has a 2-week lifespan As it degenerates, progesterone and estrogen levels decrease, causing GnRH, FSH, and LH levels to increase.

Exercise & Menstrual Cycle

1. MENSTRUATION: Training during your period can ease cramps and make you feel better as hormone levels have dropped and you may feel strong.
2. FOLLICULAR PHASE: Lifts increase alongside your energy; best for muscle building and recovery.
3. OVULATION: Feeling super strong and energetic as testosterone levels elevate with LH.
4. LUTEAL PHASE: You may feel pretty similar to pre-ovulation at the start but a little more bloated as digestion can slow thanks to progesterone. Right before the period, you may feel weak as progesterone rises and energy lowers.

Contraceptives

• All methods of contraception target one of three essential steps in reproduction:
  • Ovulation
  • Sperm and ovum transport
  • Implantation of the embryo in the uterus
• Hormonal contraceptives work by altering the normal hormone levels of the menstrual cycle.

Oral Contraceptives

• Combination of estrogen and progesterone.
• Prevents follicle development, LH surge, and ovulation.
• 97\% effective.
• Risks: increased risk of breast cancer and blood clots.
• Benefits: decreased risk of ovarian and uterine cancers.