Hormonal Control of Sexual Reproduction in Humans

Hormonal Control of Sexual Reproduction in Humans

Learning Objectives

  • List the hormones involved in human reproduction and their roles.

  • Explain the main events of the ovarian and uterine cycles.

  • Explain negative and positive feedback mechanisms in the hormonal control of female reproduction.

Glands and Hormones Involved

  • Organs:

    • Gonads (testes & ovaries)

    • Hypothalamus

    • Anterior pituitary gland

  • Hormones:

    • Oestrogen

    • Progesterone

    • LH (Luteinizing Hormone)

    • FSH (Follicle-Stimulating Hormone)

    • Testosterone

Puberty

  • Hypothalamus: Releases GnRH (Gonadotropin-Releasing Hormone).

  • Anterior Pituitary: Releases gonadotrophins (LH & FSH).

  • Pituitary Stalk: Connects the hypothalamus and pituitary gland.

  • Gonads (Ovaries and Testes): LH and FSH act on the gonads, inducing the production of steroid sex hormones (oestrogen and testosterone).

  • Steroid Sex Hormones:

    • Testosterone (Male):

      • Facial hair

      • Voice changes

      • Underarm & pubic hair

      • Maturation of reproductive organs

      • Muscle mass

      • Spermatogenesis

    • Oestrogen & Progesterone (Female):

      • Breasts development

      • Underarm & pubic hair

      • Maturation of reproductive organs

      • Menstruation

      • Ovarian and uterine cycle

Control of Reproductive Hormone Levels

  • Negative Feedback: Steroid sex hormones exert negative feedback control on the hypothalamus and anterior pituitary, usually.

Female Reproductive Cycle

  • Involves a monthly reproductive cycle of events.

  • Lasts approximately 28 days.

  • Two distinct but related cycles:

    • Ovarian cycle

    • Uterine cycle

  • Ovarian Cycle Control: LH & FSH (pituitary hormones).

  • Uterine Cycle Control: Oestrogen & progesterone (produced by the ovary).

Ovarian Cycle

  • Ovaries: Almond-shaped organs containing follicles.

  • Follicles: Sack-like structures, each containing one primary oocyte.

  • Primary Oocyte: Formed in foetus before birth, halted in prophase I.

  • After Puberty: One follicle matures each month and releases a secondary oocyte.

  • Maturation: As the follicle matures, it produces oestrogen, which halts the maturation of other follicles.

  • Corpus Luteum: After release of the secondary oocyte, the follicle turns into a yellow endocrine structure.

  • Persistence: The corpus luteum persists in the ovary until the following month and secretes hormones.

Phases of the Ovarian Cycle

  • Follicular Phase (Day 1-13):

    • Growth and maturation of one or more follicles (primary oocyte → secondary oocyte).

  • Ovulation Phase (Day 14):

    • Release of the secondary oocyte from the ovary.

  • Luteal Phase (Day 15-28):

    • Development of the corpus luteum from the remaining follicle cells.

Uterine Cycle

  • Closely linked to the ovarian cycle.

  • Consists of 3 phases:

    • Menses (Menstruation) (Days 1-5):

      • The uterine lining is shed off.

    • Proliferative Phase (Days 6-14):

      • Uterine lining starts to grow again in preparation for implantation.

    • Secretory Phase (Days 15-28):

      • Uterine lining further prepares for pregnancy.

      • Development of nutrient glands and a rich vasculature.

Hormonal Control of Ovarian Cycle

  • GnRH: Released by the hypothalamus.

  • LH & FSH: Released by the anterior pituitary.

  • Oestrogen: Produced by the growing follicle.

  • Inhibin: Levels rise with oestrogen.

  • Negative Feedback: At low oestrogen levels (days 1-12), inhibits further release of FSH/LH.

  • Positive Feedback: At high levels of oestrogen (days 12-13), causes a surge of LH to stimulate ovulation (day 14).

Ovarian Cycle: Luteal Phase

  • Corpus Luteum: Produces oestrogen and high levels of progesterone after ovulation.

  • Negative Feedback: Progesterone and oestrogen inhibit further release of FSH and LH.

  • Inhibition of Follicle Growth: This stops other primary follicles from starting to grow and mature.

  • Corpus Luteum Degeneration: If no fertilization, the corpus luteum degenerates and stops producing oestrogen and progesterone.

Hormonal Control of the Uterine Cycle

  • Hormones secreted by the ovary during the ovarian cycle regulate the uterine cycle.

Proliferative Phase

  • Rising levels of oestrogen from the developing follicle.

  • Proliferation & thickening of the uterine wall (endometrium).

Secretory Phase

  • Progesterone from the corpus luteum.

  • Stimulates further growth of the endometrium and its blood supply and development of nutrient glands.

  • Uterus prepares for implantation.

Menses

  • If no fertilization, degeneration of corpus luteum occurs.

  • Decreasing progesterone/oestrogen levels.

  • Endometrium is no longer maintained and is shed (menstruation).

The Ovarian Cycle - Summary

  • Gonadotropins (from anterior pituitary):

    • Follicle-Stimulating Hormone (FSH)

    • Luteinizing Hormone (LH)

  • Ovarian Hormones:

    • Estrogen

    • Progesterone

Fertilization

  • If fertilization takes place:

    • The dividing cells (blastocyte) and then the developing placenta secrete Human Chorionic Gonadotrophin (hCG).

    • hCG maintains the Corpus Luteum and ovarian hormones, progesterone and oestrogen, continue to be produced.

  • Progesterone Role:

    • Maintains growth of the endometrium and its blood supply (no menstruation).

    • Stimulates development of nutrient fluid glands in the endometrium.

  • Placenta Takes Over: After 3 months, the Corpus Luteum degenerates, and oestrogen and progesterone are secreted by the placenta instead.

    • Maintains endometrium throughout pregnancy.

    • Inhibits FSH preventing further development of ovarian follicles.

Oral Contraceptives

  • Oral contraceptives (and implants) contain progesterone and/or oestrogen.

  • Prevent pregnancy by preventing the release of LH and FSH (due to negative feedback), therefore preventing the maturation of follicles into oocytes.

Summary

  • The hypothalamus and pituitary gland play a key role in the regulation of reproductive events.

  • Hormone levels are generally regulated through negative feedback mechanisms.

  • Positive feedback is essential for ovulation to occur.

  • Monthly hormonal changes regulate the ovarian & uterine cycles.