Hormones and the Human Reproductive System

CHAPTER 2: Reproduction

Hormones Released by the Pituitary Gland

  • The anterior pituitary gland produces the following hormones:
      - TSH (Thyroid-Stimulating Hormone / Thyrotropin):
         - Regulates thyroid gland's endocrine functions.
      - ACTH (Adrenocorticotropic Hormone / Corticotropin):
         - Controls adrenal cortex development and functioning, influencing glucocorticoids and androgen secretion (Venes, 2021).
      - PRL (Prolactin):
         - Stimulates mammary gland development during pregnancy, initiates milk production, inhibits FSH and LH stimulation.
      - GH (Growth Hormone / Somatotropin):
         - Promotes growth and cellular reproduction, impacts height during childhood.
         - Responsible for:
           - Increased muscle mass
           - Calcium retention
           - Bone mineralization
           - Organ system growth
           - Protein synthesis
           - Stimulation of the immune system
           - Reduced glucose uptake in the liver
           - Promotion of lipolysis.
      - Gonadotropins (FSH and LH):
         - Stimulate ovaries and regulate the menstrual cycle through positive and negative feedback of estrogen and progesterone.
  • The posterior pituitary gland releases oxytocin:
      - Stimulates uterine contractions and milk release during lactation.

Areola and Montgomery Tubercules

  • Areola:
      - Pigmented area surrounding the nipple's diameter ranges from 1 to 3.9 inches (2.5 to 10 cm).
  • Montgomery Tubercules:
      - Papillae on the nipple and areola's surface that secrete a fatty substance for lubrication and protection during breastfeeding.

The Interplay of Hormones and Reproduction

  • Understanding the key reproductive hormones is crucial for grasping the female menstrual cycle.

Hormones Released by the Hypothalamus

  • The hypothalamus releases “releasing factors” that stimulate the anterior pituitary:
      - Gonadotropin-Releasing Hormone (GnRH):
         - Stimulates release of FSH and LH.
         - Involved in follicular growth, ovulation, and corpus luteum maintenance.
      - Corticotropin-Releasing Hormone (CRH):
         - Regulates ACTH secretion, activated stress response and facilitates maternal immunoprotection during early pregnancy.
      - Growth Hormone-Releasing Hormone (GH-RH):
         - Stimulates production and release of GH.
      - Growth Hormone-Inhibiting Hormone (GH-IH / Somatostatin):
         - Inhibits GH release.
      - Thyrotropin-Releasing Hormone (TRH):
         - Stimulates the anterior pituitary to release TSH and PRL.
      - Prolactin-Inhibiting Factor (PIF / Prolactostatin):
         - Inhibits PRL synthesis and release (dopamine also plays a role).

Hormones Released by the Gonads

  • Gonadal Hormones: Estrogen, progesterone, and testosterone.
      - Estrogen:
         - Promotes sexual characteristics in females (breast development, fat distribution).
         - Regulates menstrual cycle and endometrial proliferation.
      - Progesterone:
         - Prepares the uterus for fertilization and maintains pregnancy.
         - Decreases uterine motility triggered by estrogen.
      - Testosterone:
         - Enhances libido, increases energy, and contributes to secondary sexual characteristics in males.

Hormones of the Menstrual Cycle

  • The menstrual cycle is controlled hormonally through the actions of the hypothalamus, anterior pituitary gland, and ovaries:
      - FSH: Stimulates graafian follicle growth, secretes estrogen.
      - Estrogen: Stimulates endometrial proliferation.
      - LH: Releases ovum and stimulates corpus luteum development.
      - Progesterone: Prepares the uterine lining for implantation.

Menstrual Cycle Overview

  • Menstruation is the discharge of blood from the vagina, occurring in monthly cycles (28 to 32 days on average).
  • The cycle is affected by stress, exercise, and health.
  • Essential hormonal feedback regulates the menstrual cycle's subtleties with interaction among endometrial, hypothalamic-pituitary, and ovarian cycles.
Key Phases of the Menstrual Cycle
  1. Menstrual Phase (days 1 to 6):
       - Marked by bleeding due to decreased estrogen and progesterone.
  2. Proliferative Phase (days 7 to 14):
       - Endometrium thickens under estrogen influence, preparing for ovulation.
  3. Secretory Phase (days 15 to 26):
       - Changes under progesterone prepare for potential ovum implantation.
  4. Ischemic Phase (days 27 to 28):
       - Drop in hormone levels leads to menstruation.
Hormones of Female Reproduction
  • See Table 2-1 for hormone functions regarding the menstrual cycle:
      - FSH: Initiates follicle development and estrogen secretion.
      - LH: Triggers ovulation and strengthens corpus luteum.
      - Estrogen: Matures follicles and facilitates secondary sex characteristics.
      - Progesterone: Aids in implantation; inhibits contraction.
      - Inhibin: Decreases FSH secretion.
      - Relaxin: Helps with implantation and uterine relaxation during pregnancy.

Sexual Maturation: Puberty

  • Puberty: Marks transition from childhood to adulthood with body changes leading to sexual maturity.
  • Initiated by GnRH production in the hypothalamus, stimulating FSH and LH production.
  • Thelarche: Breast bud development, starting around age 13.
  • Major hormonal events like menarche are triggered by FSH, stimulating follicle and ovum maturation.
  • In males, LH prompts testosterone production and stimulates spermatogenesis.
Factors Influencing Timing of Puberty
  • Genetics, environment, exposure to light, nutritional status, and general health influence the timing and process of puberty.
Female Body Changes During Puberty (Table 2-2)
  • Changes include:
      - Growth Spurts: Height increases of 2.4 - 4.3 inches (6 - 11 cm).
      - Thelarche: Breast development.
      - Menarche: First menstrual period.

Uterine (Endometrial) Cycle

  1. Menstrual Phase: Vaginal bleeding occurs from days 1-6 due to estrogen and progesterone decline.
  2. Proliferative Phase: Days 7-14, endometrial lining increases in response to rising estrogen.
  3. Secretory Phase: Post-ovulation (days 15-26), uterine endometrium becomes suitable for implantation.
  4. Ischemic Phase: Days 27-28, low hormone levels cause the endometrium to eventually shed.

Climacteric Phase

  • Defined as the decline in ovarian function leading to decreased hormone production, often followed by menopause.
  • Perimenopause: Transition phase with reduced ovarian responsiveness leading to irregular cycles.
Menopause
  • Denotes the last menstrual cycle, typically occurs between 40 to 58 years of age, marked by complete cessation of menstruation.
Postmenopausal Phase
  • Estrogen production shifts exclusively to adrenal glands, raising some health risks associated with decreased hormone levels.
  • Hormone therapy (HT) may aid in symptom relief but must be prescribed with caution.

Menstrual Disorders

  • Amenorrhea: Absence of menstruation, with possible primary and secondary types; could indicate underlying health issues.
  • Dysmenorrhea: Painful menstruation due to myometrial contractions induced by prostaglandins. It can vary in severity and duration.
  • PMS (Premenstrual Syndrome): Symptoms related to hormonal fluctuations, which may include emotional changes and physical symptoms.

Male Reproductive System

External Structures
  • Perineum: Diamond-shaped area between the pubic symphysis and the coccyx, including the scrotum and penis.
  • Penis: Contains erectile tissue, urethra, and serves as reproductive and urinary conduit. Circumcision is the surgical removal of foreskin covering the glans.
Hormone Functions in Male Sexual Maturation
  • Testosterone: Drives the development of male sexual characteristics, stimulating libido and secondary traits (muscle growth, body hair, etc.).