LG 1

Physiology of Normal Menstrual Cycle

Course Information

  • Medical Physiology Unit
    • Medical Physiology and Microbiology Department
    • College of Medicine / HMU 2025 - 2026 UG3

Learning Outcomes

  • Define the menstrual cycle.
  • Describe the phases of menstruation
  • Explain the hormonal regulation of menstrual cycle

Introduction

  • The reproductive functions in women exhibit cyclical changes occurring regularly over about one month.
  • This cyclical change is called the menstrual cycle, which is the periodic preparation of the reproductive system for fertilization and implantation of the fertilized ovum.
  • The first menstrual cycle is referred to as menarche; the usual age for menarche is 12–14 years.
  • The menstrual cycle ceases at the age of 45 to 50 years, termed menopause.

Normal Menstrual Cycle

  • Mean Duration:
    • Average duration of the menstrual cycle (MC) is 28 days.
    • Range varies from 21 to 35 days.
  • Duration of Menses:
    • Average duration of menses is 3-8 days.
  • Estimated Blood Loss:
    • Normal estimated blood loss is approximately 50–150 ml.
  • Ovulation Occurrence:
    • Ovulation typically occurs on day 14 of the cycle.
  • Contents of Menstrual Blood:
    • Menstrual blood contains blood, tissue fluid, mucus, and epithelial cells.

Variability in the Menstrual Cycle

  • The length of the menstrual cycle usually averages at about 28 days, but the duration can vary among women and also in a single woman over time.
  • Influences on the menstrual cycle include psychological, environmental, nutritional, and social factors.
  • Physiologically, the menstrual cycle temporarily ceases during pregnancy and lactation.
  • The cycle tends to become irregular toward menopause.

Changes in Reproductive Organs During the Menstrual Cycle

  • Changes accompanying the menstrual cycle include:
    • Ovarian Changes
    • Endometrial Cycle
    • Cyclic Changes in Cervix
    • Cyclic Changes in Vagina

Phases of Menstrual Cycle

1. Follicular Phase
  • This phase lasts from the onset of bleeding until ovulation, comprising two parts:
    a) Menstrual Phase
    b) Proliferative Phase
2. Luteal Phase
  • This phase spans from ovulation to the onset of the next menstrual bleeding.
  • The length of the luteal phase remains constant, irrespective of the cycle length.

Ovarian Changes During Menstrual Cycle

  • Ovarian Cycle:
    • Refers to rhythmic changes occurring in the ovaries.
  • During each cycle, a single mature ovum is released from the ovary.
  • Divided into three phases:
    a) Pre-ovulatory Phase
    b) Ovulation
    c) Post-ovulatory Phase
Pre-ovulatory Phase
  • Extends from day 5 of the cycle until ovulation.
  • Changes in the ovary during this phase are predominantly under the influence of FSH from the anterior pituitary.
  • Luteinizing Hormone (LH) also contributes to the maturation of the follicle in the latter part of this phase.
  • Approximately 10–15 primordial follicles begin to mature, but only one follicle fully matures while the rest undergo atresia (atrophy) at different developmental stages.
  • Characteristics of a fully matured Graafian follicle include:
    • Size increases markedly.
    • Antrum becomes larger and distended with fluid.
    • Theca interna becomes prominent.
Ovulation
  • Ovulation is defined as the release of a secondary oocyte from the ovary, following the rupture of the Graafian follicle into the peritoneal cavity.
  • Typically occurs 14 days into a 28-day cycle.
  • The mechanism of ovulation is triggered by a midcycle LH surge.
  • The ovum then enters the fallopian tube, with ovulation occurring approximately 9 hours post LH peak.
Indicators of Ovulation
  • Important physiological and clinical indicators of ovulation include:
    1. Rise in basal body temperature.
    2. Fleeting lower abdominal pain (mittelschmerz), due to minor blood leakage from the ruptured follicle.
    3. Vaginal discharge (spotting).
    4. Spinnbarkeit: the stretchy cervical mucus, peaking at ovulation.
Post-ovulatory Phase (Luteal Phase)
  • Extends from the 15th to 28th day of the menstrual cycle, characterized as a constant period of about 14 days.
  • Development of the corpus luteum occurs, hence the name luteal phase.
  • Corpus luteum is a glandular yellow body formed from the ruptured Graafian follicle after ovum release, primarily secreting progesterone (dominant hormone) and estrogen.
  • Key events during this phase include the formation of:
    • Corpus hemorrhagicum
    • Corpus luteum
    • Corpus albicans
Functions of Corpus Luteum
  1. Secretion of Hormones
    • Acts as a temporary endocrine gland.
    • Secretes large quantities of progesterone and a small amount of estrogen.
  2. Maintenance of Pregnancy
    • If pregnancy occurs, corpus luteum remains active for about 3 months until placenta develops.
    • Hormones secreted maintain the pregnancy.
    • Abortion may occur if the corpus luteum becomes inactive or insufficient in the first three months of pregnancy.

Endometrial Cycle

  • Refers to cyclic changes in the endometrium during the reproductive period (menarche to menopause) leading to recurrent monthly bleeding (menstruation).
  • Cyclic changes in the endometrium occur alongside ovarian changes, driven by cyclic production of estrogens and progesterone from the ovaries.
Phases of Endometrial Cycle
  • The endometrial cycle of 28 days is divided into three phases:
    1. Menstrual Phase (1st–5th day)
    2. Proliferative Phase (6th–14th day)
    3. Secretory Phase (15th–28th day)
Proliferative Phase
  • Extends from the 6th to 14th day following menstruation, starting with a thin basal layer of endometrium.
  • The hormone responsible is estrogen, secreted by the developing Graafian follicle, leading to changes in the endometrium, including:
    • Thickening of the endometrium.
    • Angiogenesis in the stratum functional leading to proliferation of blood vessels.
    • Stimulation for growth of endometrial glands.
Secretory Phase
  • Occurs from the 15th to 28th day, also known as the post-ovulatory phase.
  • Hormones include both estrogen and progesterone from the corpus luteum, during which the uterus is prepared for potential implantation of the ovum.
  • Changes include:
    1. Elongation and coiling of endometrial mucus glands.
    2. Increased blood supply to the endometrium.
    3. Increased thickness of endometrium to 5–6 mm at the end of the secretory phase.
    4. If fertilization does not occur, the corpus luteum involutes to form corpus albicans.

Menstrual Phase

  • Occurs if pregnancy does not happen, where the thickened endometrium is shed and expelled out through the vagina, termed menstruation or menstrual bleeding.
  • This phase is called the bleeding phase and averages 3–5 days.
  • Approximately 24 hours before the menstrual cycle ends, there is a sharp decline in plasma levels of estrogen and progesterone, which triggers menstrual bleeding.

Hormonal Regulation of Menstrual Cycle

  • The hormonal regulatory system operates through the hypothalamo-pituitary-ovarian axis.
  • Key hormones involved:
    1. Hypothalamic hormone: GnRH
    2. Anterior pituitary hormones: FSH and LH
    3. Ovarian hormones: Estrogen and progesterone

Changes in Cervix

  • During menstruation, the cervical mucosa does not desquamate like the endometrium.
  • In the proliferative phase (estrogen phase):
    • Secretions become thin, watery, and alkaline, facilitating sperm transport.
    • At ovulation, cervical mucus becomes thinnest and stretches like a thread (up to 8–12 cm, known as spinnbarkeit effect).
  • In the secretory phase (influenced by progesterone):
    • Decreased quantity of cervical secretions, thickened and cellular, preventing sperm entry through the cervical canal.

Changes in Vagina

  • The vaginal epithelium undergoes cyclic changes:
    • In the proliferative phase, it becomes thickened and cornified.
    • During the secretory phase, under progesterone influence, the epithelium proliferates with leukocyte infiltration, leading to thick and viscid secretions.

Conclusion

  • The menstrual cycle illustrates complex physiological changes influenced by various hormones, ensuring the readiness of the female body for potential implantation and subsequent pregnancy.
  • The regulation relies on feedback mechanisms and interactions among hormones at the hypothalamic, pituitary, and ovarian levels.

Questions

  • Several questions may arise pertaining to the detailed physiology outlined in this study guide, promoting deeper understanding and further inquiry into the menstrual cycle dynamics and hormonal interplay.