Menstruation and Female Reproductive Physiology

Menstruation Overview

• Menstruation is a cyclical process where the female reproductive system ripens and releases one mature egg
• The average cycle length is approximately 28 days, but it can range from 21 to 40 days.
• Menstruation involves the shedding of the endometrium, which then bleeds due to hormone fluctuations.

Physiology of the Female Reproductive System

• The hypothalamic-pituitary-ovarian axis controls the female reproductive system.
• GnRH from the hypothalamus stimulates the pituitary gland to release FSH and LH.
• FSH stimulates egg production in the ovaries
• LH also directs the production of progesterone in the ovaries
• Progesterone maintains the growth of the endometrium.

Physiological Stages

• Neonatal period: Birth to 4 weeks
• Childhood: 4 weeks to 12 years
• Puberty: 12 years to 18 years
• Sexual maturation: 18 years to 50 years
• Perimenopause: Decline in ovarian function (40 years to 1 year post-menopause)

The Menstrual Cycle

• The cycle begins with menstruation (day 1), the shedding of the endometrial lining

Menstruation Details

• Endometrium sheds and bleeds due to cyclic ovulation
• The non-clotting menstrual blood comes primarily from arteries (approximately 75%).
• Typical cycle interval is 21-40 days (average 28 days).
• Duration of bleeding is typically 1-7 days
• The first day of bleeding is considered day 1.

Changes in Pituitary Hormones (Days 1-14)

• During the first half of the cycle, the pituitary releases FSH to stimulate egg production.
• FSH also triggers the release of estrogen from the ovaries.

Changes in Pituitary Hormones (Days 14-28)

• On day 14, the pituitary releases LH, which triggers ovulation.
• LH also directs the production of progesterone.
• Progesterone maintains the growth of the endometrium
• If the egg is not fertilized, progesterone levels drop, causing estrogen levels to drop, leading to menstruation.

Ovarian Cycle

• The ovarian cycle is controlled by the hypothalamus-pituitary axis.
• The cycle is divided into follicular, ovulatory, and luteal phases.
• The follicular phase involves follicle growth, estrogen production, and preparation for ovulation.
• The ovulatory phase includes the release of the egg.
• The luteal phase involves corpus luteum formation, progesterone production, and maintenance of the uterine lining for possible pregnancy.

Changes in the Ovaries

• Stage 1: Ovum begins development
• Stage 2: The egg continues to ripen; follicle grows
• Stage 3: Ovulation (fully mature egg bursts out of follicle)
• Stage 4: The egg enters the fallopian tube; the follicle forms the corpus luteum

Follicular Phase

• Several follicles start to grow after puberty.
• Most follicles become atretic (degenerate), but one matures.
• Granulosa cells create estrogen and some progesterone, which are the principle sources of estrogen within the follicle fluid.
• Theca interna cells form additional estrogens.
• Estrogen and other hormones exert negative feedback on FSH and LH

Ovulatory Phase

• The rising estrogen towards the mid-cycle triggers positive feedback on FSH and LH, resulting in follicle rupture and egg release into the abdominal cavity
• The egg enters the fallopian tube to travel to the uterus.

Luteal Phase

• Following ovulation, the ruptured follicle transforms into the corpus luteum.
• Luteal cells produce primarily estrogen and progesterone.
• This phase results in negative feedback on FSH and LH.
• If fertilization does not occur, the corpus luteum degenerates, leading to a decrease in estrogen and progesterone levels and subsequent menstruation.
• If fertilization occurs, HCG is produced to maintain corpus luteum and keep producing estrogen and progesterone.

Uterine Cycle

• The endometrial lining thickness increases during the proliferative phase in response to rising estrogen levels.
• The lining becomes thick and soft during the secretory phase as progesterone causes secretions of nutrients into the lining for a potential embryo.
• If no pregnancy occurs, the superficial layer of the uterus is shed during the menstrual phase.

Menstrual Phase

• The shedding of the stratum functionale (superficial layer of endometrium) occurs after 3-5 days because of the lack of estrogen and progesterone.
• The blood loss is generally arterial (30 mL).
• The lost blood is non-clotting due to fibrinolytic substances (fibrinolysin)
• Prostaglandins and cellular debris are also present.

Indicators of ovulation

• Rise in body temperature due to progesterone’s thermogenic effect
• LH surge (peak)
• Development of the secretory phase in endometrium
• Lower abdominal pain

Terms used to describe menstrual abnormalities

• Primary amenorrhea (absence of menstrual bleeding)
• Secondary amenorrhea (absence of menstrual bleeding after it began normally)
• Dysmenorrhea (painful menstruation)
• Hypomenorrhea (scant menstrual bleeding)
• Menorrhagia (excessive menstrual bleeding)
• Metrorrhagia (bleeding from the uterus between periods)

Additional notes on Anovulatory Cycle

• Some cycles may occur without ovulation.
• The bleeding in anovulatory cycles is caused by rising and falling estrogen levels.
• Anovulatory cycles can occur before menopause and following menarche