Menstrual Cycle / Uterine cycle (phases and hormonal control)

August 25, 2012 | By | Reply More

Menstrual Cycle / Uterine cycle (phases and hormonal control)

Uterine (Endometrial) Cycle

  • Monthly cyclical changes in the endometrium of uterus for prepration of implantation (in the event of fertilization) and for menstruation (in the absence of fertilization) is called uterine cycle.
  • The uterine (menstrual) cycle is a concurrent series of changes in the endometrium of the uterus to prepare it for the arrival of a fertilized ovum that will develop in the uterus until birth. If fertilization does not occur, the lining (stratum functionalis) of the endometrium is shed during menstruation.
  •  Menstruation is also called menstrual bleeding, menses, a period or catamenia. The flow of menses normally serves as a sign that a woman has not become pregnant.
  • The uterine endometrial cycle can be divided into three phases:

–    the proliferative phase

–    the secretary

–    the menstrual phase

A. Proliferative Phase

  • The proliferative or follicular, phase, spans from the end of the menstruation until ovulation.
  • Under the influence of increasing levels of estrogen secreted by ovarian follicles all elements of endometrium proliferate.
  • The raw surface of endometrium is again covered with epithelium which proliferates out from the remains of the stems of uterine glands. The stromal cells increase
  • Endometrial glands elongate with cells containing some glycogen. But it is not secreted during the follicular phase.
  • Spiral arteries supplying blood also elongate. Stratum functionalis thus resumed  again.

B. Secretary Phase

  • The luteal, or secretory phase, begins at ovulation and lasts until the menstrual phase of the next cycle.
  • At the beginning of the luteal phase, progesterone induces the endometrial glands to secrete glycogen, mucus, and other substances. These glands become tortuous and have large lumens due to increased secretary activity.
  • The spiral arteries extend into the superficial layer of the endometrium.
  • In the absence of fertilization by day 23 of the cycle, the corpus luteum begins to degenerate and consequently ovarian hormone levels decrease.
  • As estrogen and progesterone levels decrease, the endometrium undergoes involution.
  • Days 25-26 of the menstrual cycle, endothelin and thromboxin begin to mediate vasoconstriction of the spiral arteries. The resulting ischemia may cause some early menstrual cramps.
  • By day 28 of the menstrual cycle, intense vasoconstriction and subsequent ischemia cause mass apoptosis of the stratum functionalis.

C. Menstrual Phase

  • The menstrual phase begins as the spiral arteries rupture secondary to ischemia, releasing blood into the uterus, and the apoptosed endometrium is sloughed off. It usually lasts four days. During this period, the stratum functionalis is completely shed.
  • Arterial and venous blood, remnants of endometrial stroma and glands, leukocytes, and red blood cells are all present in the menstrual flow.

Name of phase


menstrual phase


follicular phase (also known as proliferative phase)


ovulation (not a phase, but an event dividing phases)


luteal phase (also known as secretory phase)


ischemic phase (some sources group this with secretory phase)


Hormonal control

1. Pituitary gland release Follicle Stimulating Hormone (FSH) that acts on the ovary.
2. Ovarian follicles grow and mature under influence of FSH, and produce increasing quantities of estrogen
3. When levels of estrogen reach a threshold level, a feed back process decreases the production of FSH by the pituitary.
4. Increased estrogen levels trigger the release of Luteinizing Hormone (LH) from the pituitary gland
5. Ovulation occurs, that is release of an egg from the follicle.
6. The empty follicle forms the corpus luteum, which produces progesterone which maintains the endometrium in a state of readiness to receive a fertilized egg
7. If the fertilized egg does not implant itself the progesterone level falls and mensturation commences.

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Category: Medical, Physiology

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