Menstrual Cycle

Menstruation normally occurs during cycles of 22-35 days, with menstrual discharge lasting 1-8 days. The cycle is counted from day 1 of menstrual flow until day 1 of the next period.

 

from Chris73, Wikipedia user

 

 

A woman is born with approximately 200,000 eggs, surrounded by theca lutein cells.

With each mentstrual cycle, 3-30 of these follicles are stimulated by LH and FSH, with one or more of these follicles rapidly growing and being released from the ovarian wall. Menstruation follows the release of an ovum in the absence of fertilization.

 

FSH acts on the granulosa cells to increase fluid production (and thereby follicular size) as well as estradiol production.

Acting systemically, estradiol at first causes a decrease in pituitary hormones. However, within a few days a large LH and smaller FSH surge occurs. This pulse causes ovulation - release of the ovum from the largest follicle. Large amounts of FSH, produced or injected, can cause superovulation, or relase of numerous follicles.

 

 

 

 

 

 

 

 

 

 

 

 

 

  • hormonal changes
  • ovulation
  • endometrial cycle
  • cervical cycle
  • vaginal cycle

Hormonal Changes

Ovulation

 

Ovulation animation, use permitted by Andrew R. La Barbera, PhD, University of Cincinnati, 2007

Endometrial Cycle

Menstruation is the periodic loss of uterine lining, or endometrium, including cellular remains, blood, and tissue fluid. The quantity of fluid can vary widely, and some women who have heavy periods are not, in fact, at risk of blood loss anemia.

Mean blood lost during menstruation is 30 ml, with a range of 10-80 ml.

 

Once the endometrium is shed, a rapid period of development begins. Stromal cells undergo metaplasia to form the endometrial surface, and epithelium forms numerous glands. Repair is complete within 3 days of the cessation of menstruation.

During the early proliferative phase, lasting from days 3-7, glands are straight and narrow. Proliferation increases after one week, with epithelial hypertrophy and transition from cuboidal to columnar. Stromal fibroblasts become spindle-shaped.

Arteries leave the myometrium at right angles and supply the endometrium, where they become spiralled. As the endometrium continues to grow, the arteries uncoil to continue providing vascular supply.

 

The Luteal phase marks ovulation. Glandular epithelial cells develop secretory vacuoles of increasing number, causing glands to become tortuous. Vacuoles, which are initially subnuclear but then move more apically, are filled with mucus and other molecules such as.... Peak release occurs at day 6 post-ovulation.

In the absence of pregnancy, estrogen and progesterone levels fall, increasing endometrial levels of arachidonic acid and endoperoxidases. Stromal cells then begin synthesis of prostaglandins PGF2a and PGE2, along with prostacyclin. PGF2a and PGE2 cause vasoconstriction and uterine contraction, while prostacyclin opposes these actions by causing muscle relaxation and inhibiting platelet aggregation. A relative increase in PGF2a causes arteriolar vasoconstriction and reduced capillary blood flow, shunting fluid from the endometrium into the capillaries and decreasing tissue thickness. Hypoxia and ischemic necrosis also occurs, marking the beginning of menstruation.

 

Menstrual phase

The superficial and midle layers of endometrium are shed during menstruation, while the deep layer remains. Shedding occurs in an irregular pattern, with some areas undergoing repair while others slough off. Shed tissue coagulates in the uterine cavity but is usually liquified by fibrinolysis. Fluid and tissue is discharged through the cervix by uterine contractions, mediated by prostaglandins. If increased blood loss occurs, fibrinolytic capacity may be overwhlemed and clots can be discharged.

Endometrial vasculature is sealed by hemostasis and vasoconstriction. Regeneration continues repairing the sloughed off endometrium, and a new cycle begins.

 

 

Cervical Cycle

During the follicular phase, the cervical canal begins secreting thick mucus from glands to form crosslinked mesh. Prior to ovulation, the surge in estrogen leads to action of metalloproteinases that thin the cervical mucus and lead to stranding and channel formation through which sperm can travel. Progesterone, released from the corpus luteum, again causes the cervix to be blocked by thick mucus.

Vaginal Cycle

The vaginal epithelium responds to the ratio of estrogen and progesterone. During the follicular phase, both superficial and large intermediate cells are seen, while prior to ovulation, greater numbers of superficial cells, along with leukocytes are present.

Following ovulation, increased levels of progesterone leads to large numbers of intermediate cells and leukocytes.

 

 

 

Once the ovum has been released, the follicle collapses and becomes the corpus luteum. Namned for its colour yellow, theca granulosa cells become theca lutein cells and secrete progesterone and estrogen.

Progesterone reaches a plateau 4 days post-ovulation, and progressively rises if a fertilized ovum implants into the endometrium.

An impoanted embryo secretes hCG, acting on the corpus luteum to keep it alive and producing progsterone and estradiol.

In the absence of implantation, the corpus luteum degenerates and progsterone and estradiol levels fall. This leads menstruation and to renewed secretion of FSH to begin the cycle again.