Menstrual Cycle

Both estrogen and progesterone levels swing dramatically across a female's menstrual cycle. Menstrual bleeding occurs after estrogen and progesterone levels drop in response to corpus luteum disintegration. The endometrial tissue shrinks in response to decreasing levels of steroids during the premenstrual phase and this shrinkage results in increased coiling and constriction of the spiral arteries and arterioles. After constriction the vessels dilate, fill with blood and burst resulting in menstruation. As much as three-fourths of the endometrial tissue can be resorbed by the body during the menstrual phase; the remainin tissue is cleared out of the uteral cavity by the menstrual blood. Menstrual blood contains many leukocytes and enough complement to destroy parasites in vivo.

The follicular phase of the menstrual cycle is characterized by initial rise in follicular stimulating hormone (FSH) and luteinizing hormone (LH) in response to the pulsatile secretion of gonadotropic releasing hormone (GnRH) by hypothalamus. FSH and LH stimulate the production of estrogen by the developing follicles so the estrogen levels increase rapidly during the follicular phase of the cycle. Estrogen levels are correlated with increased immune system responses and decreased eating behavior.

The follicular phase ends with ovulation. Prior to ovulation estrogen levels plummet but LH and FSH levels surge causing expulsion of the Graafian follicles, the maturing ovum. If sperm are present in the fallopian tubes, fertilization can occur at this time. Under the influence of estrogen, the mucus secreted by the endocervical glands becomes more elastic, its water content increases and it becomes more alkaline. Therefore, at the time of ovulation, the vaginal and cervical environments are less hostile to both sperm and potential pathogens. The fact that most females produce antisperm antibodies which are then neutralized by her antiidiotypic antibodies indicate the presence of adaptations to insure that sperm are not destroyed by the female's immune system. Uteral protein levels also peak at ovulation providing an environment conducive to sperm metabolism. For fertilization to occur, a female's sexual activity should increase when she is ovulatory.

Two to three days after ovulation the corpus luteum begins producing progesterone and estrogen. Estrogenic effects differ in the presence of progesterone because progesterone decreases the number of estrogen receptors in the endometrium and activates an enzyme that converts estradiol to less active forms of estrogen. Progesterone is correlated with decreased immune system responses and increased eating behavior.

If fertilization occurs, implantation follows about 8 to 10 days after ovulation and the corpus luteum is maintained by the production of human chorionic gonadotropin (hCG). In contrast, if fertilization does not occur the hCG is not produced, the corpus luteum begins to break down about days 8 to 10 and is usually completely regressed by day 14 after ovulation. Estrogen and progesterone levels both drop at this time, initiating menstrual bleeding. Prior to menstruation, during the pre-menstrual phase an infiltration of leukocytes to the endometrium occurs and the vaginal epithelium regresses allowing the escape of polymorphonuclear leukocytes.