Menstrual cycle : (mensem = month)
Human Reproduction of Class 12
In primates (human, ape, monkey) the cycle of 28 days is marked by the vaginal bleeding called menstruation or “weeping of uterus” as major event.
Depending upon the changes in ovary, uterus i.e., preparation for conception and pregnancy, and associated hormonal changes,
The cycle is divided into four phases:
- Menstrual phase (destructive phase) : If fertilization/pregnancy doesn’t take place the breakdown of overgrown uterine tissue with blood vessels begins. Discharge of blood and ruptured endometrium through vagina mark the whole phase that lasts for 3 - 5 days. Estrogen level is low, LH and progesterone also come down. Average blood loss equals 50 to 150 ml.
- Follicular phase (resting phase) – Bleeding stops; follicles start growing; level of FSH, estrogen starts rising; reformation and growth (proliferation) of uterine wall begins and thus preparation for next pregnancy sets in; also called as proliferative phase; lasts for 7 - 8 days.
- Ovulatory phase – By the end of 13th day one of the mature follicles undergoes ovulation. The estrogen level is highest hence corresponds to ‘estrus’ phase of estrous cycle.
High level of LH from pituitary induces ovulation, FSH level comes down :
Fig. Graphic representation of menstrual cycle
Luteal phase (or Secretory phase)
After ovulation till the onset (or 1st day) of next menstruation it is the longest phase of about 13 days.
Ruptured follicle under the influence of LH, develops as corpus luteum to secrete progesterone. After a fall for 2-3 days the estrogen level rises again. High level of estrogen and progesterone during this period inhibits FSH, hence, maturation of follicle stops, but the growth of uterine wall increases multifold. Menstrual cycle remain suspended during pregnancy.
- Menopause (or female climacteric) – The menstruation stops at 45 - 50 years of age, women loses reproductive ability. Urine of a menopausal women contain large amount of FSH because estrogen production by ovaries stops and pituitary gets relieved of inhibitory effect of estrogen.
- Menarche – The first menstruation which marks the onset of puberty.
- Dysmenorrhoea – Abnormal or irregular menstruation with cramps and profuse bleeding due to locally produced prostaglandins.
- Amenorrhoea – Menstruation does not occur due to many physiological/hormonal or mental defects this is the sign of sterility.