Immunology | Immune system| Immunity

Menstrual (Ovarian) cycle

Reproductive period of human female(about 30 to 33 yrs)extends from puberty(10-14 yrs) to menopause(45-55 years).During this reproductive period,ovaries and female reproductive tract undergo a series of cyclic changes which are primarily meant to prepare them for fertilization and pregnancy and collectively form the menstrual cycle.Most peculiar feature of this cycle is the periodic vaginal bleeding called menstruation which lasts for about four days.
Period: The lenght of menstrual cycle varies widely in women,but on average it is completed in 28 days .In a female ,successive cycles may vary in lenght by 1 to 2 days.It is absent during pregnancy,may be suppressed during lactation and permanently stops at menopause.
Phase: Menstrual cycle is divided into four phases:

1.Follicular or post-menstrual or pre-ovulatory phase.It follows  the menstrual phase and lasts for about 10 12 days(5th to14th day of menstrual cycle).It involves following changes:
(a) Under the stimulation of FSH-RF of hypothalamus,there is increased secretion of FSH from anterior pituitary.
(b) FSH stimulates the change of a primary follicle of the ovary into graafian follicle.
(c) Follicular cells of Graafian follicle secrete estrogens.

2.Ovulaotry phase: It involves the ovulation from the Graafian follicle of ovary.It occurs midway between two menstrual cycles on the 15th day of the onset of the menstrual cycle.It is caused by increasing turgidity and contraction of smooth muscle fibres around the Graafian follicle. Ovum is received by the fimbrae of the Fallopian tube. Ovum is viable for two days. Ovulation is controlled by the increased level of LH also starts the change of empty Graafian follicle into corpus luteum and secretion of progesterone from corpus.

3.Luteal or Progestational or Pre-menstrual or Secretory phase or Post-ovulatory phase:
It lasts for about 12-14 days and extends from 16th to 28th day of the menstrual cycle.It is characterised by following changes:
(a) Corpus luteum(yellow body) formed from empty Graafian follicle,increases in size,so is called luteul phase.
(b) Corpus luteum secretes progesterone hormone whose level in blood gradually increases.

4.Menstrual phase: It lasts for about 3-5 days and extends from 1st to 4th day of the menstrual cycle.When the ovum remains unfertilized,then the corpus luteum starts degenerating.The level of progesterone in the blood declines.The uterine tissues fail to be maintained. Then the unfertilized ovum along with ruptured uterine epithelium,about 50-100 ml of blood and some mucus is discharged out through the vaginal orifice and is called menstrual flow or menstruation.

Decrease in the level of progesterone and estrogens in the blood stimulates the hypothalamus and the anterior pituitary to release FSH-RF and FSH respectively. FSH starts the follicular phase of next menstrual cycle.
However, if  fertilization occurs then blastula undergoes implantation and a chorionic placenta is formed between the foetus and the mother. Placenta secretes Human Chorionic Gonadotrophin (HCG) which maintains the corpus luteum and the progesterone secretion continues. Progesterone maintains pregnancy and prevents ovulation during gestation period.