Menstruation

At the time the ovaries are formed in the fetus, there are approximately 6000000 primordial follicles, which decrease to about 600000 at birth, to 300000 at the first menstrual cycle, to about 10000 at the time of menopause.

The Cycle

  • The average cycle is 28 days and has two distinct phases

The Follicular Phase

  • The follicular phase starts on day one of the menstrual cycle (the first full day of bleeding)
  • The hypothalamus in the brain releases gonadotropin-releasing hormone (GnRH)
  • GnRH signals the pituitary gland to release follicle stimulating hormone (FSH)
  • FSH stimulates the eggs inside the ovaries to grow
  • About 20 immature eggs response and begin to develop within sacs known as follicles
  • Follicles provide nourishment to the eggs
  • As the eggs develop, the ovaries release estrogen.
  • Estrogen signals the pituitary gland to reduce FSH production
  • Only enough RSH is now released to stimulate one egg to continue developing, the rest of the eggs shrivel away.
  • Estrogen stimulates the lining of the uterus to thicken
  • The primary follicle contains the contains the egg that has grown the most rapidly.
  • Estrogen continues to rise until it triggers a surge of luteinizing hormone (LH) from the pituitary gland
  • LH stimulates ovualtion
  • The follicle ruptures and the egg is released along with the follicular fluid onto the surface of the ovary.

The Luteal Phase

  • The ruptured follicle continues to receive LH
  • The LH enables the follicle to turn into a small cyst known as the corpus luteum
  • The corpus luteum produces progesterone
  • Progesterone

1) builds and thickens the endometrium, developing glandular structures and blood vessels that supply nutrients to the developing embryo
2) it switches off FSH an LH
3) it raises the basal body temperature (BBT) by half a degree, warming the uterus and fertilized egg.

The Journey of the Egg

  • The egg is surrounded by the zona pellucida, a protective shell
  • The shell is surrounded by a mass of sticky cells called the cumulus oophorus
  • These sticky cells allow the finger like projections at the end of each fallopian tube, to pick up the egg and sweep it into the tube
  • The channel from the fimbriae to the uterus is lined with cilia, which together with muscular contractions move the ovum along the tube to the uterus.
  • The journey from the ovary to the uterus take about 6 days
  • If the egg is not fertilized it will disintegrate and is absorbed