Female reproductive system -nature's creativity at its best

Female reproductive system consists of uterus, a pair of ovaries, Fallopian tubes-one from each ovary joining the upper angles of uterus, cervix ending in vagina.

The uterus is a hollow muscular organ situated in the mid-line in the pelvis behind the urinary bladder.

In the virgin state it is about 7.5 cm long, about 5 cm broad and 2.5 cm thick. It weighs between 30 and 40 grams.

The uterus provides the space for the development of fetus. During pregnancy it progressively increases in size to accommodate the growing fetus.

The uterus leads into a narrow portion called the cervix which in turn is continuous with the vagina.

The Fallopian tubes start from the upper angle of the uterus and extend laterally ending in finger like projections called fimbria. Each tube is about 10 cms long. When ovulation occurs the fimbria sweep the ovum towards the mouth of the Fallopian tube so that it enters the tube.

Each ovary is about 4 cm long, 2 cm wide and less than 1cm in thickness. It weighs about 3 grams. It is attached to the uterus by the ovarian ligament. It is situated near the fimbrial end of the Fallopian tube.

Under a microscope an ovary is found to have spherically arranged groups of cells which are called follicles. Each follicle contains an ovum or egg.

The follicles mature under the influence of follicle stimulating hormone of pituitary gland. The level of follicle stimulating hormone increases during puberty.

During the reproductive age one follicle matures every month and ruptures releasing the ovum. The maturation and subsequent rupture of the follicle are brought about by the Follicle Stimulating Hormone (FSH) and the Luteinizing Hormone (LH) secreted by the Pituitary gland.

The ovum enters the Fallopian tube. Fertilization takes place in the tube and the zygote moves into the body of the uterus. It gets embedded in the wall of the uterus and develops into a fetus.

If fertilization does not take place the ovum is expelled along with the lining of the uterus during menstrual cycle.

Human beings possess 23 pairs of chromosomes in each cell (total of 46 chromosomes). A new life begins with fusion of ovum and one sperm. If each of them contains 23 pairs of chromosomes the fertilized ovum will have 46 pairs of chromosomes (total of 92 chromosomes).

Such a zygote can never develop into a human being. How does nature ensure that the zygote also has 23 pairs of chromosomes?

There are two types of cell division, mitosis and meiosis. All cells except the gametocytes (ovum and sperm) divide by mitosis. During this process all the chromosomes are replicated during cell division and the two cells formed have the same number of chromosomes.

But the cells forming the gametocytes (ovum and sperm) divide by meiosis. During this replication takes place initially as in mitosis but the cell undergoes two rounds of division resulting in four cells each having 23 chromosomes. The other 23 chromosomes are contributed by the sperm and the zygote will have exactly 23 pairs of chromosomes.

The ovaries produce the hormones estrogen and progesterone. The term estrogen refers to a group of structurally related hormones which are responsible for the development of secondary sexual characteristics in females.

It stimulates the growth of lining of uterus during the first part of menstrual cycle. It also increases bone formation and protein synthesis.

Progesterone is absolutely essential for pregnancy to continue to term. Its level increases after ovulation in the middle of menstrual cycle. If fertilization does not occur, the level of progesterone drops leading to shedding of the lining of the uterus and menstrual bleeding.

If fertilization occurs the level of progesterone remains high till the end of pregnancy. It causes thickening of the cervical mucus making it impermeable to sperms.

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