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Normal Reproduction Cycle
The reproductive cycle of a normal woman generally runs through 3 phases, which are controlled by hormonal feed back mechanisms in the brain involving the hypothalamus and the pituitary gland. It is usually cyclical and becomes irregular if the hormone balance is disturbed for any reason.
Phase I- Follicular Phase
At the beginning of this cycle, the hypothalamus signals to the pituitary gland to increase its production of Follicle Stimulating Hormone (FSH). Under the influence of FSH, 20-30 follicles become active within the ovaries.With increasing age, fewer follicles are present in the ovaries and
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correspondingly fewer follicles develop during the cycle. The developing follicles produce estrogen and as the level of this hormone increases, feedback to the pituitary gland leads to a decrease in FSH production, until only enough to encourage further development of one follicle. The increasing amount of estrogen also causes a build up in the thickness of the inner lining of the uterus (endometrium) in preparation for pregnancy and makes the cervical mucus become favourable for the passage of sperms.
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Phase II - Ovulation Phase
The high level of estrogen causes the pituitary gland to release a surge of Luteinizing Hormone (LH). LH then triggers ovulation causing the follicle to mature and release the egg. At the same time, the fimbriated end of the fallopian tube directs the egg into it.
For about 2 days before ovulation, the cervical mucus becomes conducive for the passage of sperms to pass through the cervix and uterus into the fallopian tubes, where it has the ability to survive for 2-3 days awaiting for the egg to be released. On meeting the egg, only one sperm penetrates and completes fertilization within 24 hours. The fertilized egg starts to divide into cells, the number of cells doubling with each division and becomes an embryo.
Phase III - The Luteal Phase
The embryo travels along the fallopian tube and on reaching the uterus, hatches out of its shell and implants into the endometrium by the seventh day. The follicle from which the egg was released, now known as the corpus luteum, begins to produce the hormone progesterone. Progesterone together with estrogen act on the endometrium to thicken it. This change, called the secretory phase, provides nutrition to the implanted embryos.
The cells from the embryo that subsequently form the placenta produce the hormone human Chorionic Gonadotrophin (hCG). The presence of hCG encourages the corpus lutuem to continue producing estrogen and progesterone to support the pregnancy until the placenta takes over. hCG is the hormone measured in pregnancy tests. If fertilisation does not occur, the absence of hCG leads to the demise of the corpus lutuem, hence leading to the fall in the estrogen and progesterone levels. Without these hormones, the endometrium sheds, menstruation occurs and a new cycle begins.
Reproduction in Men
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