Reproduction and basic genetics

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Menstruation refers to the process of physiological changes in women and other female primates in which an egg is produced in a process known as ovulation. The uterine lining thickens to allow for the implantation if fertilization of the egg occurs. If the egg is not fertilized, the uterine lining breaks down and is discharged during menstruation. This activity occurs on monthly basis. (Menstrual Cycle, the Free Dictionary)
The repetitive monthly production of hormones which results in the release of a mature egg, also referred to as ovum, is called the menstruation cycle. This process begins at puberty and ends during menopause. The first menstrual cycle is known as the menarche. Just above the brain lies the hypothalamus pituitary gland, which together with the ovaries, produce the hormones that control the menstrual cycle.
An egg matures in the ovary in preparation for the womb to establish pregnancy. When pregnancy is not achieved, menstruation occurs. The cycle of menstruation is divided into four phases which take approximately 28 days to complete. The first phase of menstruation lasts five days. Drops on the levels of progesterone hormone trigger menstruation. Progesterone is produced by the female reproductive system (ovaries, placenta and adrenal glands). Its main function is the regulation of the inner lining of the uterus, also known as the endometrium. (Menstruation - What is Menstruation, 2013)
In the second stage of menstruation, follicle stimulation hormone production increases in the pituitary gland. The ovary responds by beginning the egg maturation process. Between 10 and 20 eggs begin developing within the follicles of the ovaries. However, only one egg reaches maturity. Clusters of cells encase a developing egg. These cells are known as follicles. The second stage therefore is also referred to as the follicular phase. Follicles that are developing release a hormone called estrogen. This hormone stimulates the lining of the uterus, also referred to as the endometrium to grow or proliferate. This is in preparation to receive an embryo which is an egg that has been fertilized and began dividing, which establishes pregnancy. This phase lasts up to the thirteenth day of the overall cycle.
From day 15 to 28, the fourth phase takes place. In this phase, the empty follicle, now called the corpus luteum further prepares the uterus for the implantation of an embryo by the release of the hormone progesterone. The endometrium in turn thickens, because of cell growth, increases in fluids and changes that occur in blood vessels and glands. If pregnancy does not take place, a drop in the levels of progesterone signals yet another start of the menstrual cycle. On the other hand, if pregnancy takes place, progesterone levels remain high and therefore, the endometrium is not shed. (Rowland & Frey, 2005)
Two types known categories of menstruation exist. Overt menstruation where bleeding occurs through the vagina is found mainly in humans and close evolutionary relatives like the chimpanzees. The other, covert menstruation occurs in other placental mammals. The endometrium is reabsorbed by the animal at the end of the menstrual cycle. It is not released out of the body like in overt menstruation.
The Luteinizing hormone (LH), produced in both men and women, is an important horm...