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| Menstruation and Menstrual Cycle |
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| At about the age of 10 to 13 years, the ovaries of females are stimulated by the follicle stimulating hormone (FSH) of the pituitary. This is called the onset of puberty and is accompanied by release of hormones oestrogen and progesterone. These hormones control the production of ova or eggs and appearance of secondary sexual characteristics. Unlike males where sperms can be produced through out the life of man, in females the reproductive phase only lasts till the age of 45-50years. This phase is characterised by the presence of menstrual cycle. |
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| Menstrual Cycle |
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| Each menstrual cycle typically lasts for 28 days. Thus it occurs every month. Each cycle has the following phases: |
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 Menstrual phase |
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Follicular phase |
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Ovulatory phase |
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Luteal phase |
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| Menstrual Phase |
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| It lasts for the first 3-4 days. During this phase the inner lining of the uterus is shed which causes the blood vessels to rupture. This causes bleeding and is called menstruation. The first occurrence of mensuration is termed menarche. It stops by the age of 45-50 years and is called menopause. |
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| In the ovary, during this phase, the follicles where the eggs are produced are growing. Follicles are structures formed by the aggregation of the germinal epithelial cells of the ovary. |
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| Follicular Phase |
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| In this phase, the follicles grow further. The FSH stimulates one of the follicles. The stimulated follicle grows in size. |
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| T.S. of Ovary of a Mammal |
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| One of the cells of this follicle becomes bigger and separated from the rest by a follicular cavity. This cell becomes the egg. The outer layer of cells of this follicle is called theca interna. This layer secretes a hormone called oestrogen. This follicle is called the Graafian follicle. This phase lasts from the 6th to the 10th day. |
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| In the uterus, this phase sees the inner wall of the uterus being built up again in order to receive the product of fertilisation, if there is one. It is again supplied with blood vessels. |
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| Ovulatory Phase |
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| When the follicle is mature, the pituitary gland secretes another hormone called luteinizing hormone (LH). LH stimulates the follicle to rupture and release the egg. The release of egg is called ovulation and occurs between the 10th and the 16th day. The egg moves along the oviduct during this time and may be fertilized by the sperm. If not, it starts disintegrating. |
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| Luteal Phase |
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| This phase lasts between the 16th and the 28th day. Once the egg is released, the Graafian follicle re-aggregates to form corpus luteum. The corpus luteum secretes two pregnancy hormones - progesterone and relaxin. The degenerating corpus luteum is called corpus albicans. |
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| In the uterus, its lining is thickened further. |
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| At the end of 28 days, if fertilisation has not taken place, the lining is shed along with the egg. This starts a new cycle all over again. |
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