Menstruation is a visible sign of the ebb and flow of the hormones produced by the ovary. The first day of bleeding is a convenient marker to count as day one of the menstrual cycle, and it also coincides with the beginning of a new ovarian cycle.
During each cycle ovarian hormones prepare the reproductive tract for the possibility of fertilisation and pregnancy. Menstruation is evidence of these changes on the lining of the uterus – the endometrium.
The endometrium is a layer up to 6 mm thick. It is a very complex tissue, specialised for the implantation of the fertilised ovum and nurturing of the developing embryo and foetus. It consists of endometrial stromal cells and is richly supplied with small blood vessels. Its surface is covered by a single layer of columnar epithelial cells that dip into the stroma to form tiny glands.
At the end of a cycle without conception the surface two-thirds of the endometrium has died and is shed over the next several days as the menstrual flow. In the first two weeks of the next cycle, oestrogen produced by the next batch of ovarian follicles causes the endometrium to regrow from the remaining third. The stromal cells multiply, new blood vessels form, and the surface layer and its glands are restored. This is called the proliferative (growth) phase of the endometrium.
After ovulation, progesterone from the corpus luteum causes the glands to secrete substances that will nourish the embryo, especially glycogen. Progesterone also stimulates changes in the stromal cells in preparation for implantation of the embryo, and makes the endometrial blood vessels lengthen and coil and the muscle in their walls thicken. When the endometrium has been influenced by progesterone, it is described as being in the secretory phase. The hormonal changes in the endometrium are co-ordinated so that it will be in just the right state to receive and implant the fertilised egg when (and if) it arrives in the uterus six to seven days after ovulation.
Unless conception has occurred, the production of both oestrogen and progesterone in the ovary falls rapidly during the fourth week of the cycle. The fall in blood hormone levels signals to the endometrium that it is not needed for pregnancy, and triggers the changes that lead to menstruation. The muscle in the walls of the coiled blood vessels constricts so that the j blood supply to the surface two-thirds of the endometrium is pinched off. Without blood the tissue dies and is sloughed off. The dead tissue liquefies and drains from the uterus through the cervix and vagina.
*82/31/5*
No comments yet.