Menstural Cycle

THE MENSTRUAL CYCLE The female reproductive system goes through a series of changes of a cyclical nature that takes place approximately every 28 days (monthly). This is referred to as the menstrual cycle. It involves three principal organs; the pituitary gland, the ovaries and the uterus. The hormones secreted by the pituitary gland affect structures within the ovary while the ovarian hormone directly affect the endometrium of the uterus. The 1st day of the cycle is the 1st day of menstrual bleeding. On this day the pituitary gland secretes FSH (Follicular Stimulating Hormone) which causes the development of the Graafian Follicle in the ovary. The Graafian Follicle refers to the immature ovum which is surrounded by a cuff of supporting cells that both nourish and protect it. By around the 6th day, the Graafian Follicle starts secreting Oestrogen. This hormone stimulates the repair and regeneration of the endometrium. By around the 12th day, Oestrogen levels in the blood rise steeply and usually peaks on or around the 13th day. This sensitises the pituitary gland to secrete Leutenizing Hormone (LH) in a rapid burst on the 14th day. This surge in LH now causes the Graafian Follicle to rupture, releasing the now mature ovum into the oviduct. This process is called Ovulation and is often accompanied by an increase in body temperature and some possible pelvic irritation. The period of time between the 1st and 14th days of the cycle are referred to as the Follicular phase of the cycle. Following ovulation, the remnants of the Graafian Follicle contract and clump forming a tight ball of cells called the Corpus Luteum. While the ovum continues its passage down the oviduct, the Corpus Luteum now secretes Progesterone. This hormone causes further proliferation and thickening of the endometrium, in preparation for implantation and pregnancy. Progesterone levels are always maintained above that of Oestrogen, and together the two hormones maintain the thickening of the endometrium, keeping it intact. At or around the 26th day of the cycle, if fertilisation and implantation has not taken place, the Corpus Luteum starts to degenerate, causing a steep decline in the level of circulating progesterone. Towards the end of the cycle, the progesterone levels fall just below that of Oestrogen, and when the two intersect, the endometrium contracts, causing menstrual bleeding to begin, marking the onset of the next menstrual cycle.

Fertilisation The ovum is the largest cell in the human body, measuring approximately 100 microns in diameter (in comparison to a RBC which has a 7 micron diameter). Fertilisation is defined a the fusion of the nucleus of the spermatozoan (male gamete) with the nucleus of the ovum (female gamete). This takes place midway along the oviduct and is accelerated by the acrosomal reaction. Only one sperm can fuse with the ovum, as a protective membrane is secreted the moment the ovum is pierced by the acrosome of one sperm. The fusion of the two nuclei results in the formation of the diploid zygote. The zygote then undergoes rapid successive mitotic divisions resulting in a ball of cells that differentiate into the three germinative layers which are the ectoderm, mesoderm and endoderm. This is referred to as the embryoblast which is now ready for implantation. Implantation This takes place approximately 5-6 days after fertilisation. The young embryo

secretes enzymes that helps to ‘digest’ the endometrium such that it burrows into the wall of the uterus. Extra-embryonic membranes are quickly formed that penetrate the endometrium forming long projections called villi. These villi come into very close contact with the maternal circulation, forming a firm disc of connective tissues called the placenta. The Placenta This is composed of maternal and chorionic villi that are interwoven and intermeshed such that they form a thick disc of tissue. The placenta continues as the umbilical cord which contains the umbilical artery and vein which carry oxygenated blood to and from the foetal heart from the maternal circulation. Functions of the placenta: 1. Anchorage of the foetus to the uterus. 2. Surface for gas exchange between foetus and mother. 3. Diffusion of glucose and other nutrients from mother to foetus. 4. Removal of urea and other metabollic waste from foetus to mother. 5. Passage of antibodies from mother to foetus. 6. Filtration of noxious agents and pathogens from entry into the foetus. 7. Production and secretion of Oestrogen, Progesterone and other hormones that maintain pregnancy. It is important to appreciate the fact that although the foetal and maternal circulation come into very close contact the blood never mixes. The two circulations are functionally separate.