These follicles gradually grow with the resultant release of oestrogen into the blood stream. At around the 10th day one of the follicles becomes distinctly larger than the others. This large follicle is known as the dominant follicle. The other follicle stop growing and become atretic and eventually die out. Oestrogen realeased by the follicle acts on the uterine endometrium making it to proliferate. This signifies the start of the uterine cycle. The dominant follicle reaches 18-20mm in size and ruptures on the 14th -- 16th day of the menstrual cycle. This process is known as ovulation and its signals the end of the follicular phase and the beginning of the luteal phase.
The Luteal phase
Once the ovum is released certain changes take place within the follicle. The follicular cells grow, fat is deposited in them and they assume a yellowish colour. They become known as luteal cells and the whole follicle is known as corpus luteum. These cells of the corpus luteum produce the hormone progesterone which in similarity to oestrogen acts on the uterine endometrium. The levels of these hormones peak around the 24th day of the menstrual cycle. If pregnancy occurs the corpus luteum enlarges in size and produces more progesterone that is used to maintain pregnancy. If pregnancy fails to occur the corpus luteum decreases in size and becomes atretic. Consequently progesterone levels fall. If progesterone is below a certain threshold level it is unable to maintain the proliferated uterine endometrium with resultant shedding and bleeding. This shedding of the uterine endometrium is what is known as menstruation.
The hormonal regulation of the menstrual cycle
The hormone regulation of the menstrual cycle is under control of what is known as the hypothalamic pituitary ovarian axis. The hypothalamus releases a decapeptide known as Gonadotropin releasing hormone or GnRH. This is released into the hypophyseal portal system where it acts on the anterior pituitary (Sherman et al.,1979). GnRH is a neurohoermone and its released in pulsatile nature. The action of GnRH is low during childhood but its activity increases during puberty The anterior pituitary the releases two hormones. The Luteinizing hormone and the follicle stimulating hormone. The LH and FSH are released into the systemic circulation where they act on the target organs. These Hormones are known as gonadotropins because they act on the testes and the ovaries. They are glycoproteins and they are comprised of an alpha and a beta subunit. Its is the beta subunit that is unique to each of these hormones. Luteinizing hormone stimulates the secretion of sex steroids from both the testes and the ovaries. In the testes LH stimulates the synthesis of testosterone. The theca cells of the ovary also produce the production of testosterone but this testosterone is eventually converted into estrogen by the granulosa cells. FSH is important for the maturation of ovarian follicles its is also important for sperm production and maturation by its action on the sertoli cells.
Effects on other body organs
The endocrine glands oestrogen and progestron that are produced by the action of LH and FSH also act on other systems of the body. On the cardiovascular system oestrogen decreases the production of antithrombin III and increases platelet aggregation. This causes a pro-thrombotic effect and may predispose patients to deep vein thrombosis and pulmonary embolism. In the gastrointestinal tract oestrogen is known to increase the production of cholesterol and reduce the motility of the gut. Increase in cholesterol production may predispose the patient to atheroma and thrombosis. On the musckuloskelatal system oestrogen is known to increase the formation of bone and reduce the resorption of bone. Subsequently women who have low levels of oestrogen are at a higher risk of developing osteoporosis. Estrogen is also known to affect lipid and protein production. Oestrogen is known to increase the hepatic production of binding proteins and increases the production of Low density Lipoproteins while the converse is true for high density lipoproteins. Similarly progesterone is known to act on other systems apart from the reproductive system. Its is known to relax the smooth muscle and therefore reduce spasms. On the cardiovascular system its is known to maintain homeostasis with regards to blood clotting. Progestrone is also a natural diuretic a natural anti-depressant that reduces anxiety. It is also stimulates bone formation and is known to normalize zinc and copper levels.
Current research has been dedicated to the investigation of infertility problems.Some of which give no regard for the potential effects of various chemicals such as endocrine disrupters on the female reproductive system. Current research do however suggest that appropriate identification of the appropriate effect as well as the role of the chemicals (endocrine disrupter) may shed some light on the causes of unknown fertility among females (Pitsos and Stamati,2001)
The ovarian research is dedicated the examination of both the basic physiology as well as the biology of the female ovary with the intention of improving the clinical outcomes. This is mainly in the study of the polycystic ovary syndrome. The current research is dedicated to the investigation of the cellular biology. This is with a focous on the ovarian follicle. These researches include:
The investigation of the role played by leucocytes (macrophages) in the ovary as well as the way that they have an interaction with oocytes, theca and granulose layers
The investigation of the mechanism through which polycystic ovary syndrome is exclusively expressed as well as the contribution of that the androgen receptor action has in the process.
Pitsos and Stamati (2001).The impact of endocrine disrupters on the female reproductive system http://humupd.oxfordjournals.org/cgi/reprint/7/3/323.pdf
Sherman BM, Wallace RB and Jochimsen PR. (1979)Hormonal regulation of the menstrual cycle in women with breast cancer: effect of adjuvant chemotherapy. Sherman BM, Wallace RB, Jochimsen PR.
Silva et al. (2003).Calcifications in Ovary and Endometrium and Their Neoplasms.