In human Female the cycle usually lasts about 28 days. Menstruation begins about 15 days after ovulation and lasts about 2—4 days.
The beginning of the menstrual cycle marks the onset of puberty in human females. The menstrual cycle begins with the casting off of endometrial lining of the uterus and bleeding.
The casting of endometrium can be considered as periodic preparation for fertilization of the ovum and pregnancy.
Under the influence of follicle stimulating hormone (FSH), an ovum and its follicle mature, and the foilicle secretes increased amounts of oestrogen which stimulates the regrowth of the endometrium.
The increased production of oestrogen triggers the production of luteinizing hormone (an example of positive feedback) paradoxically, at the end of the cycle, LH and FSH production decline as a result of the increased concentration of progesterone and oestrogen. The menstrual cycle involves the following phases:
1. Proliferative phase:
This phase is also called follicular phase or stage of repair and proliferation. This is mainly influenced by oestrogens secreted by the follicular cells forming the theca interna.
During this phase repairing of endometrial lining of the uterus (which was sloughed off during menstruation) takes place and a functional lining is reappeared in the uterus to receive the fertilized ovum. This phase extends from the end of menstruation to ovulation and generally involves 10 days.
2. Ovulatory phase:
During this phase no conspicuous changes occur in the uterus endometrium. It occurs midway between two menstrual periods.
During this period ovulation takes place and body temperature rises which remains high until the onset of next menstrual period.
3. Secretory phase:
This phase is under the control of progesterone and oestrogens secreted by the corpus luteum, therefore it is also known as progestational or luteal phase.
It is also known as progravid phase as this phase prepares the endometrium for pregnancy and implantation.
If pregnancy does not retain it is followed by menstruation and so it is called as premenstrual phase.
This phase lasts for about 13 to 14 days and is accompanied by marked hypertrophy of uterine endometrium.
The endometrial glands become complicated and tortuous and the sub-mucoral layer becomes very vascular and edematous. The glandular secretion is greatly increased.
4. Destruction phase:
This phase is also called the stage of menstrual flow and lasts about 4 days. During this phase uterus endometrial lining is sloughed off and bleeding also takes place due to rupture of blood vessels.
This happens only when fertilization is not accompained. When pregnancy does not retain the corpus luteum is replaced by scar tissue, corpus albicans.
All these phases of menstrual cycle are due to shifting balance of the hormones. The follicle stimulating releasing factor (FSH-RF) from hypothalamus triggers the release of FSH from the pituitary.
FSH initiates the development of follicle in the ovary. The cells of follicles synthesize oestrogen, principally estradiol which stimulates the uterine epithelium to proliferate.
When the oestrogen in the blood reaches a certain concentration, it also acts on the pituitary to shut down FSH production and on the hypothalmus to result in the discharge of luteinizing hormone releasing factor (LH-RF) to the pituitary.
The pituitary now releases LH to the circulation. In combination with residual FSH, it results in ovulation.
The LH also causes the cells of follicle to alter, enlarge and fill the old follicular cavity. The structure formed is the corpus luteum (yellow body).
The cells of corpus luteum soon start secreting the progesterone and oestrogen hormones. As the levels of these hormones increase, they inhibit the production of the gonadotropic hormones from the pituitary.
Production of ovarian hormones than drops, as a result of which the support of the uterine lining fades, leading to menstruation.
However, if implantation occurs, the membranes surrounding the developing embryo embedded in the uterine lining contribute to the formation of the placenta and begin to secrete a progesterone-like hormone that finally replaces that produced by the corpus luteum.
This prevents the uterine lining from sloughing and supports pregnancy. The hormones oestrogen and progesterone, produced by the developing placenta, also stimulate enlargement of the breasts.
Prolactin and oxytocin hormones regulate the milk secretion and these are released after the inhibitions of the placental harmones are removed at birth.
When implantation does not occur than in response to low level of ovarian hormones, the level of pituiary gonadotropic hormones begins to rise again, followed by development of a new follicle and a rise in oestrogen as the next monthly cycle begins.