BIOLOGY OF THE MENSTRUAL CYCLE
The menstrual cycle or reproductive cycle refers to the cyclical process a woman undergoes that prepares her for conception, spanning from the beginning of one menses to the beginning of the next menses. Various changes occur in the female reproductive tract that culminates in the flow of menses in the absence of pregnancy at the end of each menstrual cycle. Because the most important event that occurs in each menstrual cycle is ovulation, observation of one’s menstrual cycle may suggest how often one ovulates. Below are the characteristics of a normal or ovulatory menstrual cycle.
A normal menstrual cycle is one whose cycle length, defined as the number of days from the beginning of one menstrual cycle to the beginning of the next menstrual cycle, is between 21 days to 35 days, with a menstrual flow of about 2 days to 8 days and the difference between the shortest cycle length and the longest cycle length is not more than 5 days within a period of six months.
BIOLOGY OF THE MENSTRUAL CYCLE
The Hypothalamo-pituitary-ovarian (HPO) axis consisting of the hypothalamus, the pituitary gland and the ovaries work together as a single unit with one part of the unit influencing the others by the production of various hormones. To simplify this, the hypothalamus produces GnRH which stimulates the pituitary gland to produce the hormones FSH and LH. These two hormones in turn at different part of the menstrual cycle stimulate the ovaries to produce oestrogen and progesterone respectively. However, high level of progesterone hormone inhibits or prevents the production of GnRH by the hypothalamus and the production of LH and FSH by the pituitary gland. Low levels of progesterone on the other hand increase the production of both LH and FSH by the pituitary gland. Contrastingly, low level of oestrogen hormone reduces the production of the hormone LH by the pituitary gland while its high level stimulates the production of LH but reduces the production of FSH by the pituitary gland. Basically these interactions are responsible for the smooth cyclical process of the menstrual cycle all working in concert to achieve ovulation and in the absence of pregnancy, menstruation.
Each cycle, as soon as the menstrual flow begins, due to the low level of oestrogen and progesterone hormones, the pituitary gland secretes FSH which stimulates the growth of a cohort of immature follicles each containing an immature oocyte, in response to the increased GnRH release by the hypothalamus. This is known as the follicular phase of the menstrual cycle. As the follicles get bigger, one follicle outgrows the other to become known as the dominant follicle, and produce more oestrogen the rest. The resulting high level of oestrogen results in a negative effect on the production of the hormone FSH by the pituitary gland. Because the growth of the follicels depend on the FSH level, all but the dominant follicle in growth and subsequently die off, the dominant follicle continuous to grow bigger and secrete more oestrogen.
Following menses and shedding off of the old endometrial lining of the last menstrual cycle, the high level of oestrogen produced by the follicles in the follicular phase stimulates the growth of a new endometrium in preparation for another pregnancy and causes an increased in the production of the cervical mucus especially towards the middle of the menstrual cycle. This is to enable the sperms to swim freely in the cervix towards the released ovum during ovulation. This phase of the menstrual cycle is referred to as the proliferative phase of the endometrium. The watery cervical mucus which peaks towards ovulation is used as many as a sign of imminent ovulation. Toward the middle of the cycle the high level of oestrogen stimulates the production of LH which in turn stimulates the production of progesterone. The initial low level of progesterone and the high level of oestrogen further stimulate the pituitary gland to produce a surge in the release of the LH which then induces ovulation. This surge is the basis of ovulation prediction kits which tests for imminent ovulation by testing for the sudden rise in LH in urine samples. The LH surge results in ovulation approximately 36hrs later by an inflammatory process that results in bursting of the dominant follicle causing the release of a mature oocyte or ovum which is then picked up by the fallopian tube in anticipation of fertilization by a sperm.
Afterwards, the dominant follicle having extruded the oocyte becomes the corpus lutuem, a cystic structure with yellow fluid which secretes high level of progesterone and oestrogen. The high level of progesterone acts on the endometrium, stopping its excessive upward growth, converting it into one, which secretes and stores up granules rich in food and nutrients in anticipation of a pregnancy by a process called decidualization. This phase is known as the secretory phase of the endometrium. It also changes the texture of the cervical mucus, making it scanty and thick, preventing any further ascent of sperms and possibly infection into the uterus. It also causes changes in the body such as breast tenderness, change in mood and other features seen in premenstrual syndrome. The progesterone level peaks at about 7 days before the next menses and forms the basis for the measurement of progesteroneassay as a test of ovulation. This phase, which spans from the period after ovulation till the beginning of the next menses, is known as the luteal phase of the menstrual cycle and is averagely constant at about 14 days in all women in the absence of menses.
In early pregnancy, the corpus luteum is responsible for the production of progesterone necessary for decidualization and maintenance of endometrium and hence pregnancy in the first 3 months until the fetal placenta takes over the production of the progesterone hormone. The fetus on the other hand via its trophoblastic cells produces the hormone HCG which is responsible for continuous stimulation and maintenance of the corpus luteum. In the absence of pregnancy, the corpus luteum gradually disintegrates and dies off due to falling level of LH and the absence of HCG, eventually removing the progesterone responsible for maintenance of the secretory phase of the endometrium from circulation. This results in menstruation, which is the sloughing away of the superficial layer of the dead endometrial tissue. This phase is referred to as the menstrual phase of the endometrium. As the progesterone hormone and oestrogen hormone falls, the negative effect of these two hormones on the hypothalamus and the pituitary gland are released and another cycle is initiated by a rise in GnRH secretion by the hypothalamus and FSH production by the pituitary gland. This cycle goes on and on until a pregnancy ensues or menopause is reached or an hormonal contraceptive that prevents ovulation is used.