Hormonal control of the menstrual cycle
The menstrual cycle is coordinated by glycoprotein hormones, released by the
anterior pituitary gland and by the ovaries. The anterior pituitary gland secretes
follicle stimulating hormone (FSH) and luteinising hormone (LH), which control the
activities of the ovaries. During the 28-day cycle, follicles develop, which secrete the
steroid oestrogen. After the female gamete is released from the ovary at ovulation,
the remains of the follicle secrete progesterone, another steroid hormone.
In the ovary, one follicle becomes the dominant one. The presence of FSH and LH
stimulates the secretion of oestrogen from the cells surrounding the follicle. The
presence of oestrogen in the blood has a negative feedback effect on the production
of FSH and LH, so the concentrations of these two hormones decrease. The oestrogen
stimulates the endometrium to grow, thicken and develop numerous blood
capillaries.
When the oestrogen concentration in the blood has risen to around two to four times
the level at the beginning of the cycle, it stimulates a surge in the secretion of LH and
to a lesser extent, FSH. The surge of LH causes the dominant follicle to burst, usually
14-36 hours after the surge. This follicle then collapses to form a corpus luteum,
which secretes progesterone and some oestrogen. These maintain the lining of the
uterus, preparing it to receive an embryo if fertilisation has occurred. Progesterone
also inhibits the anterior pituitary gland from secreting FSH so no more follicles
develop.
High levels of oestrogen and progesterone in the second half of the cycle inhibit the
secretion of FSH and LH so there is less stimulation of the corpus luteum and it can
begin the degenerate and secrete less oestrogen and progesterone. As the
concentration of these two hormones decreases, the endometrium is not maintained
and menstruation begins. The decrease also releases the anterior pituitary gland
from inhibition, so FSH is secreted to begin another cycle.
The birth control pill
The pill contains steroid hormones that supress ovulation. They are usually synthetic
hormones as they are not broken down so rapidly in the body and can act for longer.
The pill keeps oestrogen and progesterone levels high to supress the secretion of FSH
and LH and stop concentrations reaching high enough to stimulate ovulation.
Stopping taking the pill after 21 days allows the concentrations of oestrogen and
progesterone to fall to the point at which the uterus lining is no longer maintained
and menstruation occurs.
Pills containing only progesterone allow ovulation to take place by reduce the ability
of the sperm being able to fertilise the egg by making the mucus secreted by the
cervix more viscous so less easily penetrated by sperm.
The morning after pill works up to 72 hours after intercourse and contains synthetic
progesterone to reduce the chances of the sperm fertilising an egg.
The menstrual cycle is coordinated by glycoprotein hormones, released by the
anterior pituitary gland and by the ovaries. The anterior pituitary gland secretes
follicle stimulating hormone (FSH) and luteinising hormone (LH), which control the
activities of the ovaries. During the 28-day cycle, follicles develop, which secrete the
steroid oestrogen. After the female gamete is released from the ovary at ovulation,
the remains of the follicle secrete progesterone, another steroid hormone.
In the ovary, one follicle becomes the dominant one. The presence of FSH and LH
stimulates the secretion of oestrogen from the cells surrounding the follicle. The
presence of oestrogen in the blood has a negative feedback effect on the production
of FSH and LH, so the concentrations of these two hormones decrease. The oestrogen
stimulates the endometrium to grow, thicken and develop numerous blood
capillaries.
When the oestrogen concentration in the blood has risen to around two to four times
the level at the beginning of the cycle, it stimulates a surge in the secretion of LH and
to a lesser extent, FSH. The surge of LH causes the dominant follicle to burst, usually
14-36 hours after the surge. This follicle then collapses to form a corpus luteum,
which secretes progesterone and some oestrogen. These maintain the lining of the
uterus, preparing it to receive an embryo if fertilisation has occurred. Progesterone
also inhibits the anterior pituitary gland from secreting FSH so no more follicles
develop.
High levels of oestrogen and progesterone in the second half of the cycle inhibit the
secretion of FSH and LH so there is less stimulation of the corpus luteum and it can
begin the degenerate and secrete less oestrogen and progesterone. As the
concentration of these two hormones decreases, the endometrium is not maintained
and menstruation begins. The decrease also releases the anterior pituitary gland
from inhibition, so FSH is secreted to begin another cycle.
The birth control pill
The pill contains steroid hormones that supress ovulation. They are usually synthetic
hormones as they are not broken down so rapidly in the body and can act for longer.
The pill keeps oestrogen and progesterone levels high to supress the secretion of FSH
and LH and stop concentrations reaching high enough to stimulate ovulation.
Stopping taking the pill after 21 days allows the concentrations of oestrogen and
progesterone to fall to the point at which the uterus lining is no longer maintained
and menstruation occurs.
Pills containing only progesterone allow ovulation to take place by reduce the ability
of the sperm being able to fertilise the egg by making the mucus secreted by the
cervix more viscous so less easily penetrated by sperm.
The morning after pill works up to 72 hours after intercourse and contains synthetic
progesterone to reduce the chances of the sperm fertilising an egg.
