The menstrual
cycle is the female reproductive cycle that makes eggs
available for fertilization. Menstruation involves the
hypothalamus, pituitary and ovary, and in humans lasts roughly
28 days (this can vary greatly between individuals).
The menstrual cycle can be divided into approximately two
halves:
the follicular phase. In the follicular phase,
the biochemical and physiological events (secretions of LH,
FSH and estrogen) support the growth of the follicle
(the cell complex that surrounds and nurtures the egg) in
the ovary and build up the lining of the uterus to receive a
fertilized egg. At mid-cycle, when the egg is ready, a
chemical signal -- a surge of LH and FSH -- tells the ovary
to release the egg. This stage is called ovulation,
and is the optimum time for fertilization to take place.
the luteal phase. In the luteal phase, the
remnants of the follicle (corpus luteum) keep
secreting estrogen and progesterone to maintain the
readiness of the uterus. If the egg is fertilized, then the
luteal phase continues throughout pregnancy. If the egg is
not fertilized, then the corpus luteum dies, the uterus
sheds its lining, menses begins and another menstrual cycle
ensues.
Animation of the physiological and biochemical
changes in the menstrual cycle
Here is a very detailed look at the events that occur
during the menstrual cycle:
Menses (approx. Day 0 to 5) Typically, the
menstrual cycle begins with the onset of menses,
bleeding and loss of the tissue lining of the uterus, which
lasts from 5 to 7 days (variable among women). During
menses:
The levels of estrogen and progesterone in the blood
from the previous cycle begin to drop to their lowest
levels.
Small episodic pulses of GnRH from the hypothalamus
lead to small pulses of LH and FSH from the pituitary. LH
and FSH stimulate several follicles (each
containing an egg cell) to develop in the ovaries.
The follicles secrete estrogen. The follicle that
develops first (dominant follicle) secretes greater
amounts of estrogen than the others, and this hormones
suppresses the development of the other follicles. The
dominant follicle develops outer layers of cells, called
granulosa cells, and a fluid-filled space called
the antrum. It also continues to secrete estrogen.
Follicular Phase: Part I (approx. Day 6 to 10)
The primary follicle continues to develop and grow
larger. The antrum gets bigger, and pressure develops
inside the follicle. The follicle secretes estrogen at a
faster rate.
Estrogen levels in the blood rise. The rising estrogen
levels will eventually signal the brain that the egg is
ready to be released.
The rising estrogen levels stimulate growth in the
tissue and blood vessels that line the uterus. The uterus
lining gets thicker in preparation to receive a fertilized
egg.
LH and FSH levels remain low but steady, supporting
the development of the follicle.
Follicular Phase: Part II (approx. Day 11 to 14 )
The primary follicle continues to develop, grow larger
and is about ready to be released. The antrum grows, and
pressure is near its highest level inside the follicle.
The follicle continues to secrete estrogen at a faster
rate.
Estrogen levels are at their highest, signaling that
the egg is ready to be released.
LH and FSH levels remain low but steady, supporting
the development of the follicle.
The uterine lining continues to grow in response to
stimulation by estrogen. The mucus plug in the cervix
becomes thin and watery.
Ovulation: Primate vs.
Non-primate
In all
mammals except primates (humans are primates),
fully-developed eggs sit in the ovaries at certain
times of the year, waiting for the brain to signal
ovulation. The brain assesses information such as
temperature and day length to determine when to send
the signal. At ovulation time, the female enters a
period of estrus, or heat, and mates
with one or more males.
In primates, the ovary tells the brain when it is
ready to ovulate and mate. Primates can ovulate
monthly all year round.
Ovulation (approx. Day 15) The high estrogen
levels have signaled the brain that it is time to ovulate.
The hypothalamus secretes massive amounts of GnRH,
stimulating the anterior pituitary to secrete massive
amounts of LH and FSH (mid-cycle LH/FSH peak). This
surge lasts only 24 hours.
The LH and FSH stimulate enzymes
in the follicle. The enzymatic reactions, along with the
pressure of the antrum, release the egg from the follicle.
The egg travels into the Fallopian tube, where
fertilization can take place. The egg can survive for
about 12 to 24 hours after ovulation. The remainder of the
follicle, the corpus luteum, stays in the ovary.
The corpus luteum secretes estrogen and progesterone.
The lining of the uterus approaches its thickest and
is ready to receive a fertilized egg.
The cervical mucus is at its thinnest.
This is
the optimum time for fertilization to occur.
Luteal Phase (approx. Day 16 to 28) The corpus
luteum degenerates.
The egg gets swept along by gradual wave-like motions
of the finger-like projections in the walls of the
Fallopian tube.
LH and FSH levels fall back to their low, steady
levels.
Estrogen levels fall somewhat after the LH/FSH surge,
but rebound due to continued secretion of estrogen and
progesterone by the corpus luteum.
The uterine lining remains thick and ready to host a
fertilized egg or growing embryo.
The corpus luteum shrinks and begins to die. It is
programmed to die in 14 days unless it receives
stimulation from human
chorionic gonadotropin (hCG), a hormone secreted
from a growing embryo.
If fertilization occurs, and the embryo implants in
the lining of the uterus, hCG rescues the corpus luteum
and it continues to secrete estrogen and progesterone
throughout the pregnancy.
If fertilization does not occur, the corpus luteum
dies at the end of the luteal phase. The unfertilized
egg also dies and passes out of the uterus when menses
begins. Estrogen and progesterone levels fall, menses
starts and the uterine lining is shed. A new cycle
begins.