Women's
sexual anatomy: Female sex hormones and the
reproductive cycle
Related
pages:
The
female reproductive cycle is a highly
complex sequence of monthly changes in the
ovaries and the uterus orchestrated by
different hormones, all of which influence
each other. Each menstrual cycle takes
roughly one month to complete, or more
exactly between 24 - 35 days with an average
of 28 days. Although the cycle isn't the
easiest thing to understand, it's really
worthwhile having a look at it to help make
more sense of female fertility,
contraception, and the emotional and
hormonal changes that occur as the cycle
progresses in women. The following
description is that of an average monthly
cycle in a woman of child bearing age who is
not pregnant. (This will make a lot more
sense to you if you first read the section
on Female
sexual anatomy: Uterus, Fallopian tubes and
ovaries.)
The
two main reproductive locations where
changes take place over a cycle are the
ovaries and the uterus, so we can talk about
an ovarian cycle and a uterine cycle. The
brain, or more specifically the hypothalamus
and the anterior pituitary gland in the
mid-brain, orchestrate the whole cycle by
releasing hormones which influence the
ovaries and the ovaries' hormone production.
Other areas of the female body which are
influenced by the monthly reproductive cycle
are the breasts and the cervix of the
uterus.
Let's
get down to the details of it. We start off
with the hypothalamus in the brain, which
secrets gonadotropin releasing hormone (GnRH).
This hormone is released into the blood
stream and is taken up by the anterior
pituitary gland just downstream from the
hypothalamus. GnRH stimulates the pituitary
gland to release two hormones,
follicle-stimulating hormone (FSH) and
luteinizing hormone (LH). Both of those
hormones are transported in the blood stream
to the ovaries, where they have different
effects.
FSH
initiates the development of follicles,
which are the precursors to the female
reproductive cell, the egg. As the follicles
grow and develop they start to release
another hormone, estrogen, which is the most
important female sex hormone. LH released by
the pituitary further stimulates the growth
of the follicles and their production of
estrogen. In addition, it initiates
ovulation (see below) and the growth of the
corpus luteum after ovulation; this in turn
starts to produce further hormones, mostly
estrogen and progesterone.
Estrogen
is one of the two main female sex hormones;
it's produced by the follicular cells in the
ovaries of a woman. Estrogen functions in a
similar way to testosterone in men in that
it supports and maintains the secondary
female sex characteristics such as the
distribution of fat in the breasts, hips and
buttocks (which is what produces those
wonderful womanly curves). It also
influences the pitch of the voice and the
distribution of hair over the body. Estrogen
stimulates the production of proteins and
lowers blood cholesterol levels because it's
actually produced from cholesterol by the
body. As with the negative hormonal feedback
loop described
in men, moderate levels of estrogen in
the blood influence the hypothalamus and
anterior pituitary gland and suppress their
release of GnRH, LH and FSH hormones.
Progesterone
is the second major sex hormone in women. It
is secreted by the cells which constitute
the corpus luteum, the structure that the
cells in an ovarian follicle change into
once their egg cell has been released during
ovulation. Progesterone works together with
estrogen to stimulate the inner lining of
the uterus so it grows a thick layer of
tissue which supports the fertilized egg
cell. Progesterone also works on the breasts
to prime the mammary glands for milk
secretion. High levels of progesterone have
a similar negative feedback loop to moderate
levels of estrogen, suppressing the release
of GnRH and LH hormone from the hypothalamus
and the anterior pituitary gland.
So
far so good. If you are still with me on
this you have earned yourself a big
"well done"! Unfortunately, it now
gets more complicated as we look at how the
different elements start to influence each
other to produce the monthly cycle in a
woman's body.
The
female reproductive cycle has four phases:
the menstrual phase, the pre-ovulatory
phase, ovulation itself, and the
post-ovulatory phase. Simply by convention
the first day of menstruation marks the
first day of a new cycle. During
menstruation the thick inner lining of the
uterus is shed resulting in tissue and blood
being discharged through the vagina. This
inner lining was built up during the
previous menstrual cycle in preparation for
an egg being fertilized: if it had been, it
would have come to rest in the thickened
tissue lining the womb, in which it would
then have grown into a baby if all went
well. If fertilization does not happen, the
thick inner lining is not needed and is
therefore discarded by the body.
Menstruation normally lasts on average 5
days.
During
menstruation, the anterior pituitary gland
releases FSH and LH. This initiates the
first stage of growth of about twenty
follicles in the ovaries.
The
second phase in the reproductive cycle is
the pre-ovulation phase, which starts at the
conclusion of menstruation and ends with
ovulation. This phase is the most variable
in length, a variation which results in the
differing lengths of the reproductive cycles
of different women. During this phase the
follicles in the ovaries continue to develop
under the influence of FSH from the
pituitary gland. Each follicle starts to
produce and release estrogen. After about
six days the follicle which has grown faster
than the others establishes its dominance
over the others and releases more estrogen
and another hormone known as inhibin, which
starts to suppress the other follicles.
These eventually die off.
The
one dominant follicle matures and is now
called a Graafian follicle. It starts to
release more and more estrogen, which is
linked into a positive feedback loop with LH
from the pituitary: this means that high
levels of estrogen in the blood
increase the production of LH in the
pituitary.
During
this time the lining in the wall of the
uterus starts to thicken for a possible
pregnancy. The tissue doubles in thickness
and expands its surface area by growing
protrusions with a rich blood supply.
Now
the woman's body is ready for ovulation. The
positive feedback loop of LH means that
concentrations of this hormone rise more and
more as the Graafian follicle releases more
estrogen. High levels of LH finally
stimulate ovulation, the point at which the
Graafian follicle ruptures, releasing its
egg cell into the outside space from where
it is swept into the Fallopian tube next to
the ovary. Once the follicle ruptures and
the egg cell is released the remaining cells
of the follicle enlarge and form a corpus
luteum in the ovary.
Now
we're at the last phase of the cycle. The
post-ovulation phase starts with ovulation
and ends with the onset of menstruation.
During this time the corpus luteum becomes
the main hormone factory in the ovary. It
starts to produce estrogen and progesterone
which further stimulate the uterine wall to
thicken. However, the corpus luteum only has
a life span of two weeks, after which it
shrinks and dies. This is more than enough
time for an egg cell to be fertilized and to
travel down the Fallopian tubes - a journey
which in fact takes about seven days. As the
corpus luteum produces less estrogen
and progesterone, the production of GnRH,
FSH and LH in the brain start to rise again,
which brings us back to the beginning: GnRH,
FSH and LH stimulate the growth of a new
batch of follicles, which compete to become
dominant. Additionally, as the corpus luteum
dies and the production of estrogen and
progesterone falls, the uterine wall no
longer has any signal to sustain its
existence, so it starts to degenerate. We
are then back at the visible end of the
cycle known as menstruation.
If
an egg is fertilized it anchors itself
inside the thick folds of the uterine walls.
As it starts to grow, the embryo releases a
hormone called human chorionic gonadotropin
which stimulates and sustains the corpus
luteum in the ovary. This whole set-up
continues for nine months after which a new
human being is born...if all goes well.
Other
related topics:
Female
sexual anatomy: Vulva, vagina and the G-spot
Female
sexual anatomy: Uterus, Fallopian Tubes and
Ovaries
Source
for the facts cited in this page:
ABC
of Sexual Health (2005) Second edition
edited by John M Tomlinson, British Medical
Journal Books and Blackwell Publishing.
Principles
of anatomy and physiology (2000) Ninth
edition by Gerard J. Tortora and Sandra
Reynolds Grabowski. Publishers: John Wiley
and Sons
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