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Women's sexual anatomy: Female sex hormones and the reproductive cycle

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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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