How the female reproductive system works
By Professor Charles Kingsland, Consultant Gynaecologist & Specialist in Reproductive Medicine
FEMALE REPRODUCTIVE ANATOMY
The study of the structure and internal workings of the female reproductive system.
The female pelvis, namely fallopian tubes, uterus, cervix and vagina are formed by the fusion of two separate tubes. The ovaries are formed from a different structure. This happens in the developing baby within the first few weeks and is completed by the eighth week of pregnancy.
Occasionally, this fusion can go wrong and results in defects or incomplete fusion. Therefore, you can get abnormalities of the uterus with varying degrees of division, which at its most severe can result in two of everything down the middle. For example, a woman may have a double uterus or two vaginas, although this is rare, I have seen such occurrences many times over the course of my career.
Many of these anomalies do not cause a fertility problem; the difficulty in achieving a pregnancy. However, they may cause significant difficulty in the ability to carry a baby successfully for the full nine months of a pregnancy. If you look hard enough, most of us have got some anatomical anomaly, so it’s not unusual; indeed it’s very common to have some minor defects or pelvic abnormalities, which are of no significance whatsoever.
If, for example you have failure of fusion at the level of the uterus, then your uterus may have the appearance of having two horns; a bicornuate uterus. This can have an effect of reducing the volume inside the uterus, the endometrial cavity, so the baby cannot grow effectively within the reduced space. Often the pregnancy may be shortened, and the woman will go into labour prematurely.
During the course of investigations for infertility, coincidental abnormalities such as these can be found by chance. When this happens, it is really important that someone can tell you exactly what this means for you and the implications thereof. I have often seen patients who have been informed of some abnormality which is, in reality of no clinical significance, but they carry this information around with them as if it were some burden and a sign of impending doom.
FEMALE REPRODUCTIVE PHYSIOLOGY
The study of normal mechanisms and their interactions, which work within the female reproductive system.
There are two major differences between men and women. Firstly men are sperm factories. They have the ability to make sperm every day of the week. Some days they are good at it, some days not so. It takes about six weeks to manufacture a sperm, they are ejaculated out and then they make some more.
In theory, therefore, men can reproduce up until the day they die, a bit like most other mammals. Although men’s sperm counts go down as they age, by and large men lose their fertility by losing interest where to put the sperm. By the time men reach the age of 50, up to half have a degree of impotence.
Women on the other hand are egg warehouses. They have their full complement of eggs by the 20th week of intrauterine life. That is, a full five months before they are born. By the time a female is born, she has already lost about half the eggs she was given, they just degenerate.
All of the eggs are held in the ovary in an immature state, a bit like uncooked meat pies sitting on a shelf until puberty when they start to mature and be released. Depending on how many eggs a woman is born with, this determines the age at which she loses them unless something happens which destroys them prematurely.
WHAT FACTORS AFFECT YOUR EGGS?
Things that can affect egg numbers are illnesses such as ovarian cysts, endometriosis or more widespread chronic ongoing disorders such as Rheumatoid Arthritis or Crohn’s Disease. Some medications can also affect egg numbers, chemotherapy being the classic example. There are also various chemicals that can affect egg quality with nicotine as egg enemy in chief.
Similarly there is a lot of evidence now that being overweight can have a direct effect on egg quality. The more obese you are, the poorer your eggs tend to be. If a couple come to see me in the fertility clinic, the male’s sperm may be two weeks old, but I will know with certainty the age of the females eggs.
This is why a woman loses her fertility as she gets older. By the time she reaches the age of 40, there is a 50:50 chance she will have run out of viable, fertilisable eggs. “But I’m only 40 Professor. Why am I having so much difficulty getting pregnant?” “It’s exactly because you are 40 Mrs Smith”. There are absolutely no remedies to ageing eggs that are available to women today, irrespective of what some fertility clinics may claim.
Women are unique in the Animal Kingdom because they run out of eggs well before they die. Very few, if any animals have a period of sterility and a reduction in oestrogen production well before they pop their clogs. This time, of course, is known as the menopause, a condition unique to humans and whales, in fact.
So, to summarise, we have sperm factories and egg warehouses. Eggs run out and sperm do not. The maturing and release of eggs are controlled by a sophisticated network of hormones.
Hormones are chemical messengers that are released from one part of the body and exert their action at another distant site by attaching onto tiny areas called receptors.
In the female reproductive organs, there are three main hormone production sites; these are the Hypothalamus, the Pituitary gland and the target organ, the Ovary. They all act in synchrony and are controlled ultimately by the cerebral hemispheres, the big cauliflower looking segments we recognise as the brain in pictures.
The Hypothalamus, is an indistinct tiny area in the depths of the midbrain. It releases a ‘Releasing Hormone’, which activates the Pituitary Gland. This hormone is called Gonadotrophin.
Releasing hormone (GnRH.) It’s a very small hormone that was discovered in 1971, which’s only 40 odd years ago and shows you how recent all this knowledge is. Just imagine, only two or three generations ago we had absolutely no idea what was going on, so much of it was made up.
GnRH is a pulsatile hormone, that is, it pulses and releases 15 micrograms of hormone every 90 minutes. This hormone acts on a receptor in the Pituitary Gland, a bit lower down in the brain.
If I were to drill a hole into the bridge of your nose, I would first of all reach the optic fibres of your eyes, which cross over at that point (the left Side of your brain sees what your right eye is looking at and vice versa). Immediately after I have blinded you, I reach the pituitary gland. This gland looks a bit like a chippolata sausage, the smallest one you’ve ever eaten, and is divided into two lobes, front and back, or anterior and posterior.
The anterior lobe secretes a whole range of hormones that drive the adrenal gland, the thyroid gland and also the breast. Prolactin is a hormone that acts on the breast in pregnancy to bring about milk formation.
The posterior lobe secretes hormones that regulate part of kidney function and also a hormone called oxytocin. This hormone causes milk release when the pregnant woman’s nipple is squeezed by the suckling new-born. It also causes contractions in the pregnant uterus. Isn’t human nature really clever? Just imagine after you give birth and the uterus is bleeding, by breastfeeding your baby, the oxytocin release not only brings about milk production, but also causes contraction in the uterus, thereby slowing and stopping blood loss.
Back to GnRH, the pulses act on the anterior pituitary gland receptors of the cells that produce the hormone that brings about the maturation of an immature egg. The hormone is called Follicle Stimulating hormone (FSH). I tend to think of this as the hand that lifts the meat pie off the shelf and puts it into the oven.
FSH does two other things. Firstly it drives the granulosa cells, tiny fluffy cells that surround the egg, to produce oestrogen and secondly FSH increases the number of receptors on the theca cells in preparation for another hormone, luteinising hormone which will be secreted a bit later. Theca cells are the flat cuboidal cells that sit in the centre of the ovary.
Oestrogen is just about the most powerful hormone known to man. It is an extremely potent anabolic steroid, which makes women what they are. Oestrogen receptors are found all over the female body, in the brain, pelvis, heart and bones in particular. Moreover, Oestrogen is a happy drug. It gives women their drive. If I filled a syringe full of oestrogen and injected it into your bottom (assuming you are a woman) it would probably make you feel good. It would give you the desire to fertilise your eggs.
As oestrogen increases up to the point of egg maturity and ovulation, it makes females more subtly attractive to would-be suitors by making themselves ever so slightly more symmetrical. So, in essence you can actually measure beauty in women.
At this point we need to mention the second very important difference between men and women. We need to go back to Luteinising Hormone at this point. Luteinising Hormone is produced from the front lobe of the pituitary gland alongside FSH. The production of FSH increases the number of receptors on the Theca Cells, remember? LH attaches to these receptors and as a result the theca cells make testosterone, yes testosterone, the male hormone.
This is where the second difference occurs. Women have the ability to convert testosterone into oestrogen and men don’t. In fact, testosterone is a building block for oestrogen. Oestrogen is, therefore, produced by two separate sites in the female. Firstly in the Granulosa cells and secondly by the conversion of testerone in the theca cells. This is known as the two cell theory box oestrogen production and is very very important as I’m sure you understand. There are lots of conditions where this production can go wrong, in particular polycystic ovarian syndrome, which we can discuss in another chapter.
As you can imagine, towards the middle of the cycle, the ovary is pumping out loads of oestrogen which is preparing the body for a potential pregnancy. Not only is its secretion bringing about the changes we have discussed, oestrogen is producing a thick lining of the uterus, the endometrium to prepare the uterus for implantation, but also its making the cervical mucus, the fluid in the neck of the womb, really runny to enable any sperm which may be present in the vagina to swim very quickly and easily through the cervix and into the womb.
At a particular level of oestrogen, the pituitary gland senses the massive circulation levels and in response pushes out a bucketful (well not quite) of luteinising hormone, the so called LH surge. This is coordinated at a time when the egg is mature, and the womb lining ready to collect any egg that’s been fertilised.
The LH surge causes the little bag of fluid in which the egg has been growing to rupture. The egg is literally thrown out of the follicle together with the fluid within the follicle. Sometimes this is accompanied by some bleeding.
If you watch ovulation down a special telescope called a laparoscope that you can place within an abdomen, you can see that ovulation is in fact quite a dramatic event. No wonder it can cause pain, which can often be noticed by patients, so they know when they are ovulating. This pain is known as Mittleschmerz syndrome; pain on ovulation.
If it’s lucky, the egg gets grabbed by the fallopian tube. The end of the tube has lots of finger like processes surrounding it. If you look at the end of the tube, it has the appearance of a sea anemone, waiting within the tummy gently stroking the ovary. It’s easy to see, therefore, how the tiny little egg, which would sit quite comfortably on the sharp end of a pin, slips into the fallopian tube.
In the meantime, the follicle that has released the egg reveals and takes on a yellow appearance. This is then called a yellow body. We Doctors, however, never spurn the opportunity to complicate matters, so we give it a Latin name, Corpus Luteum.
The Corpus Luteum secretes a hormone called progesterone. Again, this is a powerful anabolic steroid, but unlike oestrogen, which is essentially a happy drug, progesterone is a drug of misery. It does not make you feel good. This is because progesterone is essentially, a drug made for babies.
Progesterone’s main role is to convert the lining of the womb, which at this stage is all thick and fluffy, into a sticky and very vascular endometrium in preparation for collecting and beginning to support the growth of any passing embryo.
The Corpus Luteum lives for exactly 14 days. If no pregnancy ensues, the Corpus Luteum dies and withers away. The progesterone levels begin to fall and the sticky endometrium sheds away together with some bleeding, this bleeding forms the period, together with those bloated and irritable feelings that progesterone decline brings about.
If, however, the egg gets fertilised and an embryo develops, the embryo starts to divide and double in size. As it does so, it gets gently wafted down the fallopian tubes by the tiny cilia, those fine hairs that can be so easily damaged. As it rolls down the tube, the embryo sends chemical messages to the endometrium to tell the uterus that it’s on its way.
The embryo enters the cavity of the uterus when it is about 64 cells big. Then one of two things may happen. The embryo may drop out of the uterus altogether or stick into the endometrium and bingo, a pregnancy.
Once the embryo has implanted, it starts to produce a hormone called Human Chorionic Gonadotrophin (HCG). This is the hormone that can eventually be measured in the urine of the pregnant woman and forms the basis of the pregnancy test. HCG has a very important function, it maintains the Corpus Luteum, which then continues to produce progesterone which, in turn, maintains and nourishes the pregnancy. It does this for eight weeks. After that, the pregnancy no longer needs the Corpus Luteum, or progesterone for that matter. The pregnancy can then go it alone, and is quite happy feeding itself.
In theory, therefore, if you remove a Corpus Luteum from a pregnant woman before eight weeks of pregnancy, she will miscarry. If however, you remove the Corpus Luteum after eight weeks, the pregnancy will continue.
You may think that this is a strange thing to say, but occasionally a woman may present to a hospital as an emergency and is found to have an ovarian cyst. The Gynaecologist must make sure that the cyst is not a Corpus Luteum, otherwise if he or she thinks the cyst needs to be surgically removed and they get it wrong, they may have a miscarriage on their hands.
Similarly, medications that oppose the action of progesterone, so called anti-progesterones form the basis of the pill used for medical abortions.