by Anjali Malpani
Every school child knows that you need eggs and sperm to make a baby. However, we need to examine the basics in greater detail , so let’s start by taking a guided tour of the reproductive system.
How does a woman's reproductive system function ?
The sexual and reproductive organs on the outside of the body are called the external genitals. There are three openings in the genital area. In front is the urethra, from where urine comes out; below this is the opening to the vagina which is called the introitus ; and the third is the anus from where a bowel movement leaves the body.
The outer genital area is called the vulva. The vulva includes the clitoris, the labia majora and the labia minora. The most sensitive part of the genital area is the clitoris. This is a pea shaped organ that's full of nerve endings since its only purpose is to provide sexual pleasure. The clitoris is protected by a hood of skin, and is the equivalent of the man's penis.
The labia majora, or outer lips, surround the opening to the vagina. They are made of fatty tissue that cushions and protects the vaginal opening. Between these outer lips are labia minora, or inner lips. These are sensitive to sexual pleasure. As they are stimulated, they get deeper in color and swell.
The vagina is a muscular tunnel that connects the uterus to the outside of the body. It provides an exit for the menstrual fluid; and an entrance for the semen. Normally flat, like a collapsed balloon, the vagina can stretch to accommodate a tampon, a penis or a baby's head. The walls of the vagina are muscular, smooth and soft. The vagina is a closed space which ends at your cervix.
The uterus, or the womb, is the place where the fertilized egg grows and develops into a baby during pregnancy. The uterus lies deep in the lower abdomen - the pelvis - and is just behind the urinary bladder. It is a hollow organ shaped like a pear and is about the size of the fist. Inside the muscular walls of the uterus is a very rich lining - the endometrium, and it is in this lining that the fertilized egg implants. If pregnancy does not occur, the lining is shed along with blood as the menstrual flow.
The neck of the uterus is called the cervix. It connects the uterus to the vagina and contains special glands called crypts that make mucus which helps to keep bacteria out of the uterus. The cervical mucus also helps sperms to enter the uterus when the egg is ripe.
The two fallopian tubes ( also known as oviducts) are attached to the upper part of the uterus on either side and are about 10 cm long. They are about as big as a piece of spaghetti . Each tube forms a narrow passageway that opens like a funnel into the abdominal cavity, near the ovaries. The ends of the fallopian tubes are draped over the two ovaries and they serve as a passageway for the egg to travel from the ovary into the uterus. The tube is lined by millions of tiny hairs called cilia, that beat rhythmically to propel the egg forward. Of course, the tube is not just a pathway - it performs other functions too, including nourishing the egg and the early embryo in its cavity. Also, the sperm fertilizes the egg in one of the fallopian tubes.
The two almond-sized ovaries are perched in the pelvis, one on each side, just within the fallopian tubes' grasp. The ovary serves two functions: the production of eggs and the secretion of hormones. Each month, at the time of ovulation, a mature egg is released by an ovary. This is "picked up " by the fimbria and drawn into the fallopian tubes.
The eggs in the ovary are stored in follicles (from folliculus, meaning sack in Latin). These cellular sacks contain the eggs; as well as granulosa cells and theca cells which nurture the egg , and produce the female hormones. The ovary has about 2 million eggs during fetal life. From that point onwards, the number of eggs progressively decreases, till only about 300,000 eggs are left at the time of birth - a lifetime's stock. During the fertile years fewer than 500 of these eggs will be released into the fallopian tubes - once in each menstrual cycle. Unlike the testis which is continually churning out billions of new sperm, the ovary never produces any new eggs. One of the existing eggs is matured for ovulation each month - and this limited supply runs out at the time of menopause.
Figure 1. Female external genitalia
Figure 2. The female reproductive system
Can you explain the menstrual cycle and its role
in fertility ?
The aspect of the reproductive system that women are most aware of is the menstrual period which they have every month. The menstrual cycle is the time from the beginning of one period to the beginning of the next one. Usually menstrual cycles last about 28- 35 days, though anywhere from 3 to 6 weeks is considered normal .
During the menstrual cycle, the uterus gets ready for pregnancy. Under the influence of the hormones estrogen and progesterone, its lining grows rich and thick to prepare for the fertilized egg. If pregnancy doesn't occur, the uterus must get rid of this lining so that it can grow a new one in the next cycle. The old lining passes out of the uterus through the vagina as the menstrual flow.
The menstrual flow thus consists of:
- the shed uterine lining
- blood (this comes from the blood vessels which are torn when the lining is shed)
- the degenerated unfertilised egg
If the menstrual flow is heavy, there may sometimes be clots in it. Sometimes the uterine lining is shed as large fragments - and these may sometimes looks like bits of pregnancy tissue to some women, who think they are miscarrying.
Many infertile women are obsessed with their menstrual periods, and they worry about every little variation – whether it’s too dark, too light, too much or too little. However, remember that the menstrual flow has no connection to your fertility and you should not be too concerned about variations, which are quite common and of little significance.
How do a woman's hormones control her fertility ?
Reproduction is like an orchestra - and the reproductive organs need to be synchronised to perform at just the right time for them to work properly. It is the fertility hormones which play the conductor's role.
Hormones are chemicals the body makes to carry messages from one part of the body to another . There are two major female hormones - estrogen and progesterone - which are produced by the ovaries.
The cycle of ovarian hormone production has two phases. In the first half called the follicular phase, estrogen plays a dominant role. During this phase the egg matures inside the ovary in its follicle. The egg; the surrounding cells (which nurture the egg and are called granulosa cells and theca cells); and the fluid (called follicular fluid) which accumulates in progressively larger amounts during this phase, is called a follicle. The follicle secretes a large amount of estrogen (produced by the granulosa cells) into the bloodstream, and the estrogen circulates to the uterus where it stimulates the endometrium to thicken.
The second phase of hormone production begins at ovulation, midway through the cycle, when the follicle changes into the corpus luteum. This produces estrogen ; and also large quantities of progesterone throughout the second half of the cycle. Travelling through the bloodstream to the uterus, progesterone complements the work begun by estrogen by stimulating the endometrium to mature and making it possible for a fertilized egg to implant in it. In case pregnancy does not occur, production of estrogen and progesterone falls 10 to 14 days after ovulation as the corpus luteum dies, and the endometrium is shed from the body as the menstrual period.
How is the release of hormones regulated by the body ? This is a complex self-regulating system, which uses negative feedback control loops, much like a thermostat for an oven does. As the temperature increases, the thermostat shuts off the heater to reduce its heat output. When the temperature falls below the thermostat's setting, the thermostat signals the heater to turn up the heat again, thus maintaining the desired temperature. A similar signaling relationship exists between the pituitary gland and the ovaries in women; and the testes in men . For example, as the concentration of gonadotropins in the blood rises, this signals the woman's ovaries to increase hormonal output of estrogen. In turn, when the blood levels of estrogen rise , the pituitary gland slows its release of gonadotropins, thus maintaining the desired equilibrium.
Fig 5. A schematic of the hormonal changes during the menstrual cycle. The interplay of the pituitary and ovarian hormones regulate the changes which occur in the uterine lining.
How does a man's reproductive system work ?
The male reproductive system begins in the scrotum, the sack behind the penis. This contains two testicles, which make men's sex cells, called sperm; and the male sex hormone, called testosterone. The testicles feel solid, but a little spongy, like hard boiled eggs without the shell. They hang from a cord called the spermatic cord. It's normal for one testicle to hang lower than the other; and for one testicle to feel slightly larger than the other.
The testicles make sperm best at a temperature a few degrees cooler than normal body temperature. This is why nature designed a scrotum - so that the testes can hang outside the body to keep them cool.
The testicles start making sperm when a young man reaches puberty. This is in response to the male sex hormone, testosterone , which starts being produced at this time. The testes keep making sperm for the rest of the man's life.
The testes have two components, the seminiferous tubules, where sperms are produced, and the "interstitium" or the tissue in between the tubules, which contain the Leydig cells which produce the male sex hormone, testosterone, which causes the male sexual drive.
Most of the testis is composed of the tightly coiled microscopic seminiferous tubule, which if uncoiled would reach a length of 70 cm. The sperms are produced inside the seminiferous tubule, and these converge and collect into a delta (like the mouth of a river) near the upper part of the testis called the rete testis which then empties through a series of very small ducts out of the testis towards the epididymis. The epididymis is an amazing structure - it is a very long tiny tubule ( about 5-6 meters long), which runs back and forth in convolutions and loops to form a tiny compact structure with a head , body and tail that sits like a cap on the top of and behind the testis . The tail of the epididymis then leads to the vas deferens - a thin cord like muscular tube, which is part of the spermatic cord and which ends at the ejaculatory duct in the prostate. Here is joined by the seminal vesicle ducts and they all open into the prostatic part of the urethra - which in turn leads to the urethra in the penis.
Mature sperm take about 75 days to develop in a process called spermatogenesis which takes place in the seminiferous tubules. The primordial germ cells in the testis, called the spermatogonia , which are "immortal" stem cells , divide repeatedly to form primary spermatocytes. These undergo meiotic ( reduction) division to form secondary spermatocytes, which differentiate to form spermatids , which then ultimately mature to form spermatozoa. Sperm production takes place as though it were on an assembly line - with the more mature sperms being passed along toward the center of the tubule from where they swim towards the efferent ducts of the testis towards the epididymis. The spermatogenic cells are supported and nourished by large cells called the Sertoli cell, which help to support sperm maturation. This can be a very "temperamental" assembly line - things often go wrong, causing low sperm counts.
When the sperm leave the testis, they are not yet able to swim on their own. They acquire the capacity to do so in their passage through the epididymis - which is like a swimming school for the sperm. They spend between 2 to 15 days here during which they attain maturity and fertilising potential. Sperm are propelled along this tunnel by frequent contractions of its thin muscular wall. Most of the mature sperm are then stored at the end of the epididymis - where they wait to be rushed through the vas deferens and ejaculated at the time of orgasm.
What happens during ejaculation ?
During ejaculation, the epididymis and vas deferens muscles contract to propel the sperm into the ejaculatory duct. Here the sperm is joined with the secretions of the seminal vesicles and prostate gland (which contribute the bulk of the seminal fluid) to form the semen. The powerful muscles surrounding the base of the urethra then cause the semen to squirt out of the penis at the time of orgasm. Semen and urine never mix in a healthy male (even though the final passage for both is common) because the bladder sphincter muscle contracts during sexual stimulation, thus closing down the exit from the bladder to the urethra during ejaculation - preventing urine from leaking forward out of the bladder during sex and also preventing semen from accidentally going backward into the bladder.
What about the penis and fertility? Most men equate their fertility potential with their virility - and therefore the size of their penis. However , the size of the penis has little to do either with fertility potential or with sexual ability. (In any case, if you worry that your penis is too small, you're not alone - most men think their penises are too small!)
During ejaculation, about one teaspoon of semen spurts out of the penis. Semen is a milky white color, the consistency of egg white. Sperm account for only about 2 to 3% of semen. Most of it consists of seminal fluid - the secretion of the seminal vesicles and the prostate gland, which provide a vehicle for the sperm into the vagina.
A normal ejaculation contains 200 to 500 million sperm. How can so many sperm fit into only a teaspoon of semen ? Simple - sperm are very tiny. If one average ejaculation filled an Olympic size swimming pool, each sperm cell would still be smaller than a goldfish. Sperms are the smallest living cells in the human body - and the egg the largest. Basically, sperms are designed so that they can deliver their contents - the male genetic material - to the egg. This is why they are designed like projectiles - the male DNA is in the chromosomes in the sperm head nucleus, and the tail propels the sperm up towards the egg.
Sperm are also very fragile. Men make so many because very few survive the swim through the female reproductive system to fertilize an egg. Perhaps the reason for this is an evolutionary hangover . Female fish deposit eggs on the sea-bed . This is why male fish need to produce millions of sperm which are sprayed into the sea water where millions will be wasted in order to ensure that some reach the eggs.
What happens to the sperms if you don't have sex for many days? Unfortunately, you cannot "store up" sperms. If ejaculation does not occur for many days, the sperms in the reproductive ducts simply die. This is why a sperm count done after many days of abstinence shows a high number of dead or immotile sperms. But just like you cannot store your sperm, you cannot run out of sperm either - masturbation and sex cannot use sperm up. The body keeps making sperm as long as a man has even one normal testicle.
Figure 3. The male reproductive system - front view
Figure 4. The male reproductive system - side view
Figure 5. A section through the testis and epididymis
How does testosterone affect male fertility ?
As already mentioned, the main male sex hormone is testosterone and this is made by the testicles, starting at puberty. Testosterone is produced by specialized cells in the testis called the Leydig cells. These are stimulated to release testosterone in response to the LH signal from the pituitary . LH is luteinizing hormone - the same hormone found in women.
In addition to testosterone, the production and maturation of sperm in the seminiferous tubules of the testis is stimulated by FSH produced by the pituitary gland - and this FSH is identical to that found in women. FSH acts on the Sertoli cells to cause them to secrete androgen-binding protein, which binds testosterone and facilitates its action on sperm production. The Sertoli cells also produce growth factors such as SGF ( seminiferous growth factor) which help to regulate spermatogenesis.
Note that there are two separate components in the testis - and that the Leydig cells are outside the seminiferous tubules where the sperms are manufactured. This explains why there is no relation between virility (which depends upon testosterone production) and fertility (which depends upon sperm production).
Testosterone does more than just allow men to make sperm. It also triggers the growth of facial hair, the deepening of men\'s voices, and the development of a male physique - all the changes which make boys into men. Testosterone is also important in creating desire for sex - it increases libido.
What happens to the sperm once they enter the woman\'s vagina ?
The sperm\'s odyssey in the female reproductive tract
A million million spermatozoa,
All of them alive;
Out of their cataclysm but one poor Noah
Dare hope to survive.
-- Aldous Huxley
When a man and woman have sexual intercourse, the man places his erect penis inside the woman\'s vagina. Here it releases millions of sperm when ejaculation occurs. Once the sperm have been deposited here they have a long and arduous journey ahead of them, like salmon entering the mouth of a river to swim upstream to spawn.
Some of the sperm swim straight up into the fallopian tubes through the cervix and uterus - and some of them are so fast, that sperms have been found in the tubes in as little as a few minutes after ejaculation. Some sperms die in the acidic vaginal fluid; and some enter the cervical mucus and cervical crypts. They are stored here and can remain alive here for as long as 48 to 72 hours.
During this time, the sperms are released in small numbers and these continue to swim towards the fallopian tubes. This is why you don\'t need to have sex every day to get pregnant even though the egg remains alive for only 24 hours.
Sperms in the female reproductive tract swim under their own steam - as a result of the whip- like activity of their tail which propels them on. Of the millions of sperms released in an ejaculate, only a few hundred will make the arduous trip upto the egg successfully. Perhaps this is why so many millions of sperms are produced in the first place even though only one is needed to fertilize the egg - because the wastage is so prodigal.
What happens to the egg when conception occurs ?
What about the other partner in this mating dance, the egg ? Remember that a mature egg is released from the ovary ( this process is called ovulation) only once during the menstrual cycle. This is the \"fertile time\", during which a pregnancy can occur.
How does the egg reach the tube ? When ovulation occurs, the mature egg is released from the follicle in the ovary. This process of follicular rupture looks a bit like a small volcano erupting on the ovarian surface. At this time, the tubal fimbria, like tentacles, sweep over the surface of the ovary, and actually \"swallow\" the egg.
The egg has a shell, called the zona pellucida, which looks like the ring around Saturn. It is surrounded by a cluster of nest cells called the corona cells which serve to nurture the egg. They form the cumulus oophorus which is a sticky gel which protects the egg and also helps the beating of the hair-like cilia of the fallopian tube to propel the egg towards the uterus - like a conveyor-belt. The egg must now wait in the protective confines of the fallopian tube, for a sperm to swim up and reach it. An egg remains alive for about 24 hours, and if fertilization does not occur, it dies.
Of the few hundred sperm which reach the egg, only one will successfully fertilize it. The process of fertilization is truly the primeval mating dance - the fertilization tango - when the mother\'s chromosomes (in the egg) and the father\'s chromosomes (in the sperm) fuse together to create a new life - one which is totally different from all others, because of its unique genetic composition. We have now learnt quite a lot about fertilization thanks to in vitro fertilization (IVF) - and it is truly one of Nature\'s miracles.
During the time the sperm spend in the female reproductive tract, while swimming towards the egg, they acquire the capacity to fertilize it - a process called capacitation. When the sperms reach the corona cells (only a few hundred successfully make the trip, guided by chemicals produced by the egg which serve as guiding beacons to the sperms) they become hyperactivated - they start beating their tails in a frenzy. This is useful because it provides the mechanical energy the sperm head needs to burrow its way through the outer shell of the egg called the zona.
The sperms disperse the cumulus oophorus (and so far it\'s a team effort ) and when they reach the egg, they first bind to the zona. A chemical is released here by the sperms in a process called the acrosomal reaction in which the acrosome (which sits like a cap on the head of the sperm and behaves much like a battering ram) is removed. The acrosomal enzymes dissolve the zona pellucida by making a tiny hole in it, so that one sperm can swim through and reach the surface of the egg. At this time, the egg transforms the zona to an impenetrable barrier, thus preventing other sperm from entering it.
The genetic material of the sperm (the male pronucleus) and the genetic material of the egg (the female pronucleus) then fuse - to form an embryo, which then divides into 2 cells. These cells in turn then continue to divide rapidly, producing a ball of cells - the embryo. The embryo then travels through the fallopian tube (which nurtures it and propels it ) into the uterus - a journey which takes about 3 to 5 days. The embryo must then break through its zona ( this is called embryo hatching); and then attach itself to the lining of the uterus in a process called implantation - and in 9 months , if all goes well, a baby is born. top | 
