by Robin Baker
The interface between mucus and semen at the mouth of the cervical trunk is not flat. ‘Fingers’ of semen enter some of the larger channels in the cervical mucus, and grow. They penetrate a short distance into the cervical trunk, stretching upwards into the mucus like the fingers of a rubber glove. Sperm frantically swim into these fingers, and from there stream into the narrower channels of the mucus, leaving the seminal fluid behind. Later we shall follow these sperm further, but for the moment our interest is in the flowback.
After a few minutes dipped into the seminal pool, the cervical trunk begins to shrink back to the roof of the vagina, metamorphosing once more from elephant’s trunk to sea anemone. It loses contact with the seminal pool and hence cuts off the upwards escape route for the sperm. Once the cervix has withdrawn, those sperm still in the pool are condemned to ejection and an early death. About fifteen minutes after ejaculation, the pool begins to decoagulate and becomes more watery again. Soon, imperceptible and unconscious muscular ripples begin to massage the mixture of old semen, mucus, sperm and other cells down the vagina. Eventually, the mixture collects in the vestibule. On average, this happens about half an hour after ejaculation, but can occur as soon as ten minutes or as long as two hours. Before this, a woman can stand up, walk around, even urinate, and she will not eject the flowback. Once the flowback has collected in the vestibule, however, any one of these activities, or even a cough or sneeze, will rid her of the unwanted material. Even if she stays asleep, the flowback becomes so liquid after about two hours that it will eventually begin to seep out anyway, producing the wet sheet.
On average, the flowback contains about half of the sperm that have been introduced – sometimes more, sometimes less. How many depends in part on the severity of the woman’s cervical filter. Quite often (about one in ten occasions) the filter is so severe that she ejects almost all of the inseminate; more rarely, her filter is so weak that she keeps nearly all. Most important of all, the proportion of sperm that she keeps is not due to simple chance. To a large extent, it is under the control of her body – and not just her cervical filter. Each time she has sex, a woman’s body decides how many sperm to keep and how many to eject. How and why, we shall see later. It won’t be long before this female ability becomes very important in the lives of our couple. But not just yet.
SCENE 4
Topping Up
Over the next two weeks the couple became quite active, sexually. The coolness of the woman’s fertile week evaporated. Both partners went through a phase of anticipating and enjoying their sexual activity more than they had for about a year. After making up on the Saturday night, they had sex twice on the Sunday, once in the morning when they first woke and again in the afternoon at about three o’clock. Half an hour later, they even tried again. He had an impressive erection but, despite ten minutes of intermittent thrusting and encouragement, he eventually had to accept that he wasn’t going to ejaculate. Then they missed a few days. Wednesday night was the woman’s weekly night out with girlfriends; Thursday night the man’s weekly night out ‘with the boys’. On both nights, when the reveller eventually crawled into bed, the partner was asleep, or at least pretending to be. On Friday night, however, they had sex and did so again on both Saturday and Sunday. The next week followed a similar pattern, until the woman’s period began on the Saturday morning. Then they abstained until the following Saturday, by which time her menstrual bleeding had finished.
Few couples have their routine sex at absolutely fixed intervals. In the four weeks we have been following this couple, they have had penetrative sex ten times and the woman has been inseminated nine times. But the time interval from one intercourse to the next has varied from as short as thirty minutes (albeit without ejaculation) or seven hours (with ejaculation) to as long as seven days.
Men get a fairly rough deal in this book. Our story will be one of men’s bodies forever trying to make the best of a bad job, while the woman’s body outsmarts and outmanoeuvres them at almost every turn. But this at first sight uninspiring scene does give us an opportunity to watch men doing something fairly impressive. A man may not look particularly sophisticated at the moment of ejaculation, but something remarkable is, in fact, taking place. Each time he has intercourse during routine sex, he introduces no more sperm than are needed to ‘top up’ his partner. How does such restraint help him in his pursuit of reproductive success? To understand what the man is trying to do, we have to follow further those sperm we last saw swimming through the channels in the woman’s cervical mucus.
A small proportion of these sperm, the vanguard, swim straight through her cervix into the womb. Except when she is pregnant, her womb is roughly pear-sized as well as pear-shaped. As with the vagina, the walls press closely together, so there is little space inside. Once in the womb, sperm swim close to the walls and are helped by the womb to reach its top, the widest part of the pear: in effect, they surf-board, carried on the crests of muscular ripples passing along the womb’s walls. At the top of the womb on each side (where horns would be if the pear shape of the womb were a bull’s face) is the opening to a narrow tube, the oviduct. Although there are two oviducts, only one will contain an egg during any given menstrual cycle. Once out of the womb, the sperm swim a short distance along an oviduct until they reach a rest area. Here they cease swimming, settle down, and await developments.
Back in the cervical mucus, another set of sperm swim along more diagonal channels and stream into tiny crypts in the wall of the cervix. These sperm also, once in the crypts, cease swimming, settle down, and conserve their energy. Over the next four to five days they will gradually wake up and re-enter the cervical channel. Then they, too, will complete their journey through the mucus, surf-board through the womb, and head for the rest area in the oviducts.
The final set of sperm simply stay in the cervical mucus. They just sit there, cluttering up the mucus channels. Eventually they die – or are killed. Their lethal assailants are the marauding hordes of white blood cells which are unleashed by the female from the walls of her womb within minutes of insemination. As they advance through the cervical mucus, these killer cells engulf and digest live and dead sperm alike. At their peak the white blood cells can match the numbers of sperm, but within twenty-four hours of insemination these hordes have gone, leaving behind much smaller numbers to complete the mopping-up operation. Multitudinous though the white cells may be, they do not pursue sperm into the cervical crypts.
An average inseminate contains about three hundred million sperm. Of these, the woman will eject about 150 million in the flowback. A few hundred sperm may go straight to the oviducts and about a million may first go into the cervical crypts to form reservoirs, leaving in order to complete their journey to the oviducts over the next five days. In all, about twenty thousand sperm from each inseminate eventually pass through the oviducts. The remainder, those not ejected in the flowback, clutter up the cervical mucus, eventually to be mopped up by white cells or carried by the slow, glacier-like flow of the cervical mucus (Scene 3) back into the vagina.
It might seem exceedingly wasteful to introduce three hundred million sperm when only about a million enter reservoirs. But all is not what it seems. As far as topping up the woman is concerned, the important point is that the size of the reservoirs depends on how many sperm the man introduces. If he puts in only two hundred million, then the reservoirs will be only half as full as if he put in four hundred million.
From every insemination, a man and woman contrive between them to produce a steady passage of fresh sperm through each oviduct over a period of about five days. This traffic probably peaks about one to two days after insemination, then gradually declines as the reservoirs of sperm in the cervical crypts slowly shrink. This is where topping up comes in. If the male can keep the reservoirs of sperm in the cervical crypts topped up, he can ensure a continuous passage of fresh sperm to the rest areas in the oviducts. If he injects more sperm than are needed, they will be wasted; the reservoirs w
ill overflow and even more sperm will simply hang around in the cervical mucus and fall prey to the female’s white cells. There is also a danger that too many sperm will arrive in the oviduct and, with a surfeit of the chemicals that they carry on their heads (Scene 7), actually kill any egg that may be present. On the other hand, if the man doesn’t fill the reservoirs enough, too few sperm will arrive in the oviduct or the reservoirs will dry up prematurely. The man’s challenge is to adjust the number of sperm he introduces according to the number needed to top up his partner’s reservoirs. This he seems to do with remarkable precision.
The adjustments we are talking about are roughly as follows. If more than a week has passed since the man last inseminated his partner, her reservoirs will be empty and he will introduce a full load of sperm, say four hundred million. Of these, perhaps just over a million will fill the reservoirs. If the gap is only three days, he will introduce about two hundred million, and half a million will top up the half-empty reservoirs. If the gap is only three hours, he will introduce about thirty million and if only a few minutes, he won’t introduce any. Even after half an hour, he may find it difficult to ejaculate, like the man in our scene. His body is effectively saying that there is no point. His partner’s reservoirs are full and any sperm ejaculated will simply be wasted.
Our couple have had penetrative sex ten times in the four weeks we have watched them. In that time, about three thousand million sperm will have passed from one to the other. So precise is the man’s ability to top up his partner that if they had doubled their rate of intercourse, or if they had halved it, it would have made scarcely any difference to the total number of sperm she would have received.
So men’s bodies inject only the number of sperm needed to top up their partner. How do they do it? To answer this, we need to understand more about both the architecture of the male reproductive tract and the mechanics of ejaculation.
Imagine that you are a doctor, sitting on a chair, and a naked man is standing in front of you waiting to be examined. His genitals are at your eye level. Note his navel, his pubic hair, and the penis hanging down, slightly askew, in front of the scrotal sac containing his pair of testes. Hold his flaccid penis in the palm of your right hand and, if he has one, make sure the foreskin is pushed back. The rather swollen knob at the end of the penis is called the glans and, right in front of you, is the vertical, slit-like opening of the urethra. Through this, the man both urinates and ejaculates. Fix the line of his urethra in your mind. It runs in a straight line from the opening slit, back through the penis shaft, into his body and up to join his bladder. Fix your eyes at the top of his pubic hair and imagine you can see inside to where the urethra and the bladder join. Just below this point, the urethra is joined from left and right by two tubes. These tubes run all the way down to the testes and each one contains, in effect, a column of sperm. Where these tubes join the urethra, they are surrounded by a walnut-sized mass of tissue. This is the prostate gland, which produces the bulk of the seminal fluid.
So where do these two columns of sperm come from? Even as the man stands in front of you, his testes are a hive of activity. Inside, cells are multiplying, growing and finally maturing into sperm. By the time the sperm are mature and fit to be ejaculated, they have already been herded into the single column of sperm from their testis. They are in the sperm tube, but they are still in, or rather on the surface of, the testis. The sperm tube changes in character along its length: in the testis the tube is called the epididymis, and from the testis up to the urethra it is called the vas deferens. Whereas the vas deferens is more or less straight, the epididymis is incredibly zigzag and convoluted.
Once the sperm are in the epididymis they are, in effect, simply queuing up to be ejaculated. Each time the man ejaculates some of his sperm, the rest shunt forward. As part of the front of each queue is lost through ejaculation, newly mature sperm join the back of the queue in the testis. Very approximately, it takes two months for a sperm to develop and travel from deep inside the testis and join this queue. Each sperm will then spend a further two weeks queuing in the epididymis and up to a further five days or so in the vas deferens. There is a little queue-jumping as some young sperm from the back of the queue are shunted ahead of older sperm at the front, but this need not concern us here.
Now, let’s follow what happens to our medical model as he goes off with his two columns of sperm and has intercourse. While he was standing in front of you he had no sperm in his urethra, but two sperm tubes containing in total up to about a thousand million sperm. Nothing will change when his penis becomes erect, not even during the early stages of penetration and thrusting. Eventually, though, sperm will be shunted out of each sperm tube and into the urethra. A round sphincter that normally prevents urine leaking out of the bladder also prevents sperm from entering the bladder. The man’s urethra is now loaded, ready to fire.
While loading, the man will feel a pleasurable urgency at the base of his penis. He will also know that ejaculation is imminent. Just how imminent is, to a limited extent, under his conscious control. When he finally ejaculates, seminal fluid pours from his prostate into his urethra. Then muscles contract and the mixture of fluid and sperm is projected in a series of spurts along the urethra and out into the woman.
Now, it is not difficult to understand how the man’s body controls the number of sperm he ejaculates. Depending on how many of his loading muscles work, and how strongly, he can shunt any length of each of his sperm columns out of their tubes and into his urethra. Even after loading, his body can change its mind. By varying the number of spurts, usually from between three and eight, he can ejaculate a different proportion of the loaded sperm. Any sperm and seminal fluid left in his urethra after ejaculation can be flushed out at the next urination.
Of course, somewhere there needs to be a link between the man’s brain, which keeps track of when he last inseminated his partner, and his genital musculature. Given such a link, it is not difficult to see how his body can top up his partner so accurately. Nobody suggests, of course, that a man has conscious control over sperm number. In the midst of thrusting and at the point of loading his urethra, he does not ask himself consciously, ‘Is this a hundred-million occasion or a four-hundred-million?’ His subconscious mind and body have done this for him. When the moments to load and ejaculate arrive, his various body parts respond accordingly. This leaves his conscious mind free to concentrate on his thrusting – and the woman.
SCENE 5
Conception
It is Friday night and twenty-one days have passed since the beginning of her last period – bleeding that signalled a second month without conception. Briefly, the couple had feared for their fertility. They were calmed, however, both by the experiences of friends and by the discovery that a couple must have unprotected sex for a whole year before they qualify for medical investigation. With these reassurances, they had put the month behind them and started afresh. Now, they have just had sex and are drifting off into post-coital sleep. Tonight, she won’t wake to go to the toilet and her flowback will wet the sheet.
In the past two weeks, their sex life had more or less followed the routine they had established since giving up contraception. Sex on Saturday, Sunday and sometimes, as this week, also on Friday had become the norm. This week, though, had been slightly different. On Wednesday night, as soon as she had got into bed after returning from her weekly night out, she had stroked his body until he awoke and had played with his penis until he was erect. Then, she had sat astride him and manhandled his penis into her vagina. He had slipped in easily, because she was very wet, and from that moment she had done all the work. Slowly he had begun to enjoy himself. They weren’t very practised at having sex with her on top, and a couple of times he had slipped out. She had had to work quite hard but eventually he had ejaculated. Tonight, they had tried the same position but this time, for some reason, it just hadn’t worked. Eventually, they had reverted to their usual missionary routine.
As she drifted into sleep, things were happening inside her body that would change her life for ever. She had ovulated earlier that evening and her egg was just reaching the place in her left oviduct where fertilisation would occur. As it arrived in the fertilisation zone, three fertile sperm arrived simultaneously. They began to burrow through the outer layers of the egg. Two of them were delayed for a few seconds as they bumped into each other, attempting to penetrate the same point in the egg’s defences. The prize of fertilisation went to the third sperm, which had a clear run, unhindered by any others. By the time another sperm arrived only a few seconds later, the egg had put up its barriers and there was no way in. The egg had been fertilised by the first sperm to get through. Three months after giving up contraception, the woman had conceived.
In twenty days’ time, an overdue period would prompt her to carry out a pregnancy test. Two hundred and fifty days after that she would give birth. The identity of the father, however, would never be known. For the sperm waiting in her oviduct that Friday night had in fact been from two different men.
There is one final phase to the odyssey of fertilisation – a phase that can be critical in a man’s and a woman’s pursuit of reproductive success.
In previous scenes, we have followed sperm from their earliest life in the testes to the moment they are shot into the woman’s vagina. We have watched them attempt to escape from the seminal pool, migrate through the narrow channels of cervical mucus and eventually make their way to a rest area in an oviduct. There we left them, waiting for the final stage of their journey.
Sperm may rest in an oviduct for as long as a day and, at any one time, up to a few thousand may be resting and waiting. One by one, they wake up and swim further along the oviduct. Their initial destination is a region in which, if an egg is present, fertilisation can occur. But usually no egg is present, and the sperm simply pass through and eventually die.
