Читать книгу The Making of You - Katharina Vestre - Страница 8

IN THE HOURS preceding conception, a race begins that is almost impossible to win. A sperm cell starts out on an intense swimming trip. It looks like a little tadpole, battling wildly upstream against the current and in unknown terrain. It has several hundred million competitors. And it must swim a distance more than one thousand times its own body length. The rules are simple: reach the finishing line first, or die.

The landscape around the sperm is confusing and inhospitable, an overgrown forest full of chaotic thickets and blind alleys. It risks being either swallowed up by immune cells or destroyed by acid on the way. It could also end up trapped in one of the deep crevices in the cervical wall. Before long, such hazards have eliminated most of the field, but our competitor is luckier: the woman’s muscle contractions help to push it upwards and it manages to enter the uterus.

It is still a long way from victory. To have any chance of winning, it must first choose where to go next: right or left? The uterus is connected to two narrow channels – the fallopian tubes – and the finish line is at the end of one of them. The walls of each tube are lined with hairs that sweep fluid down to the uterus, but the sperm cell refuses to give up. It struggles on against the flow. Somewhere up there, hidden among the deep valleys and high peaks of the mucous membrane, a round egg is about to meet its champion.

The egg cell has waited a long time for this moment. When the woman was a tiny foetus herself she’d already made the forerunners of her egg cells. Later on she began transforming them into mature egg cells. The cell now floating down her fallopian tube was one of the lucky ones. Each month, several egg cells start maturing inside every fertile female, but only one of them gets the chance to escape. The others face certain death.

To create a mature egg cell, the forerunners divide so that the chromosome pairs containing the genes from the new foetus’s grandmother and grandfather are separated. At the end, each mature egg cell has half a set of chromosomes, some from Grandma and others from Grandpa, ready to find itself a new partner. All the while, the egg cell has been packing itself with nutrients, blowing up like a giant compared with the other cells in the body. It’s actually possible to see the egg cell without a microscope: it has a diameter of around a tenth of a millimetre.

The sperm cell could not be more different. Swimming frantically with its wriggling tail, there’s barely any room for nutrients because its entire head is packed with the father’s DNA. Among the many millions of sperm cells, only one of them carries half of your specific genes; the chances of two sperms being identical are vanishingly small. Had another of your father’s sperm swum just a little faster, you would never have existed as you are now.

When your parents’ sperm and egg cells were formed, the chromosomes from your grandparents sat right next to each other; and before the chromosome pairs were separated from each other for ever, they managed to exchange small pieces of DNA. So a chromosome that was originally from a grandmother can carry some genes from a grandfather when it ends up in the sperm cell. The possible combinations are endless, and so we have to be sure we cheer on the right sperm.

Returning to our race commentary, our frenetic little tadpole is made for what it’s doing right now. It may be blind and deaf, but that doesn’t stop it making its way through a landscape it’s never even been close to before. Among other things, it can sense minute changes in temperature. Because its target is slightly warmer than its surroundings, the sperm can tell when it is getting close. In addition, the sperm is equipped with a basic sense of smell. Just as the cells in your nose do, sperm cells contain molecules called odorant receptors. Each odorant receptor is programmed to recognise a particular molecule. When air flows into your nose, the fragrance molecules become attached to different odorant receptors and create an electrical signal that is sent to the brain. In the case of sperm cells, the odorant receptors catch molecules streaming from the egg, confirming that it is on the right path.

At the finish line there are relatively few competitors left swimming, and the egg’s attractor chemicals make them travel faster than ever. Soon the egg is completely surrounded by minute tadpoles. Their tails wriggle furiously as they drive forward into the jelly-like membrane protecting their goal. From their heads they spray chemical weapons, enzymes that break down the membrane and allow them to burrow even deeper.

But only one of them is fast enough. The winner discards its tail, melts into the egg and releases its valuable cargo: twenty-three of the father’s chromosomes. At the same instant, the egg cell releases substances that create a hard capsule around it so that no more sperm can enter. There’s no time to lose: if two sperm cells were to penetrate the egg at the same time, the result would be a cell of sixty-nine chromosomes instead of forty-six. Although the egg cell does its best to avoid this, it isn’t always successful. When a group of researchers studied artificially fertilised eggs, they found that 10 per cent of them had been fertilised by more than one sperm cell. Eggs like this have no chance of developing normally, and, as we’ll see later on, they are handed a death sentence.

But for now you can relax – this time there was only one winner. The chromosomes from your mother and father are now united and your very first cell has been created. The race is over. The making of you can begin.