In the womb, your mother has prepared a thick, sponge-like membrane that the vesicle can stick to. Soon after, the vesicle expels a substance that causes the membrane to disintegrate so that it can burrow deeper. At this point, the whole scene resembles a gory horror film. Blood vessels burst. Cells die in their masses. Your ravenous cells feed on the mucosa that seeps out of the womb lining and sprout small roots that attach to your mother’s blood vessels. That is how the placenta begins.
When you are born, the placenta is a slimy, blue-red slab weighing about half a kilo. It is ejected from the mother’s womb just moments after a wriggling, screaming baby, so perhaps it’s not so strange that it doesn’t grab our attention there and then. Chubby arms and tiny little fingers are, after all, more instantly appealing. But in the past, the placenta was highly valued in many cultures. In ancient Egypt it was treated with the utmost care and then mummified. In Korea the placenta of a newborn prince or princess was placed in an ornate jar and buried. Even today, some people think it is a good idea to eat the placenta (Google, ever helpful, recently suggested I search for ‘placenta smoothies’). There are companies that, for a fee, will freeze-dry a placenta and turn it into pills for the new mother to take. Either way, the placenta deserves a little gratitude. For almost nine months this peculiar organ works tirelessly for you. Without it, you would not be alive.
Those tiny roots, however, are just the beginning. Soon the invading cells will paralyse your mother’s blood vessels and rebuild them according to their own needs. Her blood will leak out and fill up spaces in the placenta, and your veins will branch out to reach them, by winding their way through the umbilical cord. Your blood is never actually in direct contact with your mother’s, but a great many substances can pass through the thin walls separating you. Because of this, you get all the oxygen and nutrition you need from your mother, and all your waste material is sent back to her in return. But it doesn’t stop there. You also exchange hormones, and because of this, you and your mother can affect each other’s bodies. The placenta quickly begins to produce a cocktail of hormones, which keeps your mother’s blood vessels open and, among other things, makes her eat more. Furthermore, these hormones make sure that her body prepares itself for pregnancy and breastfeeding.
One of the hormones the placenta cells rapidly begin to make is called hCG. Regular pregnancy tests check a woman’s urine for this hormone. Nowadays it’s simple to take a test at home, but it wasn’t quite so easy in the past. Back then, the doctor would need to sacrifice a mouse to get to the answer. Mice react in a specific way to the hCG hormone, and early pregnancy tests would therefore involve a doctor injecting a mouse with some of the woman’s urine. A few days later, the doctor would kill the mouse and examine whether its ovaries had changed. The method was developed further in the late 1920s, and a few years later rabbits, which proved to be quicker and easier to work with, replaced the mice. The expression ‘The rabbit died’ became synonymous with ‘I’m pregnant’, although the animal bit the dust no matter what the test result was. More effective pregnancy tests – ones that did not involve animals – didn’t become available until the 1960s.
Women have developed strict vetting systems so that not just anyone can set up home in their bodies. Only if the vesicle can confirm itself by sending the correct signal will it be allowed to stay put. It’s possible that only about a third of the vesicles make it past the checkpoint, perhaps even fewer. Many pregnancies end without the mother ever realising they’d started. For example, an egg fertilised by more than one sperm cell will never pass this point. The extra chromosomes disrupt the neat web the cell normally weaves as it divides. Some of the cells end up with too few chromosomes, others with too many. If the cells aren’t already in the process of dying, they’re guaranteed to fail the stringent quality control awaiting them now. For them, it’s game over.
If the uterus has not heard otherwise, it will switch to its habitual monthly routine: the mucosa will dissolve and the woman will have her period. New cycle, new mucosa, repeat. It’s a troublesome phenomenon, which most mammals are fortunate to escape. The short list of menstruating animals includes humans, monkeys and (don’t ask me why) some varieties of bat. But why specifically us? Well, we should probably blame our greedy placentas. Most mammals produce a far more secure variant. In horses, cows and pigs, the cell vesicle sits more or less on the surface of the placenta’s mucous membrane; it then winds threads around the mother’s blood vessels without destroying them. This gives the mother a good deal of control over what is transferred to her offspring, and a lower risk of serious bleeding if the placenta should become detached. For humans, on the other hand, it was an absolute necessity to create an emergency brake. Allowing you to move in was potentially fatal for your mother. So you had to ask nicely for permission before you could begin sponging off her.
At this point you may have got the impression that we’re all little better than gruesome, greedy parasites, invading the bodies of our innocent mothers. It’s not exactly a pleasant image, so to correct this, allow me tell you about a fascinating experiment that shows another side of the story. In the darkness of the ocean the jellyfish Aequorea victoria resembles a glowing chandelier, thanks to a luminous green protein that it produces. By injecting a fertilised mouse egg with one of Aequorea’s genes, researchers were able to create male mice with luminous green cells. The scientists allowed these luminous mice to mate with normal female ones, and soon after, the female mice were pregnant. The next thing they did might sound a bit brutal: twelve days later, the researchers gave each pregnant mouse a heart attack. After which they examined the mother mouse’s heart, and noticed something quite incredible: some luminous green cells that could only have originated from the baby mice growing in her womb. It appeared that stem cells from the baby mice had found their way out of the placenta and into the mother’s bloodstream, and upon reaching the heart they had turned into pulsating heart cells to help repair the damage from the heart attack.
The same thing can probably happen with humans too. Interestingly, pregnant women who suffer heart failure are more likely to survive than those who are not pregnant. When a Spanish research group examined the hearts of two women who had suffered from severe heart failure, they found cells that originated from their sons – despite the fact that it had been more than a decade since they were born. Blood tests have also shown that mothers carry cells bearing their child’s DNA for many decades after pregnancy. Researchers have even discovered foreign cells hidden away in the brain. Could there be a tiny bit of you in your mother’s body? A single cell beating in her heart, or chatting away to the other nerve cells in her brain? It’s nice to think that you were at least a tiny bit useful while you were in there freeloading.
NATURAL CLONES AND UNKNOWN TWINS
THESE CELLS THAT burrow and murder their way into the uterine mucosa will never become an actual part of your body. The ones that will become you sit hidden within the cell vesicle. One week after conception, you consist of a bunch of stem cells that can form any body part – they could become heart muscle cells, nerve cells, liver cells or anything else. At this stage they are still so flexible that they can even create more than one body. If the cells were to detach from one another, and form two separate cell clusters instead of one, they might develop into two complete people. This is the most common way that identical twins occur, and since the placenta is already being formed, the twins will have to share it. Alternatively, the cells could have fallen apart a few days earlier, when they resembled a microscopic raspberry, in which case two vesicles will attach to the uterus and two embryos, each with their own placenta, will be created. About one third of identical twins begin like this.
Since identical, or monozygotic, twins originate from the same cell, they possess exactly the same DNA strands – they are natural clones. If one of the twins commits a crime, investigators will be unable to distinguish between them using a DNA analysis. However, if their fingerprints were examined, then the culprit would be revealed. This is because fingerprint patterns are in part shaped by the environment in the womb. The two twins occupy different spaces, and therefore experience different streams and pressures against their fingertips. In addition, because the supply of nutrients from the placenta is not evenly distributed, one can grow slightly faster than the other.