germ cells are thus immortal, although that’s an odd formulation – by that definition, we are all immortal simply by virtue of being able (if indeed we are) to produce offspring.
* * *
In the story of how to make a human “the natural way”, the fertilized egg is often portrayed as the end – at least, until the happy day that the baby emerges. All our traditional stories of people-making rely on that quantum leap from fertilization to birth. The dire moral warnings about pregnancy that loomed over adolescence (and in some cultures still do) make this the equation: bring together sperm and egg and you’ll get a baby! It’s a warning (sometimes needed, for sure) to experimenting teenagers, but becomes more like a promise in the narrative of IVF: to make that longed-for baby, all you need to do is unite the gametes. And if it doesn’t turn out that way, something has gone wrong. There is a single and inevitable road from fertilized egg to infant, and anything else is an aberration.
This is misleading. To put it starkly, most acts of non-protected penetrative sexual intercourse do not produce a baby – and when I say most, I mean 99.9 per cent. Even most fertilized eggs do not become babies – about 2 to 3 in 10 confirmed pregnancies abort spontaneously in miscarriage, but even those figures mask the 75 per cent or so of fertilized eggs that never get to the point of registering as a pregnancy at all, either because they don’t develop into a multi-celled embryo or because the embryo fails to implant in the uterus. That’s a puzzling thing about humans: we are unusually poor, within the animal kingdom, at reproducing. You have to wonder whether all the attention we give to sex is because we are so spectacularly bad at getting results from it.
Even to say “bad” is perhaps to collude with the moral imperative of the fertilized-egg-to-infant story; let’s just say that we are an anomaly, for reasons imperfectly understood. This calls into question the idea that sex really is “for” reproduction, as some religious moralists insist. If we were inclined to see procreation as a divine gift and imperative, one would at least need to grant that God expects us to have a heck of a lot of rehearsal.
The baby grows, of course, from a fetus: even children’s books tell us that. But in the common view the fetus is simply a baby – a person – that has not yet fully developed. Its proportions might be a little odd, its limbs blunter, but it is recognizably human. The classic images made in the mid-1960s by Swedish photographer Lennart Nilsson and presented in the book A Child Is Born (1965), have defined the view of our in utero existence ever since. They show the fetus floating freely in space, often lacking even an umbilical cord, like the iconic image from Stanley Kubrick’s 2001: A Space Odyssey three years later. Perhaps this “child” even sucks its thumb. But these images were actually made by artful arrangement of aborted fetuses – they were not in fact living organisms at all, much less in utero. They were curated to tell a reassuring story. (At least, so it might seem until you realize that it’s a story in which the mother has been edited out.)
By the time a fetus looks even vaguely human (which is what, loosely speaking, distinguishes it from an embryo), most of the important stuff has happened. Most of the dangerous hurdles have been cleared. And most importantly, the developing organism is already anthropomorphic, relieving us from any need to grapple with the strangeness of an entity evidently made of cells, which we might want to call human but would struggle to justify that intuition.
Yet it is the early embryo that reveals the true versatility, the genius, of our cells – and the unfamiliarity of the moment when those cells are not merely what we are made of, but what we are.
It might surprise you to discover – it surprised me – that when a woman first has a fertilized egg (a zygote) inside her body she is not technically pregnant. This is not some perverse biomedical fine print; it simply makes no sense to see things otherwise. A pregnancy test would show nothing, nor will it for the first four days or so after fertilization. The zygote divides by mitosis into two, then four, then eight cells and so on, and at this point these cells can form all the tissues needed in the embryo: they are called stem cells and are said to be totipotent.
In other words, every one of these cells could potentially become a separate embryo. In the early days of embryology, that was by no means clear. The German zoologist Wilhelm Roux thought, for example, that cells are headed towards different fates from the first division of the zygote. In 1888, he reported experiments on frog embryos at the two and four-cell stage, in which he destroyed one of the cells by lancing it with a hot needle. A single remaining cell from a two-cell embryo would then, he said, grow into a half-embryo, suggesting that it had even at that stage become assigned as the progenitor of that part of the body plan alone.
But Roux’s method was flawed, because he could not detach the remains of the ruptured cell from the intact one. This debris interfered with the subsequent growth of the embryo. In the 1920s and ’30s, German embryologist Hans Spemann performed a cleaner act of surgery on salamander embryos. By using a noose made from a single hair taken from a baby, he pinched early embryos in two and found that each of the resulting parts is able to grow into a complete embryo.7 In effect, Spemann made identical twins by artificial means. Because he produced two genetically identical embryos from a single initial one, you could also call this a process of cloning.8 Spemann and his co-workers used amphibian cells, because they are so large that the delicate manipulation could be done by hand – albeit an impressively steady one.
The ball of totipotent stem cells that is the human embryo floats freely in the fallopian tube (also called the oviduct), borne slowly towards the uterus. By day five, the embryo has become a ball of around 70 to 100 cells and has rearranged itself into a structure known as the blastocyst. By the time it arrives at the uterus, it has shed the protein coat called the zona pellucida that formed the protective shell of the original egg – it has “hatched” and is ready to implant.
The human embryo at around five days, called a blastocyst.
That ball of cells is not exactly the nucleus of a person. Most of the cells of the blastocyst became the mere housing and life support. Some of them form an outer layer enclosing a fluid-filled void: these are trophoblast cells, comprising the tissue called trophectoderm which will become the placenta. Others congregate into a clump on the inside, called the inner cell mass, which separates into the epiblast from which the fetus will grow, and the hypoblast that will eventually become the yolk sac. The epiplast consists of embryonic stem cells, capable of forming all the tissues of the body (but not the placenta): a capacity called pluripotency. Identical twins grow from two separate inner cell masses in a single blastocyst, whereas non-identical twins grow from two separate blastocysts, formed from distinct eggs fertilized by different sperms. Within a few days of implanting, the epiblast is covered in a layer of specialized cells called the primitive endoderm, derived from the hypoblast.
The human embryo at around day 10–11.
The fate of the embryo wholly depends on a successful implantation in the lining of the uterus. If this does not happen – which is the case around 50 per cent of the time – the embryo will be expelled in the menstrual cycle. Failure to implant is one of the common reasons why an IVF cycle does not work. No wonder, then, the division of labour in the blastocyst makes it seem that its priority is to those cells surrounding the epiblast, which won’t be a part of the fetus at all. For without implantation, it’s game over.
Implantation is a delicate and complex process involving a dialogue of hormones and proteins between the embryo and the cells of the uterine lining. In some ways it is more delicate and complex than fertilization itself. The placenta, for example, is made not just from the trophoblast layer of the
