The Epigenetics Revolution

By far the greatest numbers of our class are called placental (or eutherian) mammals. Humans, tigers, mice, blue whales – we all nourish our young in the same way. Our offspring undergo a really long developmental phase inside the mother, in the uterus. During this developmental stage, the young get their nourishment via the placenta. This large, pancake-shaped structure acts as an interface between the blood system of the foetus and the blood system of the mother. Blood doesn’t actually flow from one to the other. Instead the two blood systems pass so closely to one another that nutrients such as sugars, vitamins, minerals and amino acids can pass from the mother to the foetus. Oxygen also passes from the mother’s blood to the foetal blood supply. In exchange, the foetus gets rid of waste gases and other potentially harmful toxins by passing them back into the mother’s circulation.

It’s a very impressive system, and allows mammals to nurture their young for long periods during early development. A new placenta is created in each pregnancy and the code for its production isn’t carried by the mother. It’s all coded for by the foetus. Think back yet again to our model of the early blastocyst in Chapter 2. All the cells of the blastocyst are descendants of the fertilised single-cell zygote. The cells that will ultimately become the placenta are the tennis ball cells on the outside of the blastocyst. In fact, one of the earliest decisions that cells make as they begin to roll down Waddington’s epigenetic landscape is whether they are turning into future placental cells, or future body cells.

We can’t escape our (evolutionary) past

While the placenta is a great method for nourishing a foetus, the system has ‘issues’. To use business or political speech, there’s a conflict of interest, because in evolutionary terms, our bodies are faced with a dilemma.

This is the evolutionary imperative for the male mammal, phrased anthropomorphically:

This pregnant female is carrying my genes in the form of this foetus. I may never mate with her again. I want my foetus to get as big as possible so that it has the greatest chance of passing on my genes.

For the female mammal, the evolutionary imperative is rather different:

I want this foetus to survive and pass on my genes. But I don’t want it to be at the cost of draining me so much that I never reproduce again. I want more than this one chance to pass on my genes.

This battle of the sexes in mammals has reached an evolutionary Mexican stand-off. A series of checks and balances ensures that neither the maternal nor the paternal genome gets the upper hand. We can get a better understanding of how this works if we look once again at the experiments of Azim Surani, Davor Sobel and Bruce Cattanach. These are the scientists who created the mouse zygotes that contained only paternal DNA or only maternal DNA.

After they had created these test tube zygotes, the scientists implanted them into the uterus of mice. None of the labs ever generated living mice from these zygotes. However, the zygotes did develop for a while in the womb, but very abnormally. The abnormal development was quite different, depending on whether all the chromosomes had come from the mother or the father.

In both cases the few embryos that did form were small and retarded in growth. Where all the chromosomes had come from the mother, the placental tissues were very underdeveloped[62]. If all the chromosomes came from the father, the embryo was even more retarded but there was much better production of the placental tissues[63]. Scientists created embryos from a mix of these cells – cells which had only maternally inherited or paternally inherited chromosomes. These embryos still couldn’t develop all the way to birth. When examined, the researchers found that all the tissues in the embryo were from the maternal-only cells whereas the cells of the placental tissues were the paternal-only type[64].