Biological complexity: mammalian pregnancy is a war (Introduction)

by David Turell @, Saturday, November 30, 2019, 22:35 (221 days ago) @ David Turell

The problem with placental pregnancy is that the Mother and the fetus are different animals with different antigens causing immune problems, with different needs, and it is a designed battle royal:

The cells of the human endometrium are tightly aligned, creating a fortress-like wall around the inside of the uterus. That barrier is packed with lethal immune cells. As far back as 1903, researchers observed embryos ‘invading’ and ‘digesting’ their way into the uterine lining. In 1914, R W Johnstone described the implantation zone as ‘the fighting line where the conflict between the maternal cells and the invading trophoderm takes place’. It was a battlefield ‘strewn with… the dead on both sides’.

When scientists tried to gestate mice outside the womb, they expected the embryos to wither, deprived of the surface that had evolved to nurture them. To their shock they found instead that – implanted in the brain, testis or eye of a mouse – the embryo went wild. Placental cells rampaged through surrounding tissues, slaughtering everything in their path as they hunted for arteries to sate their thirst for nutrients. It’s no accident that many of the same genes active in embryonic development have been implicated in cancer. Pregnancy is a lot more like war than we might care to admit.


The situation becomes a tug-of-war. Some genes fall silent, while others become more active, counterbalancing them.

That insight led Haig to found the theory of genomic imprinting, which explains how certain genes are expressed differently depending on whether they come from your father or your mother. Armed with this theory, we can see how conflicts of genetic interest between parents play out within the genomes of their offspring.

Because both parental genomes drive each other to keep ramping up their production of powerful hormones, should one gene fail, the result can be disastrous for both mother and infant. Normal development can proceed only as long as both parental genotypes are correctly balanced against one another. Just as in a tug-of-war, if one party drops its end, both fall over. This is one reason why mammals cannot reproduce asexually, and why cloning them is so difficult: mammalian development requires the intricate co-ordination of paternal and maternal genomes. A single misstep can ruin everything.


The invading placental cells paralyse the vessels so they cannot contract, then pump them full of growth hormones, widening them tenfold to capture more maternal blood. These foetal cells are so invasive that colonies of them often persist in the mother for the rest of her life, having migrated to her liver, brain and other organs. There’s something they rarely tell you about motherhood: it turns women into genetic chimeras.

Perhaps this enormous blood supply explains why primates have brains five to ten times larger than the average mammal. Metabolically speaking, brains are extremely expensive organs, and most of their growth occurs before birth. How else is the fetus to fund such extravagance?


Fascinatingly, the intensity of the invasion does seem to correlate with brain development. Great apes, the largest-brained primates, seem to experience deeper and more extensive invasion of the maternal arteries than other primates. In humans – the largest-brained ape of all – placental cells invade the maternal bloodstream earlier even than in other great apes, allowing the foetus unprecedented access to oxygen and nutrients during early development. (my bold)


The second major consequence of the foetus’s direct access to maternal nutrients is that the foetus can also release its own hormones into the mother’s bloodstream, and thus manipulate her. And so it does. The mother counters with manipulations of her own, of course. But there is a strong imbalance: while the foetus freely injects its products into the mother’s blood, the mother is granted no such access to foetal circulation.

Comment: Note the bold paragraph. One can certainly see the advanced preparation in the great apes for the oncoming humans. The energy needs for the giant brain started in the small brained apes, by God's preplanning. Read the whole article. I've presented only a tiny portion of the battles that rage and the designed complexity that was involved in the back and forth between fetus and mother. And please remember the other birth problem that had to be solved: a growing baby head in humans requiring a bigger opening in the mothers pelvis and the two involved individuals are genetically different adversaries. Choosing a chance mechanism to develop all of this is not reasonable.

Complete thread:

 RSS Feed of thread

powered by my little forum