Case 11

While a human ovum is relatively harmless on its own, fertilisation transforms it into a ravening invasive parasite whose activities wouldn't look out of place in an Alien movie.¹ The zygote divides repeatedly, producing a relatively homogenous blob called the morula that then differentiates to form the blastocyst. This latter consists of an inner cell mass or embryoblast, which will go on to become the fœtus, and a surface layer called the trophoblast, which will undertake a ground assault on the maternal uterus and form the bulk of the placenta.

The developing embryo attaches itself to the uterine wall, and the trophoblast cells invade the endometrium, burrowing long tentacles or villi into the mother's flesh. These entwine with the maternal blood supply but do not directly connect to it. Instead, nutrients, gases and waste products are exchanged across the villous surface. The villi also secrete hormones to modify the behaviour of the maternal tissues.²

Traditionally, people thought this apposition between the two circulatory systems occurred immediately, with the fœtus consuming all the resources it could get as soon as it could get them. However, it turns out that the process is more delicately controlled than that. For the first trimester, the trophoblast grows plugs into the intervillous space to restrict the flow of oxygen, an excess of which would be harmful to the developing embryo. These are then withdrawn to allow higher levels of oxygenation as the baby grows.

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1 Obviously, since we all owe our lives to this process we prefer to think of it in rather more positive terms. Still, if any other entity took over our bodies this way we'd be horrified.
2 The process isn't purely one-way: the endometrium also secretes hormones to stop the trophoblasts from growing out of control and invading the entire uterus.