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 These are then withdrawn to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.
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.
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.
Note: there was a bunch more to this case than is presented here, and
at some level I should really make the effort to express it. But it was,
frankly, all over the place and not at all susceptible to summarisation.
Acknowledging that the failure is mine rather than the case presenters', I
abdicate that responsibility. Life's too damn short.