The placenta is a new organ. Primitive mammals like the duck-billed platypus do not have one. So it is not surprising that nature is still experimenting and the placenta of a cow looks nothing like that of a rabbit. There are differences too in how the placenta works - although it goes without saying that every placenta succeeds in fulfilling its role as a link between mother and fetus. Sufficient nourishment and oxygen are transferred from mother to baby across the placenta. The communication is not one way, however, as the placenta sends hormones to the mother’s blood that help sustain her pregnancy and prepare her to feed the baby after birth. Yet even the hormones produced by the placenta are different between, say, a rat and a human. Another thing about the placenta is that it is made up largely of trophoblast – a tissue derived from the embryo. So half of its genes come from the father and it ought to be seen as an organ transplant (the precise term is semi-allograft). Why is it not rejected by the mother’s immune system? The question was posed long ago by Sir Peter Medawar, who won the Nobel Prize for Physiology and Medicine in 1960. We still do not have the answer but it is clearly not the same for every species of mammal.
Given all these differences in structure, function and immune defence, the question arises of how the placenta evolved. When I first wrote on the topic twelve years ago, the response was quite gratifying. It seemed many of my fellow scientists had been asking the question for years. It was not apparent from their published papers because this is not the kind of thing that attracts research grants. Understanding placental evolution will not save lives; nor is it as sexy as the Higgs boson.
This blog will be a bit about the placenta, a bit about evolution and quite a bit about mammals, including obscure ones like my favourites the Malagasy tenrecs.
Thank you Pamela for persuading me to start it.