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The placenta is a multi-organ kit
The placenta forms in the first few weeks of pregnancy from both fetal and maternal cells, implanting on the wall of the uterus and enabling the fetus to get everything it needs to develop. Though it looks like an amorphous blob of tissue, it carries two separate but intimately connected blood vessel systems that enable the placenta to carry out the job of many organs, acting as a lung, liver, gut, kidney and hormone-producing endocrine system. It filters out waste products from the blood of the fetus, and provides it with antibodies for immune defence. Yet the placenta becomes redundant at the moment of birth, and is rapidly expelled from the mother鈥檚 body in what is known as the third stage of labour. We might be missing a trick, say XPrize founder Peter Diamandis and surgeon Bob Hariri. They think we should freeze placenta cells and use them later in life to stay young and healthy, and they have now set up a company to harness the cells鈥� potential.
The womb is a battleground of the sexes
Half the placenta develops from fetal cells, and so contains a mix of maternal and paternal DNA, whereas the other half originates solely from the mother. This results in a battle of the sexes. According to the 鈥減arental conflict hypothesis鈥�, the father鈥檚 genes enhance his child鈥檚 growth, improving the chance of his DNA being passed on. In contrast, it is in the mother鈥檚 interest to limit fetal growth, decreasing the risks to her and increasing her chance of being able to reproduce again. In the womb, one focal point of this conflict is IGF2, the gene for a growth factor that results in bigger babies. This gene is imprinted, which means that only the father鈥檚 version is switched on; the maternal version is off. However, the growth-inhibiting gene IGFR2 is only switched on if it comes from the mother. The size of the baby is governed by these two competing factors, and the dominance of one over the other results in overly.
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It could lead to new cancer treatments
It鈥檚 not a pleasant analogy, but there are many similarities between pregnancy and cancer. Both are characterised by rapid proliferation, invasion of the host and evasion of its immune system. Normally the mother鈥檚 immune system will respond to foreign cells or tissue 鈥� such as the placenta and fetus, which contain the father鈥檚 DNA 鈥� by releasing chemicals to destroy the invaders. But during pregnancy, this process is switched off. Understanding the mechanisms behind this could help explain how cancer also manages to evade the immune system, with the potential for new cancer treatments.
Placentas allowed mammals to conquer the world
The development of a throwaway organ that allows mammals to carry their developing young around while pumping them full of nutrients and oxygen and keeping them warm was a revolutionary innovation. It allowed mammals to give birth to well-developed offspring which, some biologists argue, is that gave mammals the edge over birds, reptiles and fish in the competition for ecological niches made available by the demise of the dinosaurs. A generous supply of oxygen and nutrients is also needed for the development of the energy-hungry mammalian brain. Mammals have certainly been extraordinarily successful, today occupying every ecological niche on land and in the sea, from the poles to the tropics, and the placenta was a key part of that.
Placentas may be the result of an ancient viral infection
Peer into the placenta and you might be surprised to find that it鈥檚 teeming with viruses. But these HIV lookalikes, called endogenous retroviruses or ERVs, are a normal part of pregnancy and are written into the DNA of every mammal. Indeed, they may have been critical for the . The idea is that, when the ability to give birth to live young evolved some 160 million years ago, it happened because an ancestral mammal made use of one of its ERV parasites to evolve a structure that could provide a connection between the fetus and the mother inside her body. 听Without , we humans might still be laying eggs.
Mammals aren鈥檛 the only animals with placentas
Many lizards and snakes give birth to live young, though few nourish their young via a placenta the way mammals do. The exceptions are a couple of species of skink, a type of lizard. Given the benefits of a placenta, it is surprising they aren鈥檛 more common in the animal kingdom. Perhaps this is due to their downsides. An embryo in close contact with its mother鈥檚 blood risks being attacked by her immune system. Male embryos could also be feminised by the mother鈥檚 sex hormones.
They鈥檙e not essential 鈥� some mammals still lay eggs
When they first evolved, the earliest mammals were egg-laying, but most mammals today nurture their offspring . Not all though. Kangaroos and other marsupials give birth to what is essentially an undeveloped embryo that matures in a pouch. And a couple of egg-laying mammals are still in existence: The duck-billed platypus and some types of spiny anteater, only found in Australia and New Guinea.
They can be star-shaped
The term 鈥榩lacenta鈥� is derived from the ancient Greek for 鈥榝lat cake鈥� due to its flattish disc shape. However, many animals鈥� placentas have different shapes, and irregularly shaped placentas are not uncommon. Cows and sheep have in their uterus, while in dogs and cats the placenta forms a band around the fetus. And sometimes human听 such as a star with 5 or 7 points.
Animals cannibalise their placentas. Should you eat yours?
The new mothers of most mammal species eat their placenta, either as a source of nutrition or to hide the evidence of birth from predators. Humans have historically been the exception.听 However, 鈥減lacentophagy鈥� is increasing in Western societies. Some mothers consume theirs raw, cooked, blended into smoothies or dried and turned into capsules. So should you eat yours? In a word, no. A found no evidence of any benefit, and that the levels of nutrients and hormones found in placenta pills were too low to be of any measurable use to the new mother. The researchers did, however, note that eating placenta tissue could increase the risk of ingesting harmful pathogens.
Read more:听笔濒补肠别苍迟补鈥檚 alarm clock signals when it鈥檚 time for birth to begin;听How the placenta adapts to make mammals a success;听Getting to grips with the 辫濒补肠别苍迟补鈥檚 real health benefits
