What are some interesting parts of our bodies that evolved over millions of years?
It is the placenta definitely!
The fetus inside his mother is nourished via a structure called the placenta. It is responsible for delivering blood, nutrients, and oxygen to him while taking away the toxic metabolites that the body no longer needs. How did vertebrates evolve to have a placenta?! Placentas probably first evolved about 150 million to 200 million years ago. In fact, without placenta, neither I nor you would have been there! no one ever would have been there! or maybe we would exist as species that still lay eggs!
How did this crazy structure evolve?
before going into the nerdy stuff, let me introduce to you some basic concepts related to the placenta and how it develops that would be useful for you for the rest of your life and for the purpose of understanding this article.
The steps of the development of the placenta can be very complex. I will make it very simple, concise, and in a few words.
We all know that the first step in making us was the fusion of daddy's sperm with mammy's ovum inside the mother’s fallopian tube. Then, this structure moves from the fallopian tube and inserts itself inside the uterine wall.
Here is a zoomed-in picture that shows the fetus (actually it is still a group of cells) inserts inside the uterine wall. The outer layer cells that we see in this picture is called the trophoblast and is the most important as it secretes enzymes that degrade the endometrium (the outer layer of the uterine) to allow for the fetus to implant itself inside the uterine wall.
And here is a photo that shows the exact stages the development before implantation
Then the trophoblast which was one layer will differentiate into 2 sperate groups of cells called the cytotrophoblast and the syncytiotrophoblast and they what make the fetal part of the placenta. They make the fetal blood vessels that take blood from the maternal blood vessels and sends it to the fetus to nourish him. The maternal part is formed by a layer from the mother’s uterine wall. Here is a diagram that illustrates how this happens. If medicine is not your carrier, it would be hard to interpret it, but don’t need at all to interpret it. Just know that cytotrophoblast and syncytiotrophoblast are what make the fetal part of the placenta.
From the previous explanation, we can see that the placenta is a very invasive structure. It invades the uterine wall, takes blood away from the host, and even protects itself from the immune system by a very sophisticated mechanism! Doesn’t this remind you of another organism? hmmm, maybe a virus? wait .. what?
It turns out that the genes responsible for this complexity of the placenta are originally derived from a virus that infected a sperm cell. When this sperm cell fertilized the ovum, the viral genes began to be expressed, and after millions of years, that was the result! A placenta!
But what genes are exactly in the cells of the placenta and what they exactly do?
It is a gene that produces a protein inside the syncytiotrophoblast cells that helps it hide from the immune system. This is very sophisticated mechanism as the mother's immune system would attack the fetus if that doesn’t happen. The protein syncytin, which makes up the physical barrier between mother and baby in the syncytiotrophoblast, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus essentially a "good" virus that helped pave the transition from egg-laying to live birth. A retrovirus that can insert its RNA inside our DNA. It turns out the gene that a previous RNA virus inserted inside our ancestor’s genome was the reason for us to exist!
Why placenta can put us at a disadvantage?
There are some cases in which the placenta can insert the wrong way inside the mother’s uterine wall. It is called placenta increta. It can be very dangerous to the mother as this can lead to bad complications like bleeding, shock, and disseminated intravascular coagulation.
Here is an interesting video that I recommend you to watch!
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