[ ♪ Intro ♪ ]
We usually make pregnancy sound like a wonderful,
9 month period of parental bliss.
But if you really think about it, pregnancy
is kind of bonkers.
When you’re pregnant, you’re essentially
growing an alien parasite inside of you, having
your entire body tweaked and drained to support
something that’s half you, and half some
other person.
Biologically, it’s quite a feat.
And yet, it happens all the time.
You, me, and every human on Earth, came from
this strange quasi-parasitic system.
So, today, we’re going in utero.
We’ll talk about how embryos develop, how
they get their food and get rid of their waste,
and do all this while staying under the radar
of their parent’s immune system.
At the center of it all is the placenta.
It’s an organ — a set of tissues dedicated
to a particular task.
And it’s actually the first organ that you
make, and the only temporary organ in the human body.
Once a baby pops out, the placenta comes out
as afterbirth.
Which, to be honest, looks a little like something
from the movie Alien.
Since nobody needs the placenta anymore, it’s
usually thrown away.
These days, though, eating it has also become
a trend, even though researchers haven’t
found any health benefits.
But this weird organ is what’s been making
all 9 months of pregnancy possible!
The placenta is made from cells from the fetus
and the pregnant parent.
And it basically serves as an interface between
the two.
It helps deliver food, dump waste, and exchange
gases like oxygen and carbon dioxide that
are critical for life.
The placenta begins to form within a week
or so of fertilization.
In case you need a recap, this is when the
sperm meets an egg to form a zygote, or fertilized egg.
The zygote divides a few times as it travels
to the uterus, or womb, and becomes a semi-hollow
ball of cells called a blastocyst.
The blastocyst is important because it’s
made of two types of cells.
There’s a clump of cells inside, which will
go on to make the embryo and eventually the fetus.
And there’s a single layer of cells on the
outside, known as trophoblasts, which will
form the fetal part of the placenta.
These outer cells invade the lining of the
uterus to make the whole thing stick, or implant,
and truly kick off pregnancy.
Different types of trophoblasts then go on
to form the amniotic sac, which is really
just a protective bag of fluid the embryo
floats in.
It’s thin, but super tough, and provides
cushioning and room to grow.
Inside the amniotic sac, the embryo gets to
work quickly, starting to develop its nervous
system, and then its heart and blood vessels,
followed by other organs.
Driving all this growth is food, thanks to
what’s essentially a big pool of parental blood.
Yep, it turns out that embryos are kind of
like vampires.
See, early on, the invading trophoblasts destroy
a bunch of the tissue in the uterine wall
and remodel the blood vessels.
Trophoblasts attack vessels from the inside
and outside, turning them into limp, open
pockets of blood.
Those pockets help make sure that there’s
a steady flow of parental blood rich in oxygen
and nutrients like glucose, amino acids, and
fatty acids — all of which the embryo needs
to grow.
Those chemicals get passed on to the embryo
through a branching network of blood vessels
that travel through the placenta and feed
into the umbilical cord.
But there’s always a membrane between parental
blood and fetal blood.
The blood pools are close enough for nutrients
and gases to get passed back and forth through
the membrane — usually through diffusion,
which is when molecules move from higher concentration
to lower concentration.
But they’re still separate.
Because direct contact could cause problems…
like total annihilation of the embryo.
We’ll get to more of this later, but because
a baby is half another person, its cells are
full of proteins that the pregnant parent
doesn’t recognize as their own.
Direct contact would mean the immune cells
in the parent’s blood would mount an attack
to get rid of the half-foreign being — like
it would stomp out a flu virus.
To make sure that doesn’t happen, we evolved
a placental system that always puts a membrane
in between the blood supplies.
So, through processes like diffusion, a parent
delivers sugar and other goodies to the baby.
And the baby can dump all of its carbon dioxide
and waste products into the parent’s blood pool.
Garbage problem solved!
Based on all of this, you can probably tell
that pregnancy is kind of blood-intensive.
And one of the biggest changes to someone’s
body, besides making space for a new human,
is to their cardiovascular system.
Blood vessels across their body widen.
Their heart rate ticks up an extra 10 to 20
beats per minute.
And ultimately, they pump 40 to 50% more blood.
You can actually see some of this extra blood
flow as a rosy blush on people’s faces,
which is sometimes called pregnancy glow.
It happens partially because of the increased
blood output.
Beyond glowing skin, though, these changes
matter to the health of both humans.
They make sure the placenta gets enough blood
to constantly refresh the blood pool — something
that happens 2 to 3 times per minute.
That way, the baby gets enough oxygen and
nutrients.
And the additional blood flow also helps the
parents’ kidneys process the extra waste
that’s getting dumped into their bodies.
They’re eating and excreting for two.
One of the ways scientists think this happens
is through a hormone called relaxin, which
dilates blood vessels to make them wider.
This allows pregnant people to safely handle
more blood pumping through their bodies, instead
of boosting their blood pressure.
Relaxin levels naturally go up in people during
ovulation, and stay high if they become pregnant.
The hormone might also help later in pregnancy
by relaxing ligaments in the pelvis to make
it easier to deliver the baby.
Other hormones, like estrogen, might also
be involved.
And while the parent’s body initially makes
most of the hormones, eventually a bunch are
made by cells in the placenta.
So basically, the fetus ends up controlling
a lot of the show.
And some of this stuff borders on mind control.
Progesterone, for instance, is a hormone that
keeps the pregnancy going, and even changes
how the parent breathes.
This hormone tells the brain to lower the
amount of carbon dioxide in the body, so pregnant
people will actually take bigger breaths.
This provides more oxygen, which the baby
needs, and also makes it easier to get rid
of the carbon dioxide, so breathing is more
efficient.
Clearly, the fetus has no qualms about doing
whatever it needs to do to get things for itself.
And by the eighth month, some researchers
think that about 25% of the proteins a fetus
gets are used to make hormones to manipulate
their parent.
Now, if the cardiovascular stuff sounded intense,
think about the fetus’s near-monopoly on glucose.
Not only does the baby want more blood, it
wants more sugar in that blood.
And one way the fetus accomplishes that is
to make their parent diabetic.
Or at least, diabetic-like.
The fetal part of the placenta releases a
hormone called human placental lactogen, among
other chemicals, that decreases the pregnant
parent’s insulin sensitivity.
Insulin is a hormone that controls how much
glucose is in your blood.
When it binds to cells, it tells them to stop
releasing sugar into the bloodstream, and
to start taking up more sugar from the blood.
So when you’re less sensitive to insulin,
you don’t clear your blood of glucose as
quickly, so you have higher blood sugar.
This can work to the baby’s advantage, which
is probably why it happens.
The fetus can take all of that extra sugar
for itself.
Now, this whole fetal exploitation of their
parent is usually okay.
But sometimes, especially in people who already
have diabetes or are at higher risk, this
can lead to something called gestational diabetes.
In gestational diabetes, the parent’s blood
sugar goes too high, and the fetus ends up
getting way more sugar than it needs.
If that happens, the baby can grow really
big — to the point where it’s not safe
to do a natural delivery, and doctors do a
C-section, which involves surgery.
There can also be other complications, for
both the parent and the baby, so doctors tend
to keep close tabs on sugar levels.
The fetus’s biggest trick of all, though,
might be its ability to stay out of the way
of the parent’s immune system.
After all, the baby is still a guest… and
kind of an interloper.
Yes, it’s walled off in a fluid-filled sac,
and protected by the placenta.
And these physical barriers go a long way
in explaining how pregnancy can even work at all.
But the fact remains that many cells of the
placenta come from the fetus, so they’re
chock full of a mix of proteins — some foreign
to the pregnant parent and some not.
But, somehow, they touch parental tissue with
little to no problem.
Immunologists have wondered about this for
more than 60 years.
At first, they assumed that pregnant people
simply didn’t make immune cells that recognize
the fetus.
But we now know they do.
So, how do developing fetuses avoid damage?
We don’t know the full story, but one big
way seems to be a surge in a type of immune
cell called a T regulatory cell.
These cells basically dial down immune responses,
rather than increasing them.
And they show up in large numbers as soon
as the blastocyst implants in the uterus,
and possibly even earlier.
Another clue comes from looking at the proteins
on the surface of the trophoblasts — the
placental cells that do the invading.
Normally, cells have proteins on their surfaces
that different immune responders can recognize.
That’s how an immune response starts, and
your body can tell you’re infected with
a bunch of bad bacteria.
But trophoblasts are missing a lot of these
surface proteins, or have slight changes to them.
And that could give them a kind of invisibility
cloak.
The immune cells in the uterus are also different.
Normally, natural killer cells do exactly
what their name implies — they kill.
But the ones in the uterine lining don’t.
Basically, they still have the proteins they
need to kill, but the cells also have inhibitory receptors.
And when those inhibitory receptors are activated,
it prevents the release of those deadly proteins.
These natural killer cells are around when
the trophoblasts are reworking the parent’s
blood vessels, and might even help by pumping
out special factors.
In mice that are engineered to lack natural
killer cells in the uterus, the parental part
of the placenta doesn’t grow properly, and
baby mice are born abnormally small.
There are other ways that researchers think
babies avoid detection, too.
And most of the time, they pull it off.
But scientists are now realizing that many
fertility problems might actually be immune
problems that crop up very early in pregnancy.
If an embryo can stick around long enough,
though, they might get the last laugh.
Because decades after pregnancy, scientists
have found fetal cells still hanging out.
This is called microchimerism, and may be
one of the strangest features of pregnancy.
It’s actually a two-way street: kids end
up with cells from their parent in them, too.
Babies can even end up with cells from their
older siblings or older generations, kinda
like a Russian nesting doll of past pregnancies!
But people finish pregnancy with far more
cells from their children than the other way around.
Doctors haven’t figured out if they’re
meaningful in any way.
Some have proposed that these foreign cells
can cause autoimmune diseases later, although
that hasn’t been fully demonstrated yet.
If you’re feeling sentimental, though, you
can think of them as a literal keepsake in
your body.
Pregnancy means you’ll always carry part
of your kid with you, whether you want to or not.
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