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From One Cell to a Body – The Science of Development · Talking Science

0:00
I want you to do something for me right now.
0:02
If you aren't driving, obviously, just take a
0:04
second and look at your hand. Like, really look
0:06
at it. Yeah, flex your fingers a bit. Exactly.
0:09
Curl your fingers into a fist and then open them
0:11
back up. Now take a slow, deep breath in and
0:15
let it out. It feels pretty mundane, right? Right.
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But here's the crazy part. Every single part
0:21
of your body that just performed those actions,
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you know, the muscle fibers contracting in your
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hand, the nerves firing that signal from your
0:29
brain, the lung tissue expanding in your chest,
0:32
all of it started as a single cell. One single
0:34
cell? Yeah, a cell no bigger than like a full
0:37
stock at the end of a sentence on a page. And
0:40
that brings us to the mission of our deep dive
0:42
today. We're trying to unpack this massive mystery.
0:46
How does one microscopic speck of matter with
0:49
no architect, no supervisor, just know how to
0:52
build a fully functional human? It really is.
0:55
I mean, it's one of biology's most extraordinary
0:56
feats. We're looking at a system that basically
0:59
assembles staggering complexity from absolute
1:02
scratch. And it does it so reliably. Breathtakingly
1:05
reliably. And to really understand how it pulls
1:07
that off, we have to look at the exact starting
1:10
point. So the story officially begins with a
1:12
process called fertilization. Right. The very
1:15
beginning. Exactly. This is the moment a single
1:17
sperm cell and a single egg cell fuse together
1:20
and they create one entirely new cell, which
1:24
in biology we call the zygote. The zygote. It's
1:28
completely wild to think that this one microscopic
1:31
speck contains the absolute complete starting
1:34
material for a whole person. Oh, yeah. Everything
1:37
is in there? I mean, my brain, my fingernails,
1:39
my heartbeat, it's all packed in there. But what
1:41
is actually inside that zygote that's driving
1:43
the whole operation? Well, it all comes down
1:46
to a famously long molecule you've definitely
1:48
heard of, DNA. Of course. Yeah, that single cell
1:52
contains a full set of genetic constructions,
1:54
which you inherit half from each parent, and
1:56
it's all wrapped up in DNA. Now, you will constantly
1:58
hear people describe DNA. As a blueprint for
2:02
a human being. Wait, blueprint of life, right.
2:05
That's basically the marketing slogan for DNA.
2:07
I feel like I saw that in every high school textbook.
2:09
It's a very catchy slogan, but we really need
2:11
to correct that misconception right away. It
2:14
gives you a deeply misleading mental image. Misleading
2:17
how? Well, think about what a blueprint is. It's
2:20
a fixed static diagram. A builder looks at a
2:23
blueprint, reads the whole thing to see where
2:25
all the walls go, and follows it once to build
2:28
a house. Okay, yeah. DNA just doesn't work like
2:30
that at all. It is much more accurate to think
2:33
of DNA as a massive encyclopedic recipe book
2:36
or like a giant construction toolkit. OK, I can
2:39
picture a giant recipe book. Right. And inside
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this massive recipe book are thousands of individual
2:45
instructions, which we call genes. The recipes
2:47
themselves. Exactly. These genes are essentially
2:50
recipes for building thousands of different proteins.
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And proteins are the tiny molecular machines,
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you know, the bricks and the mortar that actually
2:58
do the. physical work and build the structures
3:00
inside yourselves. So if we run with the cookbook
3:02
idea, if I have a massive encyclopedia of recipes
3:05
in my kitchen, I don't walk in and cook every
3:08
single recipe in the book all at once for one
3:11
Tuesday night dinner. No, that would be a disaster.
3:13
Right. I pick the pasta recipe, I completely
3:16
ignore the cake recipe, and I just use what I
3:19
need for that specific meal. And if we stick
3:21
with that kitchen idea, the secret isn't just
3:23
having the recipes, it's knowing exactly when
3:26
to cook them. Ah, timing? Yes. Not every gene
3:29
in that DNA recipe book gets used at once, and
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they absolutely aren't used the same way in every
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cell. The exquisite coordination... of switching
3:39
specific genes on and firmly switching other
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genes off at very specific moments is what actually
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drives this entire building process. And there's
3:47
a term for that switching, right? There is. We
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call this switching process gene expression.
3:52
Gene expression. Okay, so the zygote isn't just
3:54
reading a blueprint from start to finish. It's
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constantly flipping through the cookbook, deciding
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which recipes to express and which to just ignore.
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That switching on and off is the beating heart
4:04
of development. But, you know, before the zygote
4:07
can really start specializing its recipes, it
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has to tackle an almost absurdly simple first
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step. Which is? It divides in two. It just splits
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right down the middle. It does. We call this
4:16
cell division. The one cell becomes two. Then
4:19
those two cells divide to make four. Four becomes
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eight. Eight becomes 16. And it just doubles
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repeatedly. But here is the critical part to
4:27
remember. Every single time a cell divides, the
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new daughter cells receive a complete identical
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copy of that original DNA recipe book. Cover
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to cover. Cover to cover. Wow. That sounds like
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a massive logistical headache. I mean, we are
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talking about trillions of copies of an enormously
4:46
long molecule. It's really worth marveling at
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the sheer reliability of this copying process.
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It happens over and over with remarkably few
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errors. Which I guess is pretty important. Vital,
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actually. Because a serious mistake at this early
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stage would cascade into every cell that comes
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after it. So for those first several rounds of
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cell division, the cells are just multiplying.
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They don't look or behave like any particular
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body part yet. They are just building up the
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raw supply of building materials. Okay, I have
5:14
to jump in here. Because this brings up a massive
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logical hurdle for me. Let's hear it. If these
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cells are just blindly copying the exact same
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recipe book over and over, and every single cell
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is totally identical, wouldn't you just end up
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with a massive, shapeless blob of identical cells?
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Like, how do we ever get from a uniform blob
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to an eyeball or a heartbeat or a patch of skin
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if they all have the exact same manual? That
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is the ultimate hurdle the embryo has to overcome.
5:44
Because, you're right, a shapeless ball of identical
5:46
cells doesn't get you a function. human. Right.
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At a certain point, these completely identical
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cells have to start becoming different from one
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another. Some are destined to become skin, others
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muscle, others nerves. This progressive process
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of becoming different is called differentiation.
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Differentiation. Okay, so this is where the gene
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expression we talked about really kicks in. The
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cells start picking different recipes out of
6:09
the identical books they hold. They do. With
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very rare exceptions, every single cell in your
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body carries the exact same DNA recipe book that
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was in that original Zyko. Which is so counterintuitive
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to think about. My skin and my nerves have the
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exact same instructions. They do. They are astonishingly
6:24
different in how they look and function, but
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the instructions inside them are identical. The
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difference is simply which pages of the book
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they're actively reading. So how does a nerve
6:34
cell use the book compared to a skin cell? Well,
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think about what a nerve cell needs to do. It
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reads the genes required to build, you know,
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long branching cables and electrical signaling
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machinery. But just as crucially, it firmly switches
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off the unrelated genes. Like what? For instance,
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a nerve cell has the genes for making tooth enamel
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sitting right there in its DNA. It has the genes
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for making stomach acid, but it keeps those pages
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firmly closed. Oh, wow. And the skin cell does
7:03
the exact reverse. It turns on the skin -making
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genes and shuts down the nerve -making genes.
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Differentiation is the step -by -step narrowing
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of which genes are active until a cell finally
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commits to a specific permanent identity. But
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wait, if I'm a cell sitting in this growing bowl,
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And I have the whole book. How do I know which
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page to read? Ah, that's the big question. Right.
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What tells one cell to turn into skin and tells
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another cell, just a fraction of a millimeter
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away, to turn into a nerve? The answer involves
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some of the most elegant chemical choreography
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in all of biology. The embryo uses these chemical
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signaling molecules called morphogens. Morphogens.
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Kind of sounds like morphing or changing shape.
7:44
Form creators, literally. A morphogen is a chemical
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signal released from a very specific source within
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the embryo, usually just a small cluster of cells.
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Okay. These source cells pump out this chemical
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and it spreads outward, diffusing through the
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surrounding tissue, and this creates a concentration
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gradient. Concentration gradient. Let's try to
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visualize that. Yeah. Like dropping a single
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drop of dark blue dye. into a glass of still
8:10
water. That works perfectly. At the exact spot
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where the dye hits, the water is a deep, intense
8:15
blue. Right. But as it slowly spreads outward,
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you get rings of fading intensity. A little further
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out, it's medium blue. Further out still, it's
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light blue until eventually the water is barely
8:25
tinted at all. So it's almost like a cell is
8:27
listening to a radio broadcast. I like that.
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If the cell is bathed in a massive concentration
8:32
of the chemical, the signal is blasting. And
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the cell says, OK, message received. I'll turn
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into skin. Right. But if the cell is further
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away and the signal is just a whisper, it says,
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got it. I'll become a nerve. The cells are basically
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just reading a chemical dial to figure out where
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they are. They are reading a dial and they are
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remarkably sensitive to it. A cell bathed in
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a high concentration switches on one developmental
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program. A cell experiencing a middling concentration
9:00
switches on a different program entirely. That
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is incredibly economical. You don't need a million
9:05
different signals. You just need one signal at
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different volumes. It is economical and it's
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also highly collaborative. The morphogens establish
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these broad fading zones, but cells are also
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constantly talking directly to their immediate
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neighbors. Oh, they talk to each other. Yeah,
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they physically touch each other and use short
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-range signals to say, in effect, hey, I'm becoming
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skin, you should become skin too. Oh, peer pressure.
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Exactly. This neighbor -to -neighbor whispering
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takes those broad, faded zones and sharpens them
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into crisp, clear boundaries between different
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tissues. And once a cell gets that initial message...
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From the broadcaster or the neighbor, does it
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just instantly transform? No, it's a cascade.
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We call them gene cascades. One gene gets switched
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on by the morphogen and it produces a protein
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whose only job is to go and switch on five other
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genes. Oh, I see. And then those five switch
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on 20 others. So a simple early instruction ripples
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outward into this elaborate pattern. A cell doesn't
10:00
wake up and decide its fate in isolation. It
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becomes what becomes based entirely on where
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it's sitting in the embryo and what signals are
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washing over it in that exact location. So these
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fading chemical signals are based. creating a
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three -dimensional map for the cells they create
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a literal coordinate system for the body very
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early on one gradient defines an axis from head
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to tail another gradient coming from a different
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direction defines an axis from back to belly
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Eventually, another sets up left to right. Every
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cell reads its exact location on the head -to
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-tail axis and the back -to -belly axis simultaneously.
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Every cell basically has a biological GPS telling
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it its X, Y, and Z coordinates. Yes. And that
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combined GPS data tells it exactly which pages
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of the recipe book to open. If this early coordinate
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system gets set up even slightly wrong, the errors
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cascade through everything built on top of it.
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That brings up a really fascinating question
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about destiny, actually. Oh, so? If a cell's
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entire future is tied to its location and its
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GPS coordinates, is it locked in immediately?
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Like, can a cell ever change its mind if it gets
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moved? To answer that, we have to look at a vital
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concept called potency. Potency is a measure
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of how many different cell types a specific cell
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is still capable of becoming. So how much potential
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it has left before it's completely locked in.
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Exactly. The original fertilized egg, that zygote,
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and the handful of cells produced by its very
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first few divisions are what we call totipotent.
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Totipotent. Toti meaning total. Total potential,
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yes. A totipotent cell retains the remarkable
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ability to give rise to an entire, complete individual
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all on its own, including the placenta and supporting
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tissues needed for pregnancy. Wait. Really, any
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single one of those early cells could just branch
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off and build a whole person? They absolutely
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can. And we see a perfect natural experiment
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demonstrating this all the time. Identical twins.
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Oh, of course. When an embryo is still just a
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tiny cluster of these totipotent cells, if that
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cluster happens to split cleanly into two separate
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groups, each of those groups just carries on
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independently. Because they both have total potential.
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Because they are totipotent, they each have the
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power to develop into a complete individual.
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And because they split from that exact same original
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cluster, they share the exact same original genetic
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recipe book. which is why they are identical.
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Exactly, which is completely different from non
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-identical or fraternal twins. Right, those are
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different. Those occur when two separate egg
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cells are fertilized by two separate sperm cells
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at the exact same time. They are genetically
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no more similar than any standard pair of siblings,
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they just happen to share a womb. Identical twins
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are the direct result of totipotency in action.
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OK, so that a total potential is at the very,
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very beginning. But as the cells start reading
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those chemical GPS signals and, you know, cascading
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their genes, I assume they lose that total flexibility.
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They step down a level. As development progresses,
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they lose totipotency and become pluripotent.
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You have likely heard of these. They are the
12:59
famous embryonic stem cells. Right. Stem cells.
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Pluripotent meaning plural. Many potentials.
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Exactly. They have many potentials, but not total
13:08
potential. They can still become a lot of things,
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but they can't build a whole human from scratch
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anymore. Right. A pluripotent cell can still
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become essentially any cell type found inside
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the actual body, heart, liver, brain, bone, but
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it has lost the ability to build the supporting
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structures like the placenta. From this point
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on, development is a one -way street of narrowing
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potential. The sources for our deep dive used
13:31
a fantastic analogy here, describing it as...
13:34
a broad river dividing into progressively smaller
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channels. Oh, I like that. You start with this
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huge wide river of potential and then it forks.
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And once a cell goes down one fork, say the fork
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that leads to the nervous system, it can't swim
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back upstream to become a skin cell. It just
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keeps branching into smaller and smaller streams.
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The nervous system stream forks into brain cells
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or spinal cord cells. The brain cells stream
13:59
forks into different specific types of neurons.
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It funnels down step by step until it reaches
14:04
a very specific destination and becomes fully
14:07
specialized. So once the river reaches the ocean,
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you know, once a cell is fully specialized and
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you're born, is the development story totally
14:14
over? The book is closed. You would naturally
14:17
assume the process stops at birth, but a limited
14:19
version of this developmental engine keeps running
14:21
for your entire life. Adult tissues retain small
14:24
reservoirs of what are called multipotent stem
14:26
cells. Multipotent. So they have multiple potentials,
14:30
but it's highly restricted compared to the earlier
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stages. They are far more restricted. They can
14:34
only produce a limited range of related cell
14:37
types, but they are absolutely crucial for your
14:40
survival. How so? Well, your skin, for example,
14:43
is constantly shedding, so it is being continuously
14:45
renewed from multipotent stem cells deep in its
14:49
lower layers. Oh, right. Your red blood cells
14:51
only live for a matter of weeks. They are continuously
14:54
replenished by multipotent stem cells hiding
14:56
in your bone marrow. So we basically have these
14:58
little developmental toolkits left on standby
15:01
all over our bodies, just quietly doing maintenance
15:04
and repair, using the exact same logic of cell
15:07
division and differentiation. The underlying
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logic never really stops, but this brings us
15:12
to a crucial mechanical question. We've talked
15:14
deeply about the chemical signals, the recipes,
15:17
the GPS coordinates. We have a chemical map telling
15:20
a cell it needs to be part of the spinal cord.
15:22
Right. But knowing your coordinates doesn't magically
15:25
build a spine. How do these static cells actually
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construct the physical three -dimensional architecture
15:31
of a human? Yeah, that's true. Because reading
15:33
a chemical dial is great, but it doesn't explain
15:35
how you get... organs nested inside each other,
15:38
or tubes, or a heart that actually pumps. Right.
15:41
The chemistry directly drives physical action.
15:44
Morphogens don't just turn on genes that, say,
15:46
be a nerve. They turn on genes that physically
15:49
alter the cell's cytoskeleton. Cytoskeleton.
15:52
Yeah. Like it's internal scaffolding. Exactly.
15:55
This causes cells to actively migrate. Whole
15:58
groups of cells detach from one location and
16:01
migrate surprisingly long distances through the
16:04
embryo to take up residence somewhere else entirely,
16:07
helping deform the face or parts of the peripheral
16:09
nervous system. So the chemical map is literally
16:12
telling them where to drive. and how to change
16:14
shape when they get there the embryo doesn't
16:16
just blow up like a balloon in place sheets of
16:19
cells literally fold inward to form tubes like
16:23
origami basically yeah think about your entire
16:26
nervous system the spinal cord starts as a flat
16:29
sheet of cells on the back of the embryo the
16:31
chemical signals tell the cells along the edges
16:33
of that sheet to physically constrict pulling
16:36
inward like a drawstring on a bag that flat sheet
16:39
rolls up into a closed tube which eventually
16:41
becomes your spinal cord and That is staggering
16:45
to visualize. I mean, this combination of chemical
16:48
signals whispering instructions and cells physically
16:50
migrating and folding themselves into origami
16:53
shapes, it honestly sounds incredibly fragile.
16:56
It does sound like a house of cards. Yeah. Given
16:59
how many moving parts there are, cells dividing,
17:01
migrating, folding, reading chemical dials, and
17:04
over months and months across trillions of cells,
17:07
you'd think the slightest hiccup would ruin the
17:09
whole thing. One of the most striking features
17:11
of early development is how robust it actually
17:14
is. Embryos have a staggering capacity to buffer
17:17
against small disturbances. Really? Yeah. If
17:20
a signal is a little too weak or cell division
17:22
is slightly delayed or a few cells are lost along
17:25
the way, the embryo doesn't just fail. It can
17:28
usually compensate, course correct, and still
17:30
produce a normally formed individual. It's like
17:33
it has built in shock absorbers. Exactly. Evolution
17:36
has refined these gene networks over immense
17:39
spans of time. Embryos that could tolerate a
17:42
bit of chemical noise or minor mishaps survived
17:45
better than those that couldn't. That makes sense.
17:47
This creates redundancy. If one gene cascade
17:50
is slightly muted, another overlapping pathway
17:54
can often pick up the slack. That robustness
17:57
makes a lot of sense from an evolutionary standpoint.
18:00
But it makes me think about everyday pregnancy
18:03
advice. How do you mean? If the embryo is so
18:05
incredibly good at correcting its own course,
18:08
why are doctors so exceptionally strict about
18:11
what a pregnant person eats or what medications
18:14
they take, especially in the first trimester?
18:16
Because that robustness has very hard limits,
18:18
and it all comes down to a concept called a critical
18:20
window. A critical window. A critical window
18:23
is a very specific stretch of time when a particular
18:26
organ or structure is actively being formed.
18:29
When that flat sheet of cells is actively rolling
18:31
up into the spinal cord, or when the cells are
18:34
actively migrating to form the chambers of the
18:36
heart, those tissues are highly sensitive to
18:39
disruption. So it's all about the timing of the
18:41
construction schedule. Timing is everything.
18:44
A substance that disrupts development, whether
18:46
it's a specific medication, alcohol, or a virus,
18:49
is called a teratogen. A teratogen, okay. A teratogen
18:52
usually only does its severe damage if the exposure
18:56
coincides exactly with the critical window for
18:59
the structure it affects. So it's a timing issue.
19:01
Right. If the teratogen sweeps through before
19:03
the heart starts forming or after it's mostly
19:06
finished, the disturbance might be buffered by
19:08
that redundancy we talked about. But if it hits
19:12
right when those delicate morphogen gradients
19:14
are trying to tell the heart cells how to fold,
19:16
the error cascades. Wow. So standard medical
19:19
advice isn't just doctors being arbitrarily cautious.
19:22
It is directly tied to the precise week -by -week
19:26
schedule of when the body's structures are actively
19:29
pulling themselves together. It's grounded entirely
19:31
in our understanding of that building schedule
19:33
and how easily a specific step can be knocked
19:36
off course if you introduce the wrong chemical
19:38
interference at the exact wrong moment. Looking
19:40
at how complex this all is, where does the science
19:43
even go from here? Like, do we have this whole
19:45
cookbook fully translated? Do we know every gene
19:48
in every fold? Oh, not even close. This is very
19:50
much an active, unsettled frontier of science.
19:53
We still don't fully understand how a cell integrates
19:57
all these countless conflicting signals, you
19:59
know, the morphogen gradients, the neighbor whispering,
20:02
the physical tension, to make one clean, final
20:05
decision about its fate. It's just so much information
20:08
for a tiny cell to process. It is. We're also
20:11
intensely studying how much of this is a rigid,
20:13
hardwired instruction set versus cells just flexibly
20:17
organizing themselves on the fly based on local
20:19
conditions. But how do you even study that? You
20:21
can't exactly go in and tinker with the morphogens
20:24
in a developing human embryo to see what happens.
20:27
No, absolutely not. But one of the most exciting
20:29
recent developments is using pluripotent stem
20:32
cells to grow highly controlled structures in
20:34
the laboratory. Yeah, we can coax these stem
20:38
cells to mimic aspects of early embryonic organization
20:41
in a dish. It gives scientists a new, ethically
20:44
considered way to directly watch, study, and
20:47
even manipulate these earliest steps of life
20:49
without needing actual embryos. That's amazing.
20:53
But any confident claim that we have the whole
20:55
picture fully worked out should be treated with
20:57
deep skepticism. The field is moving incredibly
21:01
fast. You know, stepping back from all the specific
21:03
genes, the morphogens and the folding sheets
21:06
of cells, what I find most awe -inspiring is
21:09
that none of this requires magic. No, it doesn't.
21:12
It is just ordinary chemistry and ordinary physics.
21:15
Cells dividing, chemical signals fading out in
21:18
water. genes switching on and off like light
21:21
switches, cells physically pulling on each other.
21:24
It is all totally normal biological principles
21:27
just repeated with such staggering coordination
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that it manages to build you. It is the ultimate
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testament to the power of iteration. Simple biological
21:36
rules applied over and over and over again, eventually
21:39
creating unfathomable, beautiful complexity.
21:42
I love that. And I want to leave you with a final
21:44
thought today, something for you to mull over
21:45
that builds on what we discussed about those
21:47
adult... multipotent stem cells the ones quietly
21:51
doing maintenance in your skin your blood your
21:53
gut lining they're always at work because those
21:55
cells are constantly replenishing your tissues
21:57
based on that original dna recipe the physical
22:01
matter that makes up you today is not the exact
22:04
same physical matter you were born with that's
22:06
very true thanks to the ongoing quiet engine
22:09
of development running in the background of your
22:11
life Your body is effectively a biological ship
22:14
of Theseus. You are constantly rebuilding yourself
22:17
from the inside out, cell by cell, until the
22:20
very day you die. The next time you curl your
22:22
fingers into a fist or take a deep breath, remember
22:25
that the awe -inspiring process that built you
22:28
isn't just history. It's still happening right
22:30
now.