Hi. It's Mr. Andersen and welcome to biology essentials video 44. This is on
cellular specialization. And that's how cells become tissues. Tissues become organs. And
organs become us. In other words, it's how cells decide what kind of a cell they're going
to become. If you're looking for a really cool TED video, ted.com, I would check Anthony
Atala's TED talk on growing new organs. He basically summarizes all of the research that's
been going on for the last decade on growing new organs. Shows how he was able to create
a bladder. He has a dot matrix printer that actually prints out a heart. So it's pretty
crazy. Another bit of research going on that's pretty mind bending is that of Doris Taylor
at the University of Minnesota. What she's figured out is you can use detergents to kind
of clean all of the cells off of connective tissue. And so this right here is, it looks
like a ghost heart, it's essentially a heart that's removed all of the cells from it. So
you could take tissues or cells from a person. Grow them in culture. Put them on that and
conceivably make a heart. I know she's made a heart that can actually beat. And so it's
a really cool time as far as organ replacement and regeneration. But first let's start with
how does this all work? Where do cells start and how do they eventually become organs?
And so if I were to organize this podcast, we're first of all going to talk about cells.
So cells organize themselves into tissues. That's a bunch of cells that do the same job.
And then those eventually become organs. So example could be, one cardiac muscle cell
makes up cardiac muscle tissue which eventually makes a heart. So that's how it's organized.
How a cell decides what cell it's going to become is based on cues or based on signals
that it's getting. Now some of those signals come internally from the cell itself. A lot
of the time that's based on where the cell is. And that's going to turn genes on or turn
genes off and that's going to determine what cell it's going to become. But there's also
external signals. So there are chemicals that are coming from cells adjacent to that cell
that are also turning on those genes and determining what cell it's going to become. Another really
cool area of research is cells also get signals outside of the body. And so it's getting signals
for example on temperature. And a cell can use what are called heat shock factors to
actually make proteins to protect them from heat. Or lack of oxygen. Or who know what.
The research is just getting started. And so let's first of all start with stem cells.
And so this is what human stem cells look like. These are ones that are growing on a
medium. And so those cells have the potential or the ability to become all of these different
types of cells. And so you could guess at what these are. You probably could guess a
few. This right here is fat. This would be striated muscle tissue. These would be nerves.
So nervous tissue. And then these up here are pseudo stratified ciliated columnar epithelial
cells. So that would be like the lining of your respiratory system. And so all of these
cells have the exact same DNA. And they all started as stem cells. And so how does that
proceed? Well we have to go back to fertilization of the egg. So right here we have fertilization
of the egg by the sperm. And that creates something called a zygote. A zygote is going
to make copies of itself. And then you're eventually going to have a ball of those cells.
Now all of them have the exact same DNA inside it. At this point, when it's just made a few
divisions, each of these could separate and you could have identical twins. That's one
way that identical twins form. But once you've actually folded into this blastocyst or this
blastula which is a bunch of cells in a circle. If we collect cells at this point, those are
called pluripotent stem cells. Pluripotent stem cells are those that could become any
other cell. But they're pluripotent, not totipotent because they can't form a brand new fetus.
Because this blastula is eventually going to make an embryo, fetus and a new organism.
And so these cells have the potential to become any other cell. Be it in the circulatory,
nervous, immune system. Whatever. They can become any cell. Once they've decided on what
cell they're going to become, then they're unipotent. They can't become any other cell.
They've differentiated at that point. And it's rare that they're able to de-differentiate.
There are some organisms, amphibians, some things like in healing process, where they
can actually go from unipotent back to pluripotent. But most of our cells can't do that. That's
why it's a really neat area of research. And so how do cells decide what cell they're eventually
going to become? Well like I said there are cues internally. And so this would be that
first blastula. As that blastula starts to fold, it will eventually fold in on itself.
And it forms something called a gastrula. And if you're a cell on the outside, you form
of what's called the ectoderm. And if you're on the inside you're the endoderm. And if
you're in between you're going to form a germ cell layer called the mesoderm. And mesoderm
for example will form all of the bones and muscle inside you. And so how does this cell
figure out what type of cell it's going to become? Well, if we say this is a simple cell.
So a simple cell is going to have a nucleus. And for example let's say this one just has
two chromosomes and it has a bunch of genes on there. Basically what happens is it's only
going to express, let me get a color, it's only going to express the genes, let's say
there's a gene here, there's a gene here, and there's a gene here. And all of those
create a nervous cell. And so when it differentiates, it's going to express just those nerve genes.
And all the other ones, so all these other ones are going to methylate or they're going
to wad up and they're not going to be used. And so when a cell differentiates, decides
which one it's going to be, it's getting cues based on where it is and that's going to determine
what kind of a cell it's eventually going to turn into. Be it a nerve cell or a skin
cell or even a red blood cell. And so there are cues based on that cell that tell it inside,
internally, what it's going to become. And what are those cues? We call those transcription
factors. Remember transcription is the process where we go form DNA to RNA. And so a transcription
factor is a protein or a chemical that's going to cause DNA to make a transcript of itself.
In other words make messenger RNA. And so transcription factors are going to be chemicals
in the cell that are going to say turn that on, turn that on, turn that on, turn that
on. Right now I'm talking about like in a eukaryotic cell. This would be an example
of that. And so transcription factors can come internally. But they can also come externally.
And so right here we've got an embryo that's around 6 weeks old. And when you're 5 weeks
old an embryo, a female embryo and a male embryo look exactly the same. There's no difference
between the two. But at that point, the testes start producing a transcription factor. It's
called SRY gene or the sex determining region of the Y. And that's going to flow from cell
to cell to cell to cell. And so it starts in the testes but it's going to flow to all
cells of that organism. And it's a cascade effect. Almost like a positive feedback loop
where it's going to trigger that SRY gene. So this transcription factor, to start making
proteins that make it a boy. But it also is going to make more of that SRY. So it spreads
throughout the whole organism. So at about 6 weeks all of us are female, and then just
because of the release of this transcription factor SRY, if we're a boy eventually become
a male. And so those cues again come from inside the cell or outside the cell. But what
we're doing is we're regulating which genes are turned on and which genes are turned off.
Now a cool area of research is that that stimuli doesn't even have to come from another cell
or from within the cell. It actually can come externally. And so an example of that is the
heat shock factor. Heat shock factor, this is a picture of one of those. Heat shock factor
is a protein. And what happens is if the cell senses change in the temperature. In other
words if the temperature increases, that's going to cause transcription, this transcription
factor, heat shock factor, to actually move into the nuclei. And once it moves into the
nuclei, then it's going to create a number of proteins. And those proteins are heat shock
proteins. So those proteins are going to move into the cell. And what they're going to do
is they're going to protect the cell. And so they're going to harden that cell and protect
it from changes in temperature. And so it's actually changing the physical make-up of
that cell just as a result of this transcription factor that's moving into the nuclei making
all these proteins. What happens next? Well eventually the temperature goes down, we quite
producing that heat shock factor, and we're going to quit producing those heat shock proteins
and the cell goes back to normal. And so we find that not only in heat, but in a lack
of oxygen or hypoxia is actually going to produce these shock factors as well. And so
cells are neat. They can become whatever cell they're going to become. They can respond
to cells adjacent to them based on their location. And they can even respond to external stimuli.
So that's cellular specialization. And I hope that's helpful.