- [Voiceover] So, let me give you an analogy, here.
When you were still an adorable little baby,
you were just bursting with potential.
You could decide to be a pilot,
or a doctor, or a journalist.
You had the potential to specialize into
all sorts of different careers,
and as you got a bit older, you got more and more
committed down a certain pathway,
and the decisions that you made
moved you further and further along this pathway, right?
Well, it turns out that stem cells operate in a similar way,
going from unspecialized to more specialized
as they get older.
So, let me show you what I mean by that
over the course of this video.
And let's actually start back at the zygote, here,
the cell that results when sperm and egg fuse
because that's really where our stem cell
story kinda begins.
So, the zygote starts to divide, right,
by mitosis until it reaches the blastocyst stage,
this hollow ball of cells here is called a blastocyst.
And here, things start to get a little bit more interesting.
So, in a blastocyst, there's this little grouping of cells
down in here, referred to as the inner cell mass.
And this is a really special little bunch of cells
that go on to become the embryo.
So, these are called stem cells.
And what they can do as stem cells
is they can specialize into several other cell types.
So, we actually call them pluripotent stem cells.
Pluri meaning several and potent referring
to these stem cells' ability to actually
do this differentiation.
So, during development, these inner cell mass
pluripotent stem cells can differentiate
into any of the more than 200 different cell types
in the adult human body when given the proper stimulation.
So, it's kind of incredible to think
that every single cell in your body
can trace its ancestry back to this
little group of stem cells, here.
And actually, if you ever hear anyone
talking about embryonic stem cells,
these are the ones they're referring to,
these ICM stem cells.
So, is this the only place we can find stem cells,
here in the developmental structures?
We used to think so, but, it turns out
that in mammals, there are two main types of stem cells.
Embryonic stem cells that we just saw
and somatic stem cells which are found in every person.
So, the embryonic stem cells are used to build our bodies,
to go from one cell to trillions of specialized cells,
and the somatic stem cells are used
as sort of a repair system for the body,
replenishing tissues that need to be replaced.
And they can't repair everything,
but, there's a lot of every day repairs
that can happen because of our stem cells.
So, in skin, for example...
This outside layer is the part of our skin that we can see
and that we can touch, right?
And it's made of these waterproof,
pretty rugged epithelialor skin cells
and interestingly, although they are pretty rugged,
you're constantly shedding these skin cells.
They actually just sort of fall off or get rubbed off
during every day activities like
when you're putting your clothes on.
And then, the ones from underneath them
just sort of move up and take their place.
So, you shed them and you lose
almost 40,000 of them per hour.
So, if we wanna have any hope of keeping our skin,
we kinda need a way to replace these cells,
and that's where stem cells that live
in our skin come in.
Actually, our skin cells are shed
and replaced so often, that it only takes a month
for us to have a completely new skin.
Like, literally one month, entirely new skin.
It's outrageous.
Anyway, deep within our skin,
there's this layer of stem cells
called epidermal stem cells,
and their job is to be continually dividing.
So, you can see them dividing, here, dividing,
dividing, dividing, and making new skin cells that go on
to migrate upward as the multiple layers of our skin.
And their goal is to eventually replace
these ones up here on the outside
that get damaged or worn out and fall off.
So, it's this kind of activity here
which show off our stem cells' role
as our regenerative cells.
Now, lemme just highlight a few differences
between our mature skin cells over here
and our stem cells down here.
They are very different.
Mature cells are not the same as stem cells,
and this principle goes for really any mature cell
versus any stem cell.
So, the mature cell is already specialized,
it already has a really specific function.
For example, our outer layer of epithelial cells, here,
they have a protective function
against the outside environment.
And, you know, just thinking of other adult cell types,
right, like muscle cells have a contractile function,
and neurons have a message sending function,
and bones have a rigid structural function.
So, all these adult cells are already nice
and specialized, they've grown up and decided
what they wanna do for a living,
whereas, stem cells are not like that at all.
Stem cells are unspecialized.
But, they still have a really important job,
which is to give rise to our more specialized cell types,
like these cells here, okay?
And, actually, in order to be considered a stem cell,
and this goes for the embryonic stem cells we met
previously and the somatic stem cells we're meeting now,
to be a stem cell, you'd need to possess
two main properties.
The ability to self renew, meaning you can divide
and divide, and divide, but, at least
one of your resulting cells remains a stem cell,
it remains undifferentiated,
and you'd need to have a high capacity
to differentiate into more specialized cells
when the time comes.
So, remember, this is also referred to
as having some degree of potency.
And there's actually a few different types
of stem cells, and some of them can turn into
more types of cells than others.
Some are more potent than others.
So, this epithelial stem cell we saw here
is actually one of the less potent types of stem cell.
In other words, these stem cells can only divide
and specialize into more epithelial cells.
So, they're our source of epithelial cells, sure,
but, only epithelial cells and not any other cell type.
So, we call them unipotent, referring to their
ability to only create one type of cell.
But, lemme show you another example here
of a multipotent stem cell.
Let's look at this guy's femur, his thigh bone,
which is where our blood cells are made
inside bone marrow in our bones.
So, you might know that our red blood cells
have a life span of about four months.
So, that means that we need to be constantly replacing
our red blood cells or we'll run out, right?
Well, in our bone marrow, we have what are called
hematopoietic stem cells, which are our
blood making stem cells.
And these are pretty special,
they're multipotent stem cells,
which means they can give rise to many types of cells,
but, only ones within a specific family.
In this case, blood cells, and not, for example,
cells of the nervous system or the skeletal system.
So, our hematopoietic stem cells
are always busy churning out new blood cells,
red blood cells to carry oxygen for us,
and white blood cells to keep
our immune system nice and strong.
And for a more clinical example,
with blood diseases like leukemia,
certain blood cells will grow uncontrollably
within a patient's bone marrow,
and it actually crowds out their healthy
stem cells, here, from being able
to produce enough blood cells.
So, as part of treatment, once the leukemia cells
are cleared from the bone marrow with, usually, chemotherapy
or radiation, doctors can actually put
more hematopoietic stem cells back into the bone marrow
that then go on to produce normal amounts
of blood for the person again.
So, this is probably the most common use
of stem cells in medicine as of now.
And you can actually find these multipotent stem cells
in most tissues and organs.
So, for example, we have multipotent neural stem cells
that slowly give rise to neurons
and their supporting cells when necessary.
And we have multipotent mesenchymal stem cells
in a few different places in the body
that give rise to bone cells and cartilage cells,
and adipose cells.
So, you might be wondering after seeing our epithelial
and our hematopoietic stem cells dividing,
why aren't these cells being used up as they divide?
And that's a really good question.
So, stem cells have two mechanisms in place
to make sure that their numbers are maintained.
So, their first trick is that when they divide,
they undergo what's called obligate asymmetric replication
where the stem cell divides into one so called mother cell
identical to the original stem cell,
and one daughter cell that's differentiated.
So, then, the daughter cell can go on to become
more specialized while the mother cell
replaces the stem cell that divided, initially.
The other mechanism is called stochastic differentiation.
So, if one stem cell happens to differentiate
into two daughter cells instead of a mother and a daughter,
another stem cell will notice this
and makes up for the loss of the original stem cell
by undergoing mitosis and producing two stem cells
identical to the original.
So, these two mechanisms make sure
their numbers remain nice and strong.
So, we've looked at embryonic stem cells
and we've looked at somatic stem cells.
There's actually one more type called
induced pluripotent stem cells, or IPS cells.
It turns out that you can actually introduce
a few specific genes into already specialized somatic cells
like muscle cells, and they'll sort of forget
what type of cell they are, and they'll revert back,
they'll be reprogrammed into a pluripotent stem cell
just like an embryonic stem cell.
And this is a huge discovery.
I mean, the technique is still being perfected,
but, there's a lot of medicinal implications, here.
For example, IPS cells are basically the core
of regenerative medicine, which is a pretty new field
of medicine where the goal is to repair damaged tissues
in a given person by using stem cells
from their own body.
So, with IPS cells, each patient can have
their own pluripotent stem cell line
to theoretically replace any damaged organs
with new ones made out of their own cells.
So, not only would a patient get the new organ
they might need, but, there also won't be
any immune rejection complications
since the cells are their own.
So, there's still a ways to go here before
this type of medicine is sort of mainstream,
but, already, IPS cells have helped
to create the precursors to a few different
human organs in labs, such as the heart and the liver.
Now, before we finish up here,
I just wanna answer two questions
that might have come up for you.
So, one, what triggers our stem cells to differentiate?
Well, it turns out that in normal situations,
right, when the stem cell's just hangin' out,
not doin' too much, it actually expresses
a few different genes that helps
to keep it undifferentiated.
So, there are a few proteins floating around in the cell
that prevents other genes from being activated
and triggering differentiation.
But, when put in certain environments,
this regulation can be overridden,
and then, they can go on and differentiate
into a more specialized cell.
The type of which depends on what specific
little chemical signals are hanging around
in the stem cell's environment.
So, for example, in the bone marrow,
there are certain proteins that hang around stem cells
and induce some to differentiate
into the specific blood cell types.
And finally, what's all this stuff you might have heard,
maybe in the news, about cord blood?
Well, from cord blood, which is blood taken
from the placenta and the umbilical cord
after the birth of a baby, you can get lots
of multipotent stem cells, and sometimes,
some other stem cells that have been shown
to be pluripotent.
So, this cord blood used to just be discarded
after a baby's birth, but now,
there's a lot of interest in keeping it
because now we know it contains all these stem cells.