- [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.
