what is the stem cell

- [voiceover] so, let megive you an analogy, here. when you were still anadorable 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 furtheralong this pathway, right? well, it turns out that stemcells operate in a similar way, going from unspecializedto 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 startback at the zygote, here, the cell that resultswhen 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 reachesthe blastocyst stage, this hollow ball of cellshere is called a blastocyst. and here, things start to geta little bit more interesting. so, in a blastocyst, there'sthis little grouping of cells down in here, referred toas the inner cell mass. and this is a really speciallittle 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 intoseveral other cell types. so, we actually call thempluripotent 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 than200 different cell types in the adult human body whengiven 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 placewe can find stem cells, here in the developmental structures? we used to think so, but, it turns out that in mammals, there aretwo main types of stem cells. embryonic stem cells that we just saw and somatic stem cells whichare found in every person. so, the embryonic stem cellsare used to build our bodies,

to go from one cell totrillions of specialized cells, and the somatic stem cells are used as sort of a repair system for the body, replenishing tissuesthat 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 partof 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, althoughthey are pretty rugged, you're constantlyshedding these skin cells. they actually just sort offall 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 anyhope of keeping our skin, we kinda need a way toreplace 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 onemonth, 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 becontinually dividing. so, you can see themdividing, here, dividing, dividing, dividing, and makingnew skin cells that go on

to migrate upward as themultiple 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 highlighta few differences between our mature skin cells over here

and our stem cells down here. they are very different. mature cells are notthe same as stem cells, and this principle goesfor really any mature cell versus any stem cell. so, the mature cell isalready specialized, it already has a really specific function. for example, our outer layerof epithelial cells, here, they have a protective function

against the outside environment. and, you know, just thinkingof other adult cell types, right, like muscle cellshave a contractile function, and neurons have amessage sending function, and bones have a rigidstructural function. so, all these adult cells are already nice and specialized, they'vegrown up and decided what they wanna do for a living, whereas, stem cells arenot like that at all.

stem cells are unspecialized. but, they still have areally important job, which is to give rise to ourmore specialized cell types, like these cells here, okay? and, actually, in order tobe considered a stem cell, and this goes for theembryonic stem cells we met previously and the somaticstem 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 resultingcells remains a stem cell, it remains undifferentiated, and you'd need to have a high capacity to differentiate intomore 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 someof 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 lesspotent types of stem cell. in other words, thesestem cells can only divide and specialize into more epithelial cells. so, they're our source ofepithelial cells, sure,

but, only epithelial cellsand 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'sfemur, his thigh bone, which is where our blood cells are made inside bone marrow in our bones. so, you might know thatour red blood cells

have a life span of about four months. so, that means that we needto be constantly replacing our red blood cells orwe'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 giverise 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 systemor the skeletal system. so, our hematopoietic stem cells are always busy churningout 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 cellswill 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 marrowwith, usually, chemotherapy or radiation, doctors can actually put

more hematopoietic stem cellsback 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 findthese multipotent stem cells in most tissues and organs. so, for example, we havemultipotent neural stem cells that slowly give rise to neurons

and their supporting cells when necessary. and we have multipotentmesenchymal stem cells in a few different places in the body that give rise to bonecells and cartilage cells, and adipose cells. so, you might be wonderingafter seeing our epithelial and our hematopoietic stem cells dividing, why aren't these cells beingused up as they divide? and that's a really good question.

so, stem cells havetwo mechanisms in place to make sure that theirnumbers are maintained. so, their first trick isthat when they divide, they undergo what's calledobligate asymmetric replication where the stem cell dividesinto one so called mother cell identical to the original stem cell, and one daughter cellthat's differentiated. so, then, the daughtercell can go on to become more specialized while the mother cell

replaces the stem cellthat divided, initially. the other mechanism is calledstochastic differentiation. so, if one stem cellhappens to differentiate into two daughter cells insteadof a mother and a daughter, another stem cell will notice this and makes up for the lossof the original stem cell by undergoing mitosis andproducing 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 stemcells, or ips cells. it turns out that youcan actually introduce a few specific genes intoalready specialized somatic cells like muscle cells, andthey'll sort of forget what type of cell they are,and they'll revert back,

they'll be reprogrammedinto a pluripotent stem cell just like an embryonic stem cell. and this is a huge discovery. i mean, the technique isstill being perfected, but, there's a lot ofmedicinal implications, here. for example, ips cellsare basically the core of regenerative medicine,which is a pretty new field of medicine where the goalis 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 replaceany damaged organs with new ones made out of their own cells. so, not only would apatient 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 medicineis sort of mainstream, but, already, ips cells have helped to create the precursorsto a few different human organs in labs, suchas 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 ourstem cells to differentiate?

well, it turns out thatin normal situations, right, when the stemcell'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 proteinsfloating around in the cell that prevents other genesfrom 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 proteinsthat hang around stem cells and induce some to differentiate

into the specific blood cell types. and finally, what's all thisstuff 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 ababy, 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 usedto just be discarded after a baby's birth, but now, there's a lot of interest in keeping it because now we know itcontains all these stem cells.