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Stem cells

Stem cells, originating from zygotes, play crucial roles in body repair and regeneration. Explore embryonic, somatic, and induced pluripotent stem cells, their differentiation process, and their potential in regenerative medicine.
Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video.
Created by Vishal Punwani.

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Video transcript

- [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.