what are stem cells and why are they important

- [voiceover] you've probablyheard of stem cells by now. you probably know thatevery cell in our body, whether it's a muscle cell or a nerve cell or a skin cell or a red blood cell, or any other type of cell really, they all came from a common group of stem cells during development. all of these really,really specialized cells like this muscle cell here

with its little contractile proteins, and this nerve cell herethat can send signals, and this waterproof skin cell here, and this red blood cellthat carries our oxygen, all of these came fromthese stem cells up here, which were completely unspecialized. how does something like this happen? it's actually pretty interesting. let me first give you an analogy here.

just imagine a library, right, like the one you used to go to when you were a teenageror something like that, and the one that youhopefully still go to. it has all the booksyou can imagine, right, but depending on which books you borrow and which books you read, you are changed. you end up knowing a totallydifferent subset of stuff compared to someone who read

different books than you, right? but all the books that you both read are still in this one library. there's actually a really similar system with our genes and with our dna. recall that inside the nucleusof each cell is your dna. this is our library, this isour set of genetic instructions for building our entire body. within our dna libraryhere we have our books,

which are segments of ourdna that we call genes. genes give our cells specific instructions on how to make differentkinds of proteins. having different proteins around, that changes the way our cells look and it changes the way our cells act so it gives our cellsreally different abilities. what i mean with the exception of the red blood cells which lack nucleii,

every single somatic cell in your body contains the exact same dna. yet this muscle cell here, right, it looks and it actsdifferently to this neuron here. that's because they're each reading different books in our dna library. they're using differentgenes to make their proteins. just a bit of terminology here, when a cell is activelyusing certain genes,

it's said to be expressing those genes. a gene being expressedis said to be turned on, and one not being expressed is turned off, so just keep that in mind. why am i telling you all of this? because in the end it all relates to how our stem cells all the way up here end up differentiating into ourspecialized cells down here. the bottom line is inorder to differentiate to,

for example, specializeinto our muscle cell here, this stem cell up here turnedon its muscle cell genes. here's its dna and i'mhighlighting its muscle cell genes that it turned on right now. it also turned off some other genes. by turning on its muscle cell genes, now proteins get made within the cell that changes how the cell looks. see now it's a bit elongated, right,

this muscle cell here. it also changes its functions. now our muscle cell hascontractile proteins in it to help it be a nice useful muscle cell to help us move around, right? now our neuron here,our stem cell turned on its become-a-neuron genes here, right? it turned off some other ones, and then the cell started producing

all the proteins it neededto turn into a neuron. like the proteins that wouldmake it elongate like this and grow out these littlespiky things up here called dendrites, okay? let me also say that remember our stem cell up here was pluripotent. it could turn into any ofour somatic adult body cells. but once it's specializedinto these mature cell types, these can't go on todifferentiate into other cells.

they actually can'tde-differentiate either. they can't go backwards upto stem cells naturally, at least in us humans. so these cells stickaround to form our bodies. by now you must be wondering what determines whatgenes in the given cell are turned on or off? in other words, how theheck does this cell know it's time to specializeinto a different cell type?

it turns out that cells decide what they're going to grow up to be based on cues they get. these cues can be fromtheir internal environment or their cues can come fromtheir external environment, their outside environment. let me just show you two major ways this can happen here, these cues. in the development of lotsof different organisms,

us humans included, we start out with onecell, right, the zygote. our zygote has these little proteins called transcriptionfactors floating around in its cytoplasm. also the precursors ofthese transcription factors are there too, little bits of mrna. two things to note. first, transcription factorswill activate certain genes

and turn them on. that's what transcription factors do. second, notice that all theselittle transcription factors are clustered around in one area. this is important because whenthe zygote starts to divide, where do all thesetranscription factors end up? like you see here, theyonly end up in the cells that divided off in that original region where they all wereclustered around, right?

so these cells up here don'thave any or don't have much, and these cells down here have a whole heap of transcription factors. now you can imagine that different genes will get activated inthese different cells. that'll determine what each ofthese cells specializes into because now they're gonnamake different proteins. this mechanism here ispretty appropriately called asymmetric segregationof cellular determinants.

it's this big mouthful herebut if we break it down here, you can see asymmetric becauseit really just refers to how these transcription factors are not symmetrically distributed among the daughter cells here. this cellular determinantsbit is just referring to the transcriptionfactors or their precursors. that's one way that cells can be made to specialize into different things,

just having differenttranscription factors around. but the second way tospecialization that i'll mention is called inductivesignaling or just induction. induction is kind of likereally strong encouragement, almost like peer pressure, where one cell or actuallyusually a group of cells can induce another groupof cells to differentiate by just using some signals. the signals could bepassed a few different ways

so they could be passed by diffusion. they could be released from one group and just diffused over to the other group where they'll bind receptorson the other groups and cause the cells overthere to differentiate. or the induction could be done by direct contact between cells, right? you can see these little surface proteins on each of these cells binding each other.

that's direct contact. or you could have signalspassed through gap junctions, which are little connections, or actually i should say connexons between cells that are connected and that could inducethe cell to specialize, this cell over here. i called this a connexonbecause in cellular biology, these proteins that makeup part of a gap junction

are collectively called a connexon. anyway, induction is absolutely key in forming lots of our body parts, like our limbs are formed bypartially through induction. our ears and our eyes andlots more of our body parts are formed throughinduction in development, in embryological development. so induction is really importantin cell specialization. on that note, i'll just remindyou remember the goal here

with the cytoplasmic determinants, those transcription factorsi talked about earlier and then all these signalsthat you get in induction, remember the goal is to get cells to change their gene expression, right? to flick on or flick off certain genes, which ultimately is whatcauses cells to differentiate into other more specialized cells.