where we left off after themeiosis videos is that we had two gametes. we had a sperm and an egg. let me draw the sperm. so you had the sperm andthen you had an egg. maybe i'll do the egg ina different color. that's the egg, and we allknow how this story goes. the sperm fertilizes the egg. and a whole cascade of eventsstart occurring.
the walls of the egg then becomeimpervious to other sperm so that only one sperm canget in, but that's not the focus of this video. the focus of this video is howthis fertilized egg develops once it has become a zygote. so after it's fertilized, youremember from the meiosis videos that each of these werehaploid, or that they had-- oh, i added an extra i there--that they had half the contingency of the dna.
as soon as the sperm fertilizesthis egg, now, all of a sudden, you havea diploid zygote. let me do that. so now let me picka nice color. so now you're going to have adiploid zygote that's going to have a 2n complement of the dnamaterial or kind of the full complement of what a normalcell in our human body would have. so this is diploid,and it's a zygote, which is just a fancy way ofsaying the fertilized egg.
and it's now readyto essentially turn into an organism. so immediately afterfertilization, this zygote starts experiencing cleavage. it's experiencing mitosis,that's the mechanism, but it doesn't increasea lot in size. so this one right here will thenturn into-- it'll just split up via mitosisinto two like that. and, of course, these are each2n, and then those are going
to split into four like that. and each of these have the sameexact genetic complement as that first zygote, andit keeps splitting. and this mass of cells, we canstart calling it, this right here, this is referredto as the morula. and actually, it comes from theword for mulberry because it looks like a mulberry. so actually, let me just kindof simplify things a little bit because we don'thave to start here.
so we start with a zygote. this is a fertilized egg. it just starts duplicating viamitosis, and you end up with a ball of cells. it's often going to be a powerof two, because these cells, at least in the initial stagesare all duplicating all at once, and then youhave this morula. now, once the morula gets toabout 16 cells or so-- and we're talking aboutfour or five days.
this isn't an exact process--they started differentiating a little bit, where the outercells-- and this kind of turns into a sphere. let me make it a littlebit more sphere like. so it starts differentiatingbetween-- let me make some outer cells. this would be a cross-sectionof it. it's really going to lookmore like a sphere. that's the outer cells and thenyou have your inner cells
on the inside. these outer cells are calledthe trophoblasts. let me do it in adifferent color. let me scroll over. i don't want to go there. and then the inner cells, andthis is kind of the crux of what this video is allabout-- let me scroll down a little bit. the inner cells-- picka suitable color.
the inner cells right there arecalled the embryoblast. and then what's going to happenis some fluid's going to start filling in someof this gap between the embryoblast and the trophoblast,so you're going to start having some fluid thatcomes in there, and so the morula will eventuallylook like this, where the trophoblast, or the outermembrane, is kind of this huge sphere of cells. and this is all happening asthey keep replicating.
mitosis is the mechanism, so nowmy trophoblast is going to look like that, and thenmy embryoblast is going to look like this. sometimes the embryoblast-- sothis is the embryoblast. sometimes it's also called theinner cell mass, so let me write that. and this is what's going toturn into the organism. and so, just so you know acouple of the labels that are involved here, if we're dealingwith a mammalian
organism, and we are mammals,we call this thing that the morula turned into is a zygote,then a morula, then the cells of the morula startedto differentiate into the trophoblast, or kind of theoutside cells, and then the embryoblast. and then youhave this space that forms here, and this is just fluid,and it's called the blastocoel. a very non-intuitive spellingof the coel part of but once this is formed, this iscalled a blastocyst. that's
the entire thing right here. let me scroll downa little bit. this whole thing is called theblastocyst, and this is the case in humans. now, it can be a very confusingtopic, because a lot of times in a lot of books onbiology, you'll say, hey, you go from the morula tothe blastula or the blastosphere stage. let me write those words down.
so sometimes you'll say morula, and you go to blastula. sometimes it's calledthe blastosphere. and i want to make it veryclear that these are essentially the same stagesin development. these are just for-- you know,in a lot of books, they'll start talking about frogs ortadpoles or things like that, and this applies to them. while we're talking aboutmammals, especially the ones
that are closely relatedto us, the stage is the blastocyst stage, and the realdifferentiator is when people talk about just blastulaand blastospheres. there isn't necessarily thisdifferentiation between these outermost cells and theseembryonic, or this embryoblast, or this innercell mass here. but since the focus of thisvideo is humans, and really that's where i wanted to startfrom, because that's what we are and that's what'sinteresting, we're going to
focus on the blastocyst. now, everything i've talkedabout in this video, it was really to get to this point,because what we have here, these little green cells thati drew right here in the blastocysts, this inner cellmass, this is what will turn into the organism. and you say, ok, sal, if that'sthe organism, what's all of these purplecells out here? this trophoblast out there?
that is going to turn into theplacenta, and i'll do a future video where in a human, it'llturn into a placenta. so let me write that down. it'll turn into the placenta. and i'll do a whole future videoabout i guess how babies are born, and i actually learneda ton about that this past year because a babywas born in our house. but the placenta is reallykind of what the embryo develops inside of, and it's theinterface, especially in
humans and in mammals, betweenthe developing fetus and its mother, so it kind of is theexchange mechanism that separates their two systems,but allows the necessary functions to go onbetween them. but that's not the focusof this video. the focus of this video is thefact that these cells, which at this point are-- they'vedifferentiated themselves away from the placenta cells, butthey still haven't decided what they're going to become.
maybe this cell and itsdescendants eventually start becoming part of the nervoussystem, while these cells right here might become muscletissue, while these cells right here might becomethe liver. these cells right here arecalled embryonic stem cells, and probably the first time inthis video you're hearing a term that you might recognize. so if i were to just take one ofthese cells, and actually, just to introduce you to anotherterm, you know, we
have this zygote. as soon as it starts dividing,each of these cells are called a blastomere. and you're probably wondering,sal, why does this word blast keep appearing in this kindof embryology video, these development videos? and that comes from the greekfor spore: blastos. so the organism is beginningto spore out or grow. i won't go into the word originsof it, but that's
where it comes from and that'swhy everything has this blast in it. so these are blastomeres. so when i talk what embryonicstem cells, i'm talking about the individual blastomeresinside of this embryoblast or inside of this innercell mass. these words are actuallyunusually fun to say. so each of these is anembryonic stem cell. let me write this downin a vibrant color.
so each of these right here areembryonic stem cells, and i wanted to get to this. and the reason why these areinteresting, and i think you already know, is that there'sa huge debate around these. one, these have the potentialto turn into anything, that they have this plasticity. that's another word thatyou might hear. let me write that down,too: plasticity. and the word essentially comesfrom, you know, like a plastic
can turn into anything else. when we say that something hasplasticity, we're talking about its potentialto turn into a lot of different things. so the theory is, and there'salready some trials that seem to substantiate this, especiallyin some lower organisms, that, look, if youhave some damage at some point in your body-- let medraw a nerve cell. let me say i have a-- i won'tgo into the actual mechanics
of a nerve cell, but let's saythat we have some damage at some point on a nerve cell rightthere, and because of that, someone is paralyzedor there's some nerve dysfunction. we're dealing with multiplesclerosis or who knows what. the idea is, look, we have thesecell here that could turn into anything, and we'rejust really understanding how it knows what to turn into. it really has to look at itsenvironment and say, hey, what
are the guys around me doing,and maybe that's what helps dictate what it does. but the idea is you take thesethings that could turn to anything and you put them wherethe damage is, you layer them where the damage is, andthen they can turn into the cell that they needto turn into. so in this case, they wouldturn into nerve cells. they would turn to nerve cellsand repair the damage and maybe cure the paralysisfor that individual.
so it's a huge, exciting areaof research, and you could even, in theory, grownew organs. if someone needs a kidneytransplant or a heart transplant, maybe in the future,we could take a colony of these embryonic stem cells. maybe we can put them in sometype of other creature, or who knows what, and we can turn itinto a replacement heart or a replacement kidney. so there's a huge amountof excitement about
what these can do. i mean, they could cure a lot offormerly uncurable diseases or provide hope for alot of patients who might otherwise die. but obviously, there'sa debate here. and the debate all revolvesaround the issue of if you were to go in here and try toextract one of these cells, you're going to killthis embryo. you're going to kill thisdeveloping embryo, and that
developing embryo hadthe potential to become a human being. it's a potential that obviouslyhas to be in the right environment, and it hasto have a willing mother and all of the rest, but it doeshave the potential. and so for those, especially, ithink, in the pro-life camp, who say, hey, anything that hasa potential to be a human being, that is life and itshould not be killed. so people on that side of thecamp, they're against the
destroying of this embryo. i'm not making this video totake either side to that argument, but it's a potentialto turn to a human being. it's a potential, right? so obviously, there's a hugeamount of debate, but now, now you know in this video whatpeople are talking about when they say embryonic stem cells. and obviously, the next questionis, hey, well, why don't they just call them stemcells as opposed to embryonic
stem cells? and that's because in all of ourbodies, you do have what are called somatic stem cells. let me write that down. somatic or adults stem cells. and we all have them. they're in our bone marrow tohelp produce red blood cells, other parts of our body, but theproblem with somatic stem cells is they're not as plastic,which means that they
can't form any type of cellin the human body. there's an area of researchwhere people are actually maybe trying to make them moreplastic, and if they are able to take these somatic stemcells and make them more plastic, it might maybe killthe need to have these embryonic stem cells, althoughmaybe if they do this too good, maybe these will havethe potential to turn into human beings as well,so that could become a debatable issue.
but right now, this isn't anarea of debate because, left to their own devices, a somaticstem cell or an adult stem cell won't turn intoa human being, while an embryonic one, if it isimplanted in a willing mother, then, of course, it will turninto a human being. and i want to make one sidenote here, because i don't want to take any sides on thedebate of-- well, i mean, facts are facts. this does have the potentialto turn into a human being,
but it also has the potentialto save millions of lives. both of those statements arefacts, and then you can decide on your own which side of thatargument you'd like to or what side of that balance youwould like to kind of put your own opinion. but there's one thing i wantto talk about that in the public debate is neverbrought up. so you have this notion of whenyou-- to get an embryonic stem cell line, and when i saya stem cell line, i mean you
take a couple of stem cells, orlet's say you take one stem cell, and then you put it in apetri dish, and then you allow it to just duplicate. so this one turns into two,those two turn to four. then someone could take one ofthese and then put it in their own petri dish. these are a stem cell line. they all came from one uniqueembryonic stem cell or what initially was a blastomere.
so that's what they calla stem cell line. so the debate obviously is whenyou start an embryonic stem cell line, you aredestroying an embryo. but i want to make the pointhere that embryos are being destroyed in other processes,and namely, in-vitro fertilization. and maybe this'll be my nextvideo: fertilization. and this is just the notion thatthey take a set of eggs out of a mother.
it's usually a couple that'shaving trouble having a child, and they take a bunch ofeggs out of the mother. so let's say they takemaybe 10 to 30 eggs out of the mother. they actually perform a surgery,take them out of the ovaries of the mother, and thenthey fertilize them with semen, either it might comefrom the father or a sperm donor, so then all of thesebecomes zygotes once they're fertilized with semen.
so these all become zygotes,and then they allow them to develop, and they usually allowthem to develop to the blastocyst stage. so eventually all of theseturn into blastocysts. they have a blastocoel inthe center, which is this area of fluid. they have, of course, theembryo, the inner cell mass in them, and what they do is theylook at the ones that they deem are healthier or maybethe ones that are at least
just not unhealthy, and they'lltake a couple of these and they'll implant these intothe mother, so all of this is occurring in a petri dish. so maybe these four look good,so they're going to take these four, and they're going toimplant these into a mother, and if all goes well, maybe oneof these will turn into-- will give the couple a child. so this one will develop andmaybe the other ones won't. but if you've seen john & kateplus 8, you know that many
times they implant a lot ofthem in there, just to increase the probability thatyou get at least one child. but every now and then, theyimplant seven or eight, and then you end up witheight kids. and that's why in-vitrofertilization often results in kind of these multiplebirths, or reality television shows on cable. but what do they do with allof these other perfectly-- well, i won't say perfectlyviable, but these are embryos.
they may or may not be perfectlyviable, but you have these embryos that have thepotential, just like this one right here. these all have the potentialto turn into a human being. but most fertility clinics,roughly half of them, they either throw these away,they destroy them, they allow them to die. a lot of these are frozen, butjust the process of freezing them kills them and then bondingthem kills them again,
so most of these, the process ofin-vitro fertilization, for every one child that has thepotential to develop into a full-fledged human being, you'reactually destroying tens of very viable embryos. so at least my take on it isif you're against-- and i generally don't want to take aside on this, but if you are against research that involvesembryonic stem cells because of the destruction of embryos,on that same, i guess, philosophical ground, youshould also be against
in-vitro fertilization becauseboth of these involve the destruction of zygotes. i think-- well, i won't talkmore about this, because i really don't want to take sides,but i want to show that there is kind of an equivalencehere that's completely lost in this debateon whether embryonic stem cells should be used becausethey have a destruction of embryos, because you'redestroying just as many embryos in this-- well, i won'tsay just as many, but
you are destroying embryos. there's hundreds of thousands ofembryos that get destroyed and get frozen and obviouslydestroyed in that process as well through this in-vitrofertilization process. so anyway, now hopefully youhave the tools to kind of engage in the debate around stemcells, and you see that it all comes from what welearned about meiosis. they produce these gametes. the male gamete fertilizesa female gamete.
the zygote happens or getscreated and starts splitting up the morula, and then itkeeps splitting and it differentiates into theblastocyst, and then this is where the stem cells are. so you already know enoughscience to engage in kind of a very heated debate.
