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Course: HS biology (archived) > Unit 19
Lesson 5: Stem cellsEmbryonic stem cells
An overview of early development of a zygote to an embryo. Embryonic and somatic stem cells. Created by Sal Khan.
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- In a stem cell line, what prevents the cells from differentiating? Theoretically, shouldn't they just turn into random cells (muscle, nerve, etc) and not more stem cells?(31 votes)
- there are signaling pathways that basically send signals to the cells to differentiate - and these signals are only turned on by a certain event or reason. for example, if there was damage to a ventricle of the heart and embryonic stem cells were transplanted there, the heart would be sending signals to the stem cells saying, we need you to turn into this specific kind of heart cell because we need more healthy ones! and therefore the cells would differentiate. i hope this helped a little bit, but i am only barely learning about this entire process right now so my understanding is a bit rough. :)(31 votes)
- If one sperm goes in the egg and no other sperm cells can go in, how do twins happen. Do they go in at the same time in the same place? What about triplets and so on? I have been wondering about this for ever(11 votes)
- Twins (and triplets, etc.) can also occur in cases where there is more than one egg present - sometimes there is more than one ovulation! Or, of course, in the case of in vitro fertilization, when multiple eggs are implanted and more than one implant.(15 votes)
- Why not just take the leftover Embryos from in-vitro fertilization and use them?
At least that way they're being used.(16 votes)- it's already done in some laboratories in the countries where it's not prohibited(14 votes)
- Can't embryonic stem cells be taken from the eggs of women who have had a miscarriage or are infertile? If so, why is it still seen as immoral by the Catholic Church to use these eggs? They would not have had the chance to become a human, would they?(11 votes)
- I think that possibly they might be afraid out of thinking of what that human could go on to do. That human could live a happy life. Therefore its not a rational fear, but a rational consideration.(2 votes)
- Why and how do the cells differenciate?(5 votes)
- That's a good question, with a very long and complicated answer. In fact, a lot of research that's being done in biology today has to do with exactly this question. But the short version is that there are regulatory genes responsible for switching other genes on or off. But activating the correct genes, the cells become specialized to accomplish whatever they need to do.(6 votes)
- 1. Can embryonic stem cells be used to make humans? [even though you're killing an embryo that could eventually become a human anyway...]
2. Do other animals have cells similar to stem cells to repair animal injuries?(3 votes)- 1. Theoretically it's possible, but the political and ethical climate would have to change for research in that area to move forward.
2. Any organism with differentiated cells that comes from a single-cell zygote has stem cells. This means that most multicellular life has stem cells. Whether or not these cells can be used to repair injuries is a technological question and it varies on a case by case basis. You should know that these days there are a lot of scams where people are selling miracle cures to diseases that don't have a cure yet. They cloak these scams in scientific and medical language using concepts that lots of people have heard of but don't understand, like stem cells. They prey on people who are desperate.(7 votes)
- As Sal had said by doing in vitro fertilization for 1 living child 10s of them are "killed". But the one that they are not planing to use still contain embryonic stem cells right? then instead of discarding the non-selected cell why don't they distract the stem cells from then since they will be discarded anyway?(7 votes)
- A major issue with using embryos left over from in-vitro fertilization is that fertility clinics often times don't regularly collect informed consent to use the embryos for research from parents when doing the procedures. Since the embryos would've been their potential children, some parents don't feel comfortable with allowing them to be used for research. There are thousands of leftover embryos from fertility clinics with no one paying for their storage, but fertility clinics are debating over whether it's legal to treat them as abandoned property and donate it to research without informed consent from the parents.(1 vote)
- What exactly happens in the case of identical twins? Are their DNA sequences exactly the same? Are there different situations that have to happen to have a mixed boy-girl vs a same-sex boy-boy or girl-girl identical twin situation? What causes them to be become less "identical" as they grow up?(4 votes)
- No, identical twins do not have identical DNA -- it is quite close, but not exact. Specifically, their mitochondrial DNA is different. Identical twins, scientifically called monozygous twins, come from a single egg and a single sperm, thus they will always be the same sex.
Some traits are not genetic, but environmental and developmental, which is why monozygous twins start to look more and more distinct as they age.
Fraternal twins, scientifically called dizygotic twins, develop from two eggs and two sperm, thus they are no more similar than ordinary siblings. They may both be the same sex, or they may be one of each sex.
There is a third kind of twin, exceedingly rare and not well understood. This is caused by one set of genes from the mother being fertilized by two sperm. Thus, these twins are identical on their mother's side but non-identical on their father's side. Exactly how this happens is not known, but it is thought that it might be from the ovum's division that normally creates the second polar body and the viable egg, instead splits to form two genetically identical eggs. But, this is exceedingly rare, so it is hard to get good data. I am only aware of one set of twins that has been shown to be authentically semi-identical, though there are other suspected cases.(5 votes)
- why is the trophoblast located on the edge?(4 votes)
- It is located at the edge because it attaches with Chroinic Villi and forms placenta.(4 votes)
- So is that placenta the same as blastosphere?(4 votes)
- The placenta is the organ that connects the developing foetus to the uterine walls for nutrient uptake, waste removal etc. The blastophere is the early stage of an embryo produced by the cleavage of an ovum.(3 votes)
Video transcript
Where we left off after the
meiosis videos is that we had two gametes. We had a sperm and an egg. Let me draw the sperm. So you had the sperm and
then you had an egg. Maybe I'll do the egg in
a different color. That's the egg, and we all
know how this story goes. The sperm fertilizes the egg. And a whole cascade of events
start occurring. The walls of the egg then become
impervious to other sperm so that only one sperm can
get in, but that's not the focus of this video. The focus of this video is how
this fertilized egg develops once it has become a zygote. So after it's fertilized, you
remember from the meiosis videos that each of these were
haploid, 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 fertilizes
this egg, now, all of a sudden, you have
a diploid zygote. Let me do that. So now let me pick
a nice color. So now you're going to have a
diploid zygote that's going to have a 2N complement of the DNA
material or kind of the full complement of what a normal
cell in our human body would have. So this is diploid,
and it's a zygote, which is just a fancy way of
saying the fertilized egg. And it's now ready
to essentially turn into an organism. So immediately after
fertilization, this zygote starts experiencing cleavage. It's experiencing mitosis,
that's the mechanism, but it doesn't increase
a lot in size. So this one right here will then
turn into-- it'll just split up via mitosis
into two like that. And, of course, these are each
2N, and then those are going to split into four like that. And each of these have the same
exact genetic complement as that first zygote, and
it keeps splitting. And this mass of cells, we can
start calling it, this right here, this is referred
to as the morula. And actually, it comes from the
word for mulberry because it looks like a mulberry. So actually, let me just kind
of simplify things a little bit because we don't
have to start here. So we start with a zygote. This is a fertilized egg. It just starts duplicating via
mitosis, and you end up with a ball of cells. It's often going to be a power
of two, because these cells, at least in the initial stages
are all duplicating all at once, and then you
have this morula. Now, once the morula gets to
about 16 cells or so-- and we're talking about
four or five days. This isn't an exact process--
they started differentiating a little bit, where the outer
cells-- and this kind of turns into a sphere. Let me make it a little
bit more sphere like. So it starts differentiating
between-- let me make some outer cells. This would be a cross-section
of it. It's really going to look
more like a sphere. That's the outer cells and then
you have your inner cells on the inside. These outer cells are called
the trophoblasts. Let me do it in a
different color. Let me scroll over. I don't want to go there. And then the inner cells, and
this is kind of the crux of what this video is all
about-- let me scroll down a little bit. The inner cells-- pick
a suitable color. The inner cells right there are
called the embryoblast. And then what's going to happen
is some fluid's going to start filling in some
of this gap between the embryoblast and the trophoblast,
so you're going to start having some fluid that
comes in there, and so the morula will eventually
look like this, where the trophoblast, or the outer
membrane, is kind of this huge sphere of cells. And this is all happening as
they keep replicating. Mitosis is the mechanism, so now
my trophoblast is going to look like that, and then
my embryoblast is going to look like this. Sometimes the embryoblast-- so
this is the embryoblast. Sometimes it's also called the
inner cell mass, so let me write that. And this is what's going to
turn into the organism. And so, just so you know a
couple of the labels that are involved here, if we're dealing
with 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 started
to differentiate into the trophoblast, or kind of the
outside cells, and then the embryoblast. And then you
have this space that forms here, and this is just fluid,
and it's called the blastocoel. A very non-intuitive spelling
of the coel part of blastocoel. But once this is formed, this is
called a blastocyst. That's the entire thing right here. Let me scroll down
a little bit. This whole thing is called the
blastocyst, and this is the case in humans. Now, it can be a very confusing
topic, because a lot of times in a lot of books on
biology, you'll say, hey, you go from the morula to
the 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 called
the blastosphere. And I want to make it very
clear that these are essentially the same stages
in development. These are just for-- you know,
in a lot of books, they'll start talking about frogs or
tadpoles or things like that, and this applies to them. While we're talking about
mammals, especially the ones that are closely related
to us, the stage is the blastocyst stage, and the real
differentiator is when people talk about just blastula
and blastospheres. There isn't necessarily this
differentiation between these outermost cells and these
embryonic, or this embryoblast, or this inner
cell mass here. But since the focus of this
video is humans, and really that's where I wanted to start
from, because that's what we are and that's what's
interesting, we're going to focus on the blastocyst. Now, everything I've talked
about in this video, it was really to get to this point,
because what we have here, these little green cells that
I drew right here in the blastocysts, this inner cell
mass, this is what will turn into the organism. And you say, OK, Sal, if that's
the organism, what's all of these purple
cells out here? This trophoblast out there? That is going to turn into the
placenta, and I'll do a future video where in a human, it'll
turn into a placenta. So let me write that down. It'll turn into the placenta. And I'll do a whole future video
about I guess how babies are born, and I actually learned
a ton about that this past year because a baby
was born in our house. But the placenta is really
kind of what the embryo develops inside of, and it's the
interface, especially in humans and in mammals, between
the developing fetus and its mother, so it kind of is the
exchange mechanism that separates their two systems,
but allows the necessary functions to go on
between them. But that's not the focus
of this video. The focus of this video is the
fact that these cells, which at this point are-- they've
differentiated themselves away from the placenta cells, but
they still haven't decided what they're going to become. Maybe this cell and its
descendants eventually start becoming part of the nervous
system, while these cells right here might become muscle
tissue, while these cells right here might become
the liver. These cells right here are
called embryonic stem cells, and probably the first time in
this video you're hearing a term that you might recognize. So if I were to just take one of
these cells, and actually, just to introduce you to another
term, 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 kind
of embryology video, these development videos? And that comes from the Greek
for spore: blastos. So the organism is beginning
to spore out or grow. I won't go into the word origins
of it, but that's where it comes from and that's
why everything has this blast in it. So these are blastomeres. So when I talk what embryonic
stem cells, I'm talking about the individual blastomeres
inside of this embryoblast or inside of this inner
cell mass. These words are actually
unusually fun to say. So each of these is an
embryonic stem cell. Let me write this down
in a vibrant color. So each of these right here are
embryonic stem cells, and I wanted to get to this. And the reason why these are
interesting, and I think you already know, is that there's
a huge debate around these. One, these have the potential
to turn into anything, that they have this plasticity. That's another word that
you might hear. Let me write that down,
too: plasticity. And the word essentially comes
from, you know, like a plastic can turn into anything else. When we say that something has
plasticity, we're talking about its potential
to turn into a lot of different things. So the theory is, and there's
already some trials that seem to substantiate this, especially
in some lower organisms, that, look, if you
have some damage at some point in your body-- let me
draw a nerve cell. Let me say I have a-- I won't
go into the actual mechanics of a nerve cell, but let's say
that we have some damage at some point on a nerve cell right
there, and because of that, someone is paralyzed
or there's some nerve dysfunction. We're dealing with multiple
sclerosis or who knows what. The idea is, look, we have these
cell here that could turn into anything, and we're
just really understanding how it knows what to turn into. It really has to look at its
environment 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 these
things that could turn to anything and you put them where
the damage is, you layer them where the damage is, and
then they can turn into the cell that they need
to turn into. So in this case, they would
turn into nerve cells. They would turn to nerve cells
and repair the damage and maybe cure the paralysis
for that individual. So it's a huge, exciting area
of research, and you could even, in theory, grow
new organs. If someone needs a kidney
transplant or a heart transplant, maybe in the future,
we could take a colony of these embryonic stem cells. Maybe we can put them in some
type of other creature, or who knows what, and we can turn it
into a replacement heart or a replacement kidney. So there's a huge amount
of excitement about what these can do. I mean, they could cure a lot of
formerly uncurable diseases or provide hope for a
lot of patients who might otherwise die. But obviously, there's
a debate here. And the debate all revolves
around the issue of if you were to go in here and try to
extract one of these cells, you're going to kill
this embryo. You're going to kill this
developing embryo, and that developing embryo had
the potential to become a human being. It's a potential that obviously
has to be in the right environment, and it has
to have a willing mother and all of the rest, but it does
have the potential. And so for those, especially, I
think, in the pro-life camp, who say, hey, anything that has
a potential to be a human being, that is life and it
should not be killed. So people on that side of the
camp, they're against the destroying of this embryo. I'm not making this video to
take either side to that argument, but it's a potential
to turn to a human being. It's a potential, right? So obviously, there's a huge
amount of debate, but now, now you know in this video what
people are talking about when they say embryonic stem cells. And obviously, the next question
is, hey, well, why don't they just call them stem
cells as opposed to embryonic stem cells? And that's because in all of our
bodies, 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 to
help produce red blood cells, other parts of our body, but the
problem with somatic stem cells is they're not as plastic,
which means that they can't form any type of cell
in the human body. There's an area of research
where people are actually maybe trying to make them more
plastic, and if they are able to take these somatic stem
cells and make them more plastic, it might maybe kill
the need to have these embryonic stem cells, although
maybe if they do this too good, maybe these will have
the potential to turn into human beings as well,
so that could become a debatable issue. But right now, this isn't an
area of debate because, left to their own devices, a somatic
stem cell or an adult stem cell won't turn into
a human being, while an embryonic one, if it is
implanted in a willing mother, then, of course, it will turn
into a human being. And I want to make one side
note here, because I don't want to take any sides on the
debate of-- well, I mean, facts are facts. This does have the potential
to turn into a human being, but it also has the potential
to save millions of lives. Both of those statements are
facts, and then you can decide on your own which side of that
argument you'd like to or what side of that balance you
would like to kind of put your own opinion. But there's one thing I want
to talk about that in the public debate is never
brought up. So you have this notion of when
you-- to get an embryonic stem cell line, and when I say
a stem cell line, I mean you take a couple of stem cells, or
let's say you take one stem cell, and then you put it in a
Petri 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 of
these and then put it in their own Petri dish. These are a stem cell line. They all came from one unique
embryonic stem cell or what initially was a blastomere. So that's what they call
a stem cell line. So the debate obviously is when
you start an embryonic stem cell line, you are
destroying an embryo. But I want to make the point
here that embryos are being destroyed in other processes,
and namely, in-vitro fertilization. And maybe this'll be my next
video: fertilization. And this is just the notion that
they take a set of eggs out of a mother. It's usually a couple that's
having trouble having a child, and they take a bunch of
eggs out of the mother. So let's say they take
maybe 10 to 30 eggs out of the mother. They actually perform a surgery,
take them out of the ovaries of the mother, and then
they fertilize them with semen, either it might come
from the father or a sperm donor, so then all of these
becomes zygotes once they're fertilized with semen. So these all become zygotes,
and then they allow them to develop, and they usually allow
them to develop to the blastocyst stage. So eventually all of these
turn into blastocysts. They have a blastocoel in
the center, which is this area of fluid. They have, of course, the
embryo, the inner cell mass in them, and what they do is they
look at the ones that they deem are healthier or maybe
the ones that are at least just not unhealthy, and they'll
take a couple of these and they'll implant these into
the 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 to
implant these into a mother, and if all goes well, maybe one
of these will turn into-- will give the couple a child. So this one will develop and
maybe the other ones won't. But if you've seen John & Kate
Plus 8, you know that many times they implant a lot of
them in there, just to increase the probability that
you get at least one child. But every now and then, they
implant seven or eight, and then you end up with
eight kids. And that's why in-vitro
fertilization often results in kind of these multiple
births, or reality television shows on cable. But what do they do with all
of these other perfectly-- well, I won't say perfectly
viable, but these are embryos. They may or may not be perfectly
viable, but you have these embryos that have the
potential, just like this one right here. These all have the potential
to 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, but
just the process of freezing them kills them and then bonding
them kills them again, so most of these, the process of
in-vitro fertilization, for every one child that has the
potential to develop into a full-fledged human being, you're
actually destroying tens of very viable embryos. So at least my take on it is
if you're against-- and I generally don't want to take a
side on this, but if you are against research that involves
embryonic stem cells because of the destruction of embryos,
on that same, I guess, philosophical ground, you
should also be against in-vitro fertilization because
both of these involve the destruction of zygotes. I think-- well, I won't talk
more about this, because I really don't want to take sides,
but I want to show that there is kind of an equivalence
here that's completely lost in this debate
on whether embryonic stem cells should be used because
they have a destruction of embryos, because you're
destroying just as many embryos in this-- well, I won't
say just as many, but you are destroying embryos. There's hundreds of thousands of
embryos that get destroyed and get frozen and obviously
destroyed in that process as well through this in-vitro
fertilization process. So anyway, now hopefully you
have the tools to kind of engage in the debate around stem
cells, and you see that it all comes from what we
learned about meiosis. They produce these gametes. The male gamete fertilizes
a female gamete. The zygote happens or gets
created and starts splitting up the morula, and then it
keeps splitting and it differentiates into the
blastocyst, and then this is where the stem cells are. So you already know enough
science to engage in kind of a very heated debate.