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particular, we have a class quiz up there. Take a moment if you
haven't. You've heard a lot of these terms before. One you
haven't. I have a particularly wonderful item to make
up for the tough exam. It is a flashing,
isn't that so cool? It's a flashing jellyfish ball. OK,
so let's see what we can do. Yes, I know, this is a post-test item. OK,
settle down. Please, we have a lot to cover. You've
heard most of these terms before. This lecture is all about these
terms phrased in a different way. So I want to make sure we are
together before we start. [Student speaks] Yes, ma'am? OK, so the final function of
a cell, OK. Potency in the pink top. I saw your hand first,
yeah? Yeah, OK, potency. This is a very big one in stem cells,
the number of possible fates that a cell can assume,
commitment, decision to make a cell type, differentiation,
someone on the side of the room must have some thoughts. Yeah?
OK, I'll give that to you, to the final transition here.
Do you want one that's orange rather than pink? God knows I don't want
to be genderist here, but there we go. The
differentiation, the process by which a cell assumes
its final fate. And what we are going to discuss today
is the kind of gradations of commitments in differentiation,
commitment particularly, some of the steps of commitment.
The last one we have not discussed in class, but if anyone wants to
have a go, I will, yeah? Lineage, the term lineage in
biology.
Ish, ish. Let's try another one. Good, OK, a group of cells or a
group of cell types that descends from a common precursor,
so, this is a funny term, lineage, because it's used a lot in
stem cell biology. Let's think about it for a moment. The
definition I have is the set of cell types that normally arise from a
precursor cell. Now, of course, if you think hard,
all cells arise from a fertilized egg from a zygote. So,
really, all cell types are of a common lineage.
And this gets to the question of semantics, and where you put your
definition boxes. OK, in stem cell biology, and a lot of
biology, the term lineage refers to a subset of cells arising from a
common precursor that is somewhat arbitrarily defined. All right,
so this diagram which you've seen over several lectures
is really important. That's just what we've been talking
about, the progression from uncommitted cells through committed
cells through differentiated cells. And so, we are in the how-to two
module, talking about stem cells. Stem cells are in the news all the
time, and I want to, today, take you beyond the
newspapers, beyond the magazines, and tell you what I think stem cells
are all about. So, if you look at our game board of
life, we are, I would say, halfway plus through the semester,
and we've gone through all of these things, sticking out here into stem
cells. So, where are we? OK, so what's a stem cell?
Let me go back for one moment, and let's talk about stem cells.
For those of you who don't have a handout, you really should get it.
It's two pages up front here. So, let's do some board stuff,
stem cell, which I will heretofore abbreviate SC is a cell type that I
am going to refer to as somewhat committed. So that's deliberately
vague. This is not a committed cell type, but it's somewhat committed.
It knows more or less what it's going to become.
It can be either unipotent or pluripotent or totipotent. And that
stem cell, by definition, will go on and make an asymmetric
cell division. So, it will give rise to itself,
another, not itself, but another stem cell.
And it will also give rise to a committed, and here's jargon you
should know, progenitor. And so, this is an asymmetric with one S,
asymmetric cell division. So, you'll end up with two daughters
that are different from one another, a committed cell, and a stem cell.
And so, the stem cell in a sense is self renewing,
or is self renewing. And then, this is one of the big deals about
stem cells. That committed progenitor will go on to divide,
and it eventually gives rise to one or more differentiated cell types.
OK, in those differentiated cells will come from a specific lineage.
So, the stem cell is
undifferentiated. The committed progenitor is undifferentiated,
and eventually you'll end up with a differentiated cell. You can look
at your first handout. Here it is; I've drawn it for you out. You need
to know this, so I've done it in two places. There is the asymmetric
division stem cell plus progenitor, and the progenitor goes on to give
these differentiated cell types. There is no magic about this. This
is the same process we've been talking about all along except for
the fact of the self renewing thing. So, in the normal embryo,
cells go on down the pathway of commitment towards differentiation.
But they generally do not renew themselves.
So, a zygote, for example, although it's totipotent is not
considered a stem cell because it doesn't self renew. And that's true
of most embryonic cells. They do not self renew and so they are not
considered stem cells. So, what's the deal with stem cells,
and why, when I Googled stem cells in Google News last night did I come
up with several thousand entries for the previous week?
This is the deal. One of the great goals of biologists is to repair
organs, to repair damaged tissues. And this has been very difficult to
do. We touched on this a few lectures ago.,
stem cells have some kind of promise a some kind of universal repair kit.
And the idea is this, that one could isolate some kind of
magic stem cell, or some kind of stem cell,
and it would be autologous, which means it would be matching
your own cells. OK, so you isolate some kind of
autologous, self matching stem cell. That stem cell under ideal
conditions is pluripotent, or that set of stem cells, and can
be coaxed to form many different cell types depending
on how you treat it. So, you can imagine adding some kind
of factor. And if you like, you can call that an
inducer as we have been talking about secreted factors
in developmental biology. And that inducer would turn those stem
cells into progenitors. And those progenitors would
have a specific --
-- future fate by virtue of which factor you treated the stem cells
with. You would then take those progenitors, inject them into
someone whose body needed repair --
And the notion is that these progenitors would go on to
differentiate and repair whatever needed repairing.
OK, so this is the dream. And
this is why you can find headlines like this everywhere. These are
some of the ones I pulled out last night: Stem Cells May Help Repair
Stroke Damage. Stem Cells May Repair Broken Bones. Note the use
of the term may. OK, there's a lot of hype about stem
cells and not much that's been proven. There's a lot of money
involved in stem cells, trying to isolate stem cells to
repair people, and there is also,
there are also advertisements where you pay people to take your stem
cells, and to store them. So, poured blood refers to the blood
of the umbilical cord of a newborn, which is believed to be, it was
known to be full of stem cells of a certain kinds. And
you can pay someone a couple of thousand dollars to freeze that
group of cells and keep it in case the child needs some kind of stem
cell therapy in the future. So, what's the deal? Do stem cells
actually exist?
Well, yes, OK.
Do stem cells exist or are they just hype? They do exist,
and the idea is that sometime in embryogenesis,
as cells are normally going on and making the different cell types,
some cells are put aside, that the body is going to use later on for
repairing itself. And there are some examples,
and I will tell you a couple which are really fantastic illustrations
of this. So, do stem cells exist?
Yes. Where? Likely in the older embryo, and the adult,
and the particular organs that contains stem cells is not clear.
There are several very good examples, but it's not clear whether or not
some, yes, all, not clear. It's not clear whether all organs
contain stem cells. So, let's look at this a bit more,
and let's go back to talking about how stem cells were discovered. Why
are we having this conversation?
And I'm going to be referring to number two on your handout. So,
let's talk about the discovery of stem cells. The discovery of stem
cells was an accident, and it came about when people
started looking to see how long cells lived in a particular organ.
And the way you do this is by using a protocol called a pulse
chase experiment.
And a pulse chase experiment gets to the question of turnover rate,
or if you want, half-life of cells in an organ.
OK, this is the way it goes. You've got this as number two on your
handouts. You take a cell population, or you just take the
whole organism if you like, you feed it something called
bromodeoxyuridine. It doesn't have to be this, but this is a good one.
Bromodeoxyuridine is a nucleotide. Deoxyuridine, so,
you know uracil is normally an RNA, but if you make the deoxy form,
it gets incorporated into DNA. The bromo part allows it,
later on, to be detected by various colorimetric assays,
and you can add BRDU to an organism for a short time. This is called a
pulse. The BRDU is incorporated into DNA of those cells that are
undergoing DNA synthesis, and then by various means you wash
out the BRDU. And what you get is the labeled cell
population that had undergone DNA replication during this pulse
period. And then, if you follow this group of cells
over a period of hours or days or weeks or months or years,
you can look and see what happens to those labeled cells.
So, in my example, I started off with for labeled cells
over some period of time, the number of labeled cells per
total unit number of other cells decreases by half. And that gives
you the half-life of the population. And if you do this for many organs,
you find that adult organs do not just sit there with a cohort of
cells that doesn't divide. There is an enormous amount of cell
division in adult organs. And we talked briefly about this
previously. So, for example, red blood cells have a
half-life of about 120 days. And that actually means that there are
more than ten to the 7th new cells produced per day.
The intestine is something we touched on previously. Cells in the
intestine, some of the cells in the intestine had a half-life of three
to five days, and you're producing about ten to the tenth new cells per
day. Obviously, the number that you're producing per
day depends on the total size of the population.
Skin has a half-life of about 14 days, hair on your head,
a half-life of about four years, and your eyebrows and eyelashes,
half life of about 30 days. One of the mysterious half-lives are the
neural cells, your nerve cells. It was believed for a long time that
nerve cells never divided, and once you've got all your nerve
cells by about age two, you never made any more. In fact,
that doesn't seem to be true. And there are certainly populations of
neurons that divide. But we don't really know the half-lives for those
cells. So, this was very interesting data,
and it said that there had to be some way that the organism was using
to replenish these cells that were dying, and to repopulate the organs
so that things functioned properly. And in fact, you can go further
than this because you can not only count the labeled cells,
you can ask what those labeled cells become. So, you can look at your
labeled cell population and assay the fate of those labeled cells.
And if the cells want to differentiate from an initially
undifferentiated population, you know that these differentiated
cells must have been derived from stem cells or from progenitors
by definition. OK, so what organs do we know have
got stem cells? Let's talk a bit about this. The
test is a great example, and the spermatogonia,
the diploid precursors of the spermatozoa are dividing cells that
divide throughout life, and go on to give rise to themselves
so they can replenish themselves. And they go on to give to these
primary spermatocytes, which are the first step in the
cascade or in the lineage of cells that are going to differentiate as a
spermatozoa. OK, and you know you can do these. You
can look very clearly, and mathematically look and see the
number of cells, do this pulse chase analysis,
and know that the spermatigonea must be stem cells. This is a very
important stem cell lineage. It's the hematopoietic lineage.
In the bone marrow, there is some kind of pluripotential
hematopoietic cell that gives rise to all of myeloid progenitors,
so all the red blood cells, and the various other cells in the blood
stream as well as to all the immune cells. And those come from a
single progenitor, a single pluripotent cell. This is
called the hematopoietic lineage. And we will talk more about
that in a moment. This is an example that I mentioned
to you many lectures ago. Newt limbs, if they are amputated, will
regrow. The reason that they regrow is that there seem to be a
population of cells in the lab which are called blastema cells,
and those are the cells which can go on and to form the entire limb
again. And that's something obviously we can't do,
but people are very interested in. That's been a tough system to look
at. This might be a better system to try and understand the
details of regeneration. So, this is a planarian. This is a
flatworm. These are little guys. They can grow up to a couple of
centimeters, or a few centimeters. They are very pervasive animals in
the animal kingdom. And they have this extraordinary property,
as many simple animals do, that you can cut them into pieces,
and they will regenerate the whole animal. So, you can take out the
head. And over time, it will regenerate the tail. You
can take off the middle; it will regenerate a whole animal,
and so on. And this bottom picture is a planarian that's stained for
BRDU incorporation, or for another marker as well.
And each of these dots is a cell called a neoblast.
The neoblasts are the stem cells of planaria that can regenerate the
whole animal. And if you look right up front here,
it's not very distinct, but you will see a region where
there are no neoblasts. And that is the one region of the animal
that cannot regenerate. So, if you cut off the very tip of the
sort of nose equivalent region. It will not regenerate a new animal
because there are no neoblasts. And the system is being studied
here at MIT by Professor Reddien of the Whitehead Institute,
who is a new faculty member, and who some of you might went to Europe
with sometime. OK, so let's talk about isolating stem
cells, and how you do this. If one is going to use stem cells for
repair, you've got to be able to isolate these things.
And the challenge of isolating stem cells is that they are rare. For
example, in the bone marrow, the hematopoietic stem cells,
which I'm abbreviating HSC, comprise about 0.01% of the bone marrow.
And I would say it's fair to say
that no one has really seen a cell and said, oh, this is a stem cell.
It's hard to pinpoint exactly what a stem cell really looks like. It's
just a cell. But it's got particular properties,
and you need the appropriate way to look at the cells and see these
properties. One way that's been used to isolate stem cells is
something called FACS, or fluorescence activated cell
sorting. I'll talk about it in a moment. And there are two ways
that FACS is being used to isolate stem cells. One is by getting a
group of cells called SP cells where the SP stands for side population.
We'll talk about that more in a moment. And the other is through
the use of cell surface proteins that are characteristic,
or enriched in stem cells. And this has been many decades of work
by many people to come up with a set of criteria by which one can enrich
for stem cells in a particular population.
So, here's the way. Actually, before I go through that, let me go
through the assays, and then we will go through all the
slides together. How do you assay for stem cells?
Well, one of the ways you know you have a stem cell is if you have
something that can act as a stem cell. And there are two assays
that you should be aware of. One of them is a repopulation assay usually
done by transplant, and very often you remove some
endogenous group of cells, and then try to replace that group
of cells by using stem cells. So, you remove some set of
differentiated cells.
And then, you try to replace with transplanted stem cells. And
the other way you can do this is in some kind of in vitro culture assay
which I will talk about later on. All right, so with this in mind,
let's go through some of the slides. You've all heard of bone marrow
transplants, which people who have leukemia and other associated
disorders undergo to repair themselves to get rid of the
leukemia cells to get rid of the cancer.
This is how it works in a mouse. And this is a repopulation assay.
I'm going to use bone marrow transplants as an example. You take
your mouse, or your person if you are undergoing bone marrow
transplants. And you irradiate to destroy the bone
marrow. The reason you do that is to make space for new cells to come
in to expand and grow. If you just put your new cells into
an animal with an intact bone marrow, they kind of disappear
amidst the masses. So, you have to give the cells your
assaying a chance. And then, that irradiated mouse would die.
That irradiated person would die. But you get them,
now, an injection of normal bone marrow. And if things go well,
the mouse or the person lives. And there was a Nobel Prize given
out some years ago for developing bone marrow transplantation. One of
the gold standards in the stem cell field is asking whether or not this
rescued mouse has regenerated stem cells because you can imagine that
what you're doing in this case is giving cells that are the
progenitors. They are one step down from the stem cells. Or they might
even be partly differentiated. So, you can imagine that you are
restoring this mouse is life by giving cells that are
not self-renewing. Excuse me, one of the gold standards
in the stem cell field is to take this rescued mouse,
isolate more stem cells or more putative stem cells,
and take those and then tried to rescue another mouse. And
in the hematopoietic system, you can do this over and over again.
So, how many stem cells do you need to repopulate a mouse?
Actually, you need one, and I will tell you how this is
done. So, the idea of the first thing when you are trying to do
assays to figure out how many stem cells you need for rescue. You take
your bone marrow; you stain it for some stem cell marker. You have
this in front of you. OK, this is number nine of the first
page of your handout. You stain somehow for a stem cell marker.
I'll tell you in a moment how you sought the stained cells through
this fluorescence activated cell sorter.
And you isolate from it a pure-ish population of stem cells,
an enriched population of stem cells. And then,
you do a dilution assay where you inject different
numbers of cells into a recipient irradiated mouse. Now,
you don't actually injects one, ten, and 100 cells. You inject one
cell, and millions of carrier cells to help those cells along,
OK? The one cell would just disappear in your syringe. So,
you've got to give it some companions. But
the bottom line is you really only need one stem cell to rescue the
life of that mouse. It can go and repopulate the entire
hematopoietic system, all the blood cells, all the immune
cells. What is this fluorescence activated cell sorter?
Fantastic machine. It looks like this. Again, you have this in front
of you, so look up here. The idea is that you take a reservoir of
cells that you have labeled with particular antibodies. And these
are living cells. OK, you label them in certain ways,
and they can be labeled with fluorescent antibodies or
fluorescent dyes. I'll tell you a dye example in a moment. And
you put them in a reservoir, and you trip them out so that one drop of
liquid contains one cell on average, or zero cells. And you let those
cells drip through a laser and a fluorescence detector. The laser
activates the cells. They fluoresce, and you set the detector to activate
a charging collar at a specific wavelength of
fluorescence. And when the charging collar is
activated, it will activate some deflecting plates which will give
charge to cells of particular colors, and move them into particular tubes
so that they are sorted on the basis of their color. OK,
this is done one cell at a time but it's really quick. And
you can purify millions of cells to do these kinds of assays.
This kind of thing is not done for human bone marrow transplants.
There, you take a much bigger population of cells from the bone
marrow, and you generally don't purify them very much. But
if you want to do specific stem cell assays, and many others,
the fax machine is really fantastic. So, what do you get out of this?
Well, you can plot what the cells look like. So,
here in this example, I've got cells that are labeled in
red and green. And you can label them according to this
plot as to whether they have no label, red label,
green label, or both red and green. This is a real example of an
experiment that was done here at MIT a decade ago in Richard Mulligan's
lab. And this is what a real FACS plot looks like. OK,
it's a mess. Every little is a cell. But for reasons known only to
the Mulligan lab, Peggy Goodell who is now a
professor in her own laboratory, they assayed this little region of
cells in the bottom left-hand corner for their stem cell properties.
They called these SP cells or side population cells,
and they found to their surprise that these SP cells were highly
enriched for hematopoietic stem cells 1,000 fold or more. And
in fact, if you take the very bottom left-hand corner where there are
almost no cells, you get a 10,000 fold enrichment.
So, the way they sorted these was with a dye called Hest 3342.
This is a vital dye. It stains the DNA but it doesn't tell the cells.
And somehow, these SP exclude the dye, or efflux it,
remove it from the cell. And it's really not clear what that has
to do with stem cellness, but this is still one of the very
best ways to isolate stem cells from almost every organ. These SP cells,
these things that don't stain with these DNA dyes seem to be the ones
that are stem cell-like. OK, so let's move on to the question
to several questions that I want to discuss with you that fall under the
umbrella of regulation and control of stem cell fate. And
there's several questions I like to pose to you. Firstly,
what makes a stem cell self renewing? What's the molecular basis for that?
What makes a stem cell decide whether it's going to make
progenitors or not?
And the big one for the stem cell field, what controls the potency of
a stem cell? Well, this is where we step back into the
developmental biology that we've been talking about because it all
controls at some level by gene expression.
And in particular, there are both intrinsic and
extrinsic factors that seem to control certainly the first two
points on my list, the self renewing and the progenitor
aspect, and perhaps also the potency also.
Intrinsic factors, cell autonomous factors,
determinants --
-- and extrinsic factors, non autonomous factors, secreted
ligands, inducers, and these and extrinsic factors have
been given a special name in the stem cell field just for argument's
sake. They are called the niche where the niche contains all the
cells that influence stem cell activity, cells that influence
stem cell activity. OK, this is just a term. You should
know it because if you read it you will know it. So,
here's how it works. The surrounding cells in the niche are
cells that seem to maintain stem cells usually in acquiescent state.
So, it's believed that in most organs, stem cells are
sitting there quietly. They're not dividing very much,
but they can be stimulated to divide, and this is on the second page of
your handout. They can be stimulated to divide by some kind of
environmental input, and this changes the surrounding
cells. And the environmental input could also be a secreted ligand for
example. And the surrounding cells then induce
the stem cells to be activated. And they go on to make progenitors,
and of course also to self renew. This is a fancy way of saying that
cell fate is controlled by induction. OK,
so you should be nodding. The should not be anything new for you
at this point. It's phrased a little differently,
but that's all. This is a very interesting example of control of
cell fate by surrounding cells. This is from my colleague,
Professor Fuchs at Rockefeller who studies the hair follicle,
and who has shown that this region called the bulge is the
source of stem cells. So, this brown thing is the hair
that sits in a shaft of cells that got a bunch of interesting cells
around it. In particular, there's a rather inconspicuous group
of cells on one side called the bulge. And some years ago,
there is another group of cells. I'm also going to refer to two at
the bottom, which is called the dermal propeller.
But some years ago, Professor Fuchs took the cells of
the bulge and she transplanted them into a mouse that didn't have any
hair. It's called a nude mouse. It has lots of problems including no
hair. But when she did that, here's the control and here's a
mouse into which these stem cells have been transplanted. And
you can see all of a sudden this poor little nude mouse has got tufts
of hair. OK, and in fact, these hairs actually
glow-in-the-dark. They've been labeled with green. It's
really cool, OK? The cells that were transplanted,
this is something you know, as well. They were lineage labeled,
and so, you could prove that these hairs came from the transplanted
cells. All right, so let's look. So,
during the hair cycle, so you're hairs grow cyclically.
And there are a whole bunch of processes including growth,
regression, induction of growth, and new growth. And during that
period of time, the bulge and the dermal papillae
are in relatively different positions.
So, during a process of growth, they are far away from each other,
and then as the hair and growth regresses, they come closer until
they're actually touching each other during the process of induction of
new growth. And it's at that time that new growth in
the hair is stimulated. And it's clear that the dermal papillae
is signaling to the bulge cells. And here's what it's using to
signal. It's using something called the wind pathway,
and in particular, a molecule called beta-catenin. If
you think back several lectures, we talked about beta-catenin is one
of the things that told the embryo to make its back rather than its
belly. So, here's a different use of the same molecule in stimulating
hairs to grow. And so this is a particularly cool example of cells
coming together at particular points to stimulate stem cells to go on and
to make progenitors. All right, let's move on to
something important called embryonic stem cells And all right --
-- also called ES sells. So,
although I told you that most adult organs are likely to have stem cells,
and this has been very clearly shown for many, there are many organs
where it's not clear whether they have stem cells,
or it's very difficult to isolate them. Stem cells are rare in all
organs, and things like neural stem cells in the nervous system seemed
to be a exceedingly rare and difficult to isolate. So,
the push has been to try to find a source of stem cells that would be
more plentiful and more useful for repairing lots of different organs.
And that's where the embryo comes in this particular kind of stem cell
called an embryonic stem cell. And the idea is if one takes an embryo
or part of it, puts it into culture,
after many steps you get out cells that are called ES cells. And
these ES cells are pluripotent. They are not totipotent. But they
are very, if I can be forgiven, they are very pluripotent bouquet,
and they are pluripotent, and you can control their cell fate.
So, let me going to the slides, and we will talk more about this
through the slides. So, in mammalian development,
and we're going to talk about mammals here specifically,
mammalian development, the blastula forms. And at a certain point in
development, a group of cells called the inner cell mass segregates from
the rest of the cells which form a shell around the embryo.
We mentioned this previously. This yellow stuff is fluid,
and this little group of cells called the ICM,
or inner cell mass, is the thing that's going to give
rise to the embryo proper. The cells surrounding are going to give
rise to the placenta and the other extra embryonic components. So,
the idea in trying to get these ES cells is to take the inner cell mass
of an early embryo, take it out of the embryo,
and put it in a Petri dish that's got nutrients and various factors to
disperse the cells such that you've got single cells dispersed in the
plate, and give them nutrients. And over time,
those cells will grow, and they will form clumps,
each of which is derived from a single embryonic cell. OK,
now, normal embryonic cells do not do this. OK, they will normally go
on and differentiate, and stop dividing, but there's
something that happens during this culture process that is abnormal.
And it turns the cells into groups of cells that can self renew. OK,
so you've turned these cells into self renewing cells. And
each of these groups of cells or colonies that you get may be able to
grow into a stem cell line. And I'll talk about cell lines in a
moment. I'll talk about cell lines now. So, you might not be familiar
with the term cell line. What is the cell line?
A cell line is a cell population, a homogeneous cell population that
could grow continuously in culture. So, it's self renewing. OK,
so all cell lines are self renewing. A stem cell line is a cell line
that has the capacity to go on and differentiate into specific normal
cell types. So, there are many cell lines that will
grow continuously in culture, but they will never go on and
differentiate as anything. They are very abnormal cells.
They are useful for many studies, but they are not stem cells. The
stem cells not only can grow continuously, but they can go on and
differentiate. But you are dealing with an abnormal cells here. This
is not a normal embryonic cell. OK, how do you test the potency of these
ES cells done in the following way? The ES cells from a mouse, a black
mouse, are taken, and they are injected into an early
embryo of an embryo derived from white parents.
And if you do that, the ES cells incorporate into the
embryo. You then take the embryos, and put them into a surrogate
mother. And when the mice are born, when the babies are born,
you can see that they often are not just pure white. They often got
black stripes, and you can look at the various
organs and show that these ES cells have incorporated into various
organs. So, these ES cells are highly potent. And
the idea is that you can take these ES cells and add to them
various factors. So, you can add a particular red
factor that will turn an ES line into heart muscle cells or pancreas
cells or cartilage cells. And you can use those cells in your stem
cell repair assays. OK, and this is true. You can take ES
cells, and you can do exactly this to them. And then,
they will become these different kinds of differentiated derivatives.
So, this is really cool. And the push has been to try to get
this to work not for mice but for humans. And this is where the huge
controversy in the stem cell field comes from. So,
the controversy comes from the fact that you need embryos to get these
stem cells. You get the embryos from in vitro fertilization that we
touched on a few lectures ago. Eggs are isolated by ovarian stimulation.
They are fertilized in vitro, and they are allowed to grow in
vitro for a week or so. And at that point,
there are harvested or killed to make the ES line. And
this is the very controversial point, whether or not it is ethically OK to
harvest these embryos, and turn them in to stem cell lines
or not. Presently, there is no federal funding that is
allowed to be used to make human ES cell lines. There are some that
exist, and President Bush has told scientists that they need to use the
ones that exist. However, they are not very good cell
lines, and so scientists have used private funding to make new human ES
lines that hopefully will be more useful. I don't think there's any
right or wrong answer whether this is OK or not. My opinion is that
this is an OK thing to do, with all due respect to the embryo.
I think one can save people's lives, well, and the embryo at this point
is not a differentiated entity. But it is an embryo. And so,
this is an ethical issue. There's opinion here, and it's good for you
to think about what your opinion about this type of research is. OK,
so let's move on. And I'm going to, in the last minute,
just touch on something called stem cell plasticity.
One of the things that has come out
of the human ES work or all the ES work, and that has come out of the
quest not to use embryos for stem cells is to ask whether or not one
can turn one stem cell line into another. So, it's easy,
or relatively easy, to get hematopoietic stem cells. And
wouldn't it be wonderful if you take those hematopoietic stem cells and
turn them into brain stem cells, and fix people who have Parkinson's
or Alzheimer's? OK, wouldn't it be wonderful to fix
people who have muscular dystrophy by turning hematopoietic stem cells
that you can get lots of into muscle cells? So, the idea is maybe you
could turn something from one lineage, a hematopoietic stem cell
into a stem cell from another lineage and get it to
do something else? And the hypothesis,
then, is that if you do appropriate experiments, you may be able to
figure out where the stem cells from one lineage can contribute to
another. This is the second to last slide on your handout. So,
this is the way the experiments were done. It's the last thing I'm going
to tell you. Bear with me. You can take a mouse that's been dyed green,
and its hematopoietic stem cells are expressing GFP,
which is a green fluorescent protein. You use that mouse as a
daughter for bone marrow transplants.
You put in the great cells. Not only do you rescue the bone marrow,
you also ask whether other organs have got green cells in them,
indicating that the hematopoietic cells can contribute to other
lineages. And when you do that,
initially people got very excited because you saw results like this.
And this is data from my colleague, Dr. Camargo over at the Whitehead
Institute. When this experiment was done, he could show that the liver
of such an animal had lots of green cells, suggesting that the
hematopoietic stem cells could also be liver stem cells.
But when he looked more closely, this is a complete artifact. And
what had happened was that the hematopoietic stem cells had
actually fused with the liver cells, making the liver cells green. And
in fact, presently, there is no data to suggest that you
can interconvert stem cell lineages. And I'll stop there. Thank you.