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visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: The thing we
have to talk about here is multicellular life. Cells, although we can
think of them as entities, are generally not the
unit of a whole organism. There are many organisms
whose unit is a single cell, but the really interesting
ones are the ones that have got lots of cells. And so life has
developed in a way that most organisms
are multicellular. And we can pose the question
of why that might be. My trite phrase, while
it's more interesting, is actually a very poor answer. There's a much better answer. And that is if
you have something made of lots of cells,
different parts of that organism can specialize and
do different things. And so you can get a
diversification of function, which allows the
organism to colonize new parts of the earth,
interact with its environment in different ways, interact with
other members of its species in different ways,
and it does become-- allow a much greater
complexity of life. So I would say--
the answer I would give you is it allows
new functions to evolve and greater complexity
of life to develop. If we consider the human
adult, and look and see what we're made of, we contain
about 400 different kinds of cells. We can call those cell
types where each cell type has a specialized function. Those cells are
organized together, and they're located
in specific places. So the cell types are
organized into groups. They are organized into
three dimensional structures. One of the things that is
most interesting about living organisms is that they're 3D. Think of it if we were all
sitting there as flat sheets, you know, you get the idea. The 3D-ness is extraordinary. And it's one of the things
that being multicellular allows you to do. So cells are
organized into groups, they are organized
into 3D structures, and this notion of groups
of cells and structures is that they work together. Groups of cells arranged
in a certain way work together to form
a whole, an organ, that has got an even greater
specialized function. So these work together
to make organs. And in particular, as
we'll talk about next time, it's the groups of cells
and their 3D structures which makes the organs. And the organs themselves can be
organized into super structures called organ systems. And we'll work on
all of these things over the next
couple of lectures. Today we're going to
talk about cell type. Here is-- before-- I'm thinking if I should
show you a slide first, or if I should write
this on the board. Let me show you a slide first
because I want these next two boards to follow
one from another. All right. Different cells. Oh! There we go, OK. Different cell types. These are micrographs
of different cell types. Here are red blood cells, which
regulate oxygen transports for metazoa involved
in reproduction, and neurons involved
in communication. Each of them has got the
basic cellular functions that we've talked about over the
last lectures, but each of them is obviously
morphologically different. They look different and they
carry out different functions. The cell types get
organized in specific ways. This is a really
extraordinary example, which is the retina of your
eye, the light sensitive part of your eye, which contains
a number of different kinds of cells. And these cells are
organized in layers, and so they're color coded here. And you can see the different
layers are grouped together, but then the different layers
communicate with one another. And this communication, and
this organization is key. You can have all the different
cell types in the retina, but if they're not arranged
in this rigid structure, or in this specific structure,
the retina doesn't function. 3D structures. We'll talk next time about
engineering structure out of cells. And the heart is
one thing you have to think about engineering. The only raw material that you
have to use for the engineering is cells, and so how
do you get something that looks like a human
heart, and carries out the exquisitely regulated
pumping function of the heart. And here's another one
that I'll touch on, which is the
question of position. It's not that we
just are made up of lots of different kinds of
cells that are grouped together in organs. It's that they're
also positioned in the correct place. And in this plasticized human-- actually, I think it's
a fully plastic human-- if you open up the cavity
and look into the abdomen, you can see these organs
that are arranged packed so beautifully like this. It's no accident. They get there, because
they're told to get there. There is a process
that positions the organs in the precise way. And if they're
positioned incorrectly, there are medical consequences,
which are very severe. All right, so all of these
things we need to think about. But let's go back to
the board, and you will recognize
this diagram, which is pivotal for what we're
going to talk about today. Here is a mantra that
I've mentioned before. And you need to know and
really need to understand this. All cells contain the
same set of genes. Professor Jacks will give you
one exception to this rule, but it's the exception. All cells in your body
contain the same set of genes, but not all those genes are
used in every cell type. But each cell type uses-- and you know the
word expresses now-- each cell type
expresses a subset. And it's a unique subset-- well, let me put
unique in parentheses, and you'll see what I mean-- a unique subset of the genome. And this set of genes and
the products of those genes make the cell type what it is. The products of those genes,
usually proteins and some RNAs, give the cell type
it's function. So there are two
corollaries here. One, you have to understand
how the expression of genes is controlled, and that
is pivotal to this list. And two, this set of
genes that makes each cell type, the set of active genes
that makes each cell type what it is, forms a kind of a
combinatorial code for a cell type. So let's just write this down. So control of gene
expression is crucial. And there is a
combinatorial code of gene expression
for each cell type. A combinatorial code
of expressed genes for each cell type. You will not find this notion
of a combinatorial code in your book. But I think if you talk to
any life scientist who's doing research right
now, they would agree that that
is the correct way to be thinking about cell type. So let's explore
this a bit more. And let's write out three
different cell types. And I'm going to introduce
the word here cell fate. And both cell type and cell
fate can be used interchangeably with the term function. And let's pick neurons, muscle,
and the epidermis, which is the outer layer of the skin. And let's consider
the genes that are present in each cell type. And let's consider
the genes that are expressed in each cell type. The genes that are
present in each cell type as I've just told are the same. And so let's make them
A, B, C, D, and E. And each of those genes is
present in each of these cell types, but only some
of them are used. And so let's say in the
neurons A, B, and E are used, in muscle A, C, and D
and A, B, and C in epidermis. So look at what
I've written there. And you can pick
out some patterns. You can pick out a
gene that is expressed in all of the cell types. So gene A is expressed
in all of the cell types. And gene A exemplifies a gene
that we term ubiquitously expressed, sometimes
termed a housekeeping gene. I really dislike that
term, but you will see it. So ubiquitous expressed,
maybe you'll see housekeeping. You can come to
office hours, and I'll tell you why I hate that term. And then you can
pick out a gene, which is expressed in some of
the cell types, but not others. So let's look at B.
Here's B expressed in the neurons in the epidermis,
but not in the muscle. And B would be referred
to-- and actually C is the same way, isn't it? C is in muscle and epidermis. B and C would be
referred to as genes with restricted expression. So B and C would have
restricted expression. And then there's two
genes there E and D, which are only expressed in
one of their own cell type, in neurons or in muscle. So D and E would be cell
type specific genes. And from this simple example,
you can see a number of things. Firstly, you can get
a combinatorial code that is specific for a cell
type without any genes that are specifically expressed
in the cell type. If you look in the
epidermis example, A, B, and C are expressed. None of them are a
cell type specific. But nonetheless, they give
the combinatorial code that is the epidermis. And then the other examples,
they each got a cell type specific gene. What is a combinatorial
code really look like? Well, to be honest we
don't actually know. There is no cell type for
which the combinatorial code is being worked out. But you know now there were
about 20,000 human genes. And probably about
half of them are expressed in most cell types. So the combinatorial code
for any given cell type is going to be thousands
and thousands of genes, which are expressed
or not expressed and which are also expressed
at different levels. And we have to take
that into account. So really finding the
combinatorial code is incredibly difficult.
And we don't know any for any cell type, but
the notion is exactly the same. I want to remind you
here with this diagram that the control
of gene expression to give you the final
expressed product can be anywhere all the way
from chromatin structure through transcription all the
way through protein processing modification and localization. Good. All right, so now
we have a framework by which we can think
of what a cell type is. And the question, of
course, that you're asking is, so how does that combine
tutorial code get expressed in each of the cell types? Let's pose that question. How does a cell type
express its code? And there's a couple of answers. The global answer that I'm going
to give you to this question is stepwise. And let's have
stepwise answer one. The idea-- and we can do this-- I'm going to do this two ways. I'm actually going to do this-- no, I'm not going
to do this two ways. Before we get there,
this is a great slide, which will show you two things. One, it will show
you the expression patterns of two different
genes in the whole organism. So here is a gene called myoD. I've shown this to you before
that is cell type specific. It's just expressed in
these kind of chevron shape things, which are the
developing skeletal muscles, the voluntary muscles. And here are a
couple of genes that are expressed in large regions
of the developing animal. And you can see they're
expressed, because of these colors that
are there and the colors are indicative of where
the RNA for that gene is. The technique that allows
you to look in a whole animal and ask where the RNA is for
particular genes are found, is called in situ
hybridization, up here at the top of the screen. And the idea is that
you take animals-- here, I've said embryos-- developing animals, and you fix
them, which means you kill them and you permeabilized them. You make holes in
them, and then you use the principles of base
pairing, where you look and see where the RNA is
using a probe, which is an antisense RNA for a
particular gene of interest. And you label this
antisense RNA. You mix it with the
embryo that's got holes in or the animal that's got holes,
where the RNA for gene x is. It will base pair to
your antisense probe. You then wash out the extra. And you look and see where the
color that comes from the label is. And that color tells you where
the RNA for a particular gene is. So these colors in
the developing animal tell you where
particular RNAs are. It's a very powerful technique. And it allows us to figure
out which genes are cell type specific and which genes are
more generally expressed. Let's look at your
first handout. And I'm going to
write it on the board, as well, because this
is really important. And you're going to need to
know this for this lecture and when we get to
stem cells, as well. So you can look on the screen. You can look on your hand-out. And I would suggest
you write it, as well. The notion when we're
thinking about cell type is that we start off with cells
that don't know who they are. And they're called
uncommitted cells. They're undecided. And as they go through
life, they get some inputs. We'll be vague about those. They're up there. I'm not going to
write them here. And at that point, they
become committed cells. They are sometimes
called determined cells. And at this point,
the cells have decided what they're going to become. And later on, those committed
or determined cells will go on. And they will become
differentiated cells, where they have their final
fates or their final function. There's a time metric
on this progression. And the notion really is that as
cells go through this decision making process, they
change which genes they are expressing, such
that at the culmination-- but if you think about this
or come and talk to me, you'll find it's
more complicated. At the culmination,
they will be expressing their combinatorial code. Uncommitted cells, as they
transition to committed cells, activate a set of genes that I'm
going to call regulatory genes. They're the
transcription factors, the translation factors, the
protein processing factors. And these regulatory
genes will then go and activate a set of genes
that I'll call effector genes. And the effector
genes are the ones that are actually carrying
out the function of the cell. They are the globin
that's carrying the oxygen around the body. They're the
neurofilaments that are making the neurons long
and strong and able to transmit a signal. They are the cartones, which
make hair cells able to secrete the hair that actually you see. So the effector genes are the
functioning, the functional mediators. And it's this mix of
regulatory genes, which I've written as R, an
effector genes, which I've written as E, which
form the combinatorial code for a cell. So that's one answer. And you have more
up there, which we'll come to in a moment. But let me give
you another answer. It's also the same
answer stepwise, but this answer has to do with
the history of an organism. I've avoided talking
about embryos, until now, because I
wanted you to think about the outcome,
the cell types. But actually, all
of this starts-- cell type formation starts
right at the beginning of an organism's life,
when two haploid cells, the egg and the sperm, magically
get together and join to form a diploid cell, the zygote. This zygote, which
is also a single cell is itself a magical
cell, because it contains all the information necessary
to form whatever organism is going to be the outcome. The zygote goes on
to form an embryo, also diploid that
contains many cells. And the embryo goes on to
form the diploid adult, also with many cells. And in humans,
there are about 10 to the 14 cells in
the human adult. And during this process
of two single cells, two dying cells, the
egg and the sperm are with a very
finite lifetime, when those cells fused
with one another-- and we're not going to
talk about this any more than that then this discussion
because of time constraints. When they fuse to
form the zygote, there is an extraordinary
process where the zygote is now resurrected in its life,
and it has the capacity to give rise to
the whole organism. What happens during
this process? Well, firstly, I've
pointed out, cells divide. There's a lot of cell division. And that's really key to
getting different cell types. You have to have
something to work with. And as they divide,
they become different. According to the list on the
screen and on the board above, the egg, the sperm, in fact,
are differentiated cells, but let's start with
the zygote, which is undetermined or uncommitted
and undifferentiated. And I'm using determined
and committed deliberately interchangeably so you get
the idea that the terms are interchangeable. So the zygote is undetermined
and undifferentiated. And as it goes through
its embryonic stage, determination starts,
continues into the adult. And later during late
stages of embryogenesis and into the adult cells
differentiate and become their final thing. So determination
starts in the embryo and sometime later the
process of differentiation starts and continues. And so that's a second way
of actually writing out the stepwise phenomenon of
how cells become different from one another and how
different cell types are formed. Let's take a look
at some slides here. And let's take a look
at a couple of movies. This is a movie of the first
few weeks of human development. And what you will be
able to see from this is the enormous increase in
size of a human embryo that's coupled with cell division and
also with cell determination. And it's going to play again. And what you can see,
up until about day 56, we all had very nice tails,
and then they disappeared, unlike other animals. But all of these are
taken at the same scale. And so you can get a sense
of the huge amount of cell division that's going on during
these first few weeks of life. Here's a second one. This is a zebrafish embryo. And I want you to
watch this embryo as it develops very rapidly
much more rapidly than a human during the first 19
hours of development. The fish embryo is kind
of like a chicken embryo. There's a big cell
called a yolk cell. And on top of this yolk cell
sits a little other cell, which is the embryo itself. And it's this little top cell. Here it's already
divided to give rise to two cells from which the
embryo is going to arise. And so as you watch
the movie, you'll see these two cells dividing
into four, into eight, and so on. And then you'll be able to
see the beginnings of the fish emerge. And I'll play it
a couple of times and point out some
things to you. Here's the cell
division, taking place. It's not as rapid as
this, but it's of course, a very rapid process. And now you have a little
cap of cells on the yolk, and watch what happens. That cap of cells spreads
out to cover up the yolk. And a lot of cells
move to that side of the embryo, the right
hand side of the screen. Here's the eye
emerging in the brain. And here are the
muscles of the fish. And let's watch it again. Isn't that cool? So let me stop it. Here at this stage, you have
got several hundred cells that are sitting on
top of this yolk cell. This pointer seems to have
died, but they're sitting on top of the yolk cell. And you will be able to see
when I start the movie again how those cells
spread out to cover. And they are actively doing it. They know to do it. They actively
spread out to cover the surface of the embryo. And then actively
a group of them moves to one side of the embryo
to form most of the embryo, including all the nervous
system, the muscles, the intestines, and so on. So let's start it again. There you can see these
cells spreading out. They've spread it
out to this-- they've spread out to this point. Let's let them
spread out some more. Here they are. They've completely
closed up the yolk. And if we stop now, you'll
see on the right hand side of the screen. There's a much thicker
group of cells. They're thousands and thousands
of cells now that are there. Now at this point, if you
look at gene expression in the embryo, you can pick
out many different regions of the future brain. You can pick out the
future intestines and the future muscles, but
there is no differentiation at this point. The cells do not know. The cells have not
finished becoming what they are becoming. And here as we go on a bit
more, here is the eye up front. And these bumps of the brain. And then these little
chevron shape things there are the future
muscles, your fish fillet. And there is the fish moving on. It's really a fantastic process. Very good. Let's go back to
the board and talk about this more theoretically. We have sort of answered
a question here. How does a cell type
express it's code? Well, not all at once,
over time and over a series of many steps. But that's not actually
the whole on answer. And so let's rephrase
the question to make it a little more precise. And I've rephrased it by asking,
what tells a specific cell type to express its code? What tells a specific cell
type to express its code? And the answer is-- it's just not going to
be really helpful to you, but it will in a moment. The answer is that there
are a bunch of inputs. There are a bunch
of instructions that the cell gets. And let's write this out in
a kind of theoretical way. We'll get to the
molecules in a moment. Here again are our
uncommitted cells. And they can be exposed to a
number of different molecules. The inputs-- let me
just come clean here. The inputs are
specific molecules. And as you'll see in
a moment, they all have got something to do with
cell signaling or regulating gene expression. These uncommitted
cells can be exposed to many different inputs. Let's take three
different inputs. We'll call them
input 1, 2, and 3. And these inputs through
many steps and changes in gene expression. And the inputs can be composite. They don't have to
be a single thing. We'll take those cells into
differentiated cell type 1, cell type 2, or cell type 3. And cell type 1-- you know, to belabor
this, we'll express code 1, cell type
2 code 2 et cetera. As these decisions are
made similar to the kinds of decisions we've looked at
in biochemistry previously. There can be interactions. And so it may be that as cell
type 1 develops, it's actually inhibits the formation
of cell type 2. And cell type 2,
in turn, might be an inhibitor of the
formation of cell type 3. So there are interactions
between cells. So the inputs can be composite. They can composed of several
molecules factors if you like. And there's crosstalk,
just like the crosstalk between the receptors
that we talked about in cell-cell signaling. And that's not surprising,
because in fact the inputs includes cell-cell
signaling molecules. So there's ligands and receptors
that we spoke about previously. So what are the inputs? And there are two. One are signals that
act between cells, just like we've been
talking about in cell biology, cell-cell signaling. And wherever you talk
about cell-cell signaling, it is implicit that you're
talking about interactions between cells. Ligands, which in development
are sometimes called inducers, acts on the receptor. And the outcome is
to change cell fate. This is a subset of the
signaling interactions we talked about previously,
where the response here or the response
previously was manyfold, the response here would
be to change cell fate. But there's a second
kind of input. And those are the molecules
that are cell autonomous, also referred to as
being inherited factors and referred to in
development as determinants. And these determinants-- this
is what cell autonomous means-- act within cells. And so they're not going to
be ligands some receptors. They will be
things, for example, like transcription factors. Furthermore, both
determinants and inducers can act in a concentration
dependent way so that a small amount
would give you one cell fate and a larger amount would give
you a different cell fate. So it can be
concentration dependent. And in the cases where a
signal or a determinant is concentration dependent,
it gets a special name. It's referred to as a
morphogen, for historical, not particularly logical reasons. But if you see the
term you'll know. And finally, before
we go to some slides, one can find groups of
these inputs, groups of these regulatory molecules. So all of these inputs, all
of these other regulatory molecules or regulatory factors. And they can act
spatially in groups. So you can find regions of
the developing embryo, where there are groups of these
molecules acting together. And where you find these
groups, that particular region of the embryo may have a
powerful effect in influencing the cells which form around it. And that region, that group
of factors in one region that can influence
its surroundings is called an organizer. So groups of signals,
which are localized, that is in one place,
in one group of cells can influence the
surrounding cells and is termed an organizer. And there are many organizers
in the body both in the embryo and in ourselves
as we'll discuss when we talk about stem cells. Examples of organizers,
I'll show you in a moment. There's something called
the Spemann organizer, which is very famous. And some of you may have
learned about that previously. And then there are
regions, for example, in the forebrain, the
developing cerebrum, where there is an organizer
that actually tells the different parts of you
developing higher cortical function to form. So let's look at some slides
and some of your handouts. And I drew these
for you, because I thought it was really
important that you got them. Here is localized determinants,
localized regulators called determinants. And you see the idea here. This is important. This isn't on the board. Here's the mother
cell with these boxes, where these boxes represent
some kind of regulatory factor. And you can see I've drawn
them on one side of the cell, such that when that cell
divides one of the daughters doesn't get them and
one of them does. And if the boxes are
regulatory factors, the daughter's cell that gets
them will go on to do something different than the
one that doesn't. And you can get two different
cell types coming out of this. And I've listed examples
of the many different kinds of factors that can
be determinants. Here's a real example. This is an early worm embryo. Remember, I told you
about Professor Horvitz, who got the Nobel Prize
for discovering cell death processes. This is the same
animal he works on. And on the top panels are
nuclei stained in blue with this dye called dappy. Here's the zygote. Here's the two cell embryo
and the 32 cell embryo. And you can see that there's
a nucleus in every cell. On the bottom are
some determinants that are called pea granules. Pea granules are composites
of protein and RNA. And they are regulators of
where the future germ cells will form, the future egg and sperm. Look how these germ-- look
how these pea granules segregate during development. Here they are on one side of
the embryo, even in the zygote, and then the zygote splits. Look, one of the two
cells gets them more and the other does nothing. And at the 32 cell stage, there
is one cell out of 32 that has all of the pea granules. And they've been
excluded or degraded from the rest of the embryo. And that is a really
beautiful example, perhaps the most
beautiful example of determinants segregating. Here's another one,
the signaling factor secreted by neighboring cells. Uncommitted to cells over
time will do something. There's enough transcription
factors and regulatory factors in all cells that
over time they'll go on to differentiate
into something if they're normal cells. But there's a signaling cell
telling an uncommitted cell to activate a signaling pathway
and go on and make cell type 2. And these are the
inducers, which are ligands binding to
receptors and changing sulfate. These signaling pathways can
act in a concentration dependent way, as seen on that screen. And here is the notion
of a morphogen, where a high ligand
concentration will give an output of cell type 2
and low ligand concentration and output of cell type 3. And we touched on how this works
molecularly, previously, it's not well understood
and is complicated. This is the most
famous example of cells that can go on and tell
other cells what to become. It's a group of cells that was
termed the organizer before it was clear that there were
actually lots of organizers. And they organized was defined
by a graduate student Hilde Mangold, together with
her advisor, Hans Spemann. Mangold, unfortunately, went on
and was killed in an explosion. Spemann went on to
get the first Nobel Prize for developmental
biology in 1935. It's never seemed fair. It isn't fair, anyway. That is a sad story. Spemann was the preeminent
developmental biologist in the 1920s. And this is the finding
that Dr. Mangold made. She took an embryo and she
removed from the embryo-- so you not only do
you see my calendar-- you get an insight into
the rest of my life. My husband is a professor
over at that other university. So we have competing
teaching schedules today. So Dr. Mangold took a group
of cells from the future back of the embryo-- we'll talk
about this next time-- and she transplanted
them into another embryo. She transplanted
them into the belly. So she took back
cells and put them in the future belly
of a host embryo. So there's a donor and a host. And she could see the difference
between these embryos, because they had
different pigmentation. They were different colors. And then she let them
go on and develop. And this was a hugely
difficult experiment in those days for
technical reasons. But over a period of
a couple of months, these embryos developed. And what she found was
this peculiar embryo, which is a conjoined twin. And she could show
by looking to see where the cells she
had transplanted in were, that there was this host
embryo that was made of all the original host tissue. And there was another
embryo joined to it in this orientation, but
that only a little bit of the second embryo actually
came from the donor cells. Most of the second embryo
came from the host cells. And this was epiphanal,
because she could understand that those donor cells
had told the host tissue to make another embryo. It was an extraordinary
unprecedented finding, and it gave the notion
that cells can tell other cells what to become. Does this happen in
other animals for sure? This is from a colleague
of mine Jerry Thompson, who made these conjoined twin
frog embryos by organizer transplants. Does that happen in humans? It does. Conjoined twins come from,
we believe, organizers that have split
and have given rise to two embryos, which
don't separate properly from one another. And we will stop there
and continue on Monday.
