What I want to talk to you today about is the creation of
3-dimensional structures. And I want to emphasize their
connection to the function of the organisms and the organ.
Let's look, for example, at the kidney. This is a fantastic
example of structure- function relationships.
The kidney comprises thousand of tubules that are involved in
filtering the blood and collecting urine for excretion.
Now, not only do these tubules function to transport the urine as
it is being processed, they also are connected very closely
to various cell types that are involved in this filtration
process. So the 3-dimensional structure of
the kidney is integral, is required for its normal function.
And this is true in essentially every organ. So it's very important
to think about the relationship of different cells to one another in a
particular organ. And if you're thinking about,
for example, using tissue engineering to develop an artificial
kidney and making some kind of pastiche of cells and polymers and
putting that together in some functional way --
-- it's important to understand these structure-function
relationships in the normal organ before you try to engineer something
that is a surrogate. Individual cells also have
particular shapes, and this is integral to their
function. We'll talk a bunch about neurons, cells that have very long
processes that transmit nervous impulses or transmit electrical
impulses and communicate in that way.
The shape of the neuron, the structure, the 3-dimensional
structure is absolutely required for its function. Tom,
I think we could have a little more light at the back there.
It looks really dark if you guys are trying to take notes.
Thank you. OK. We have, you've seen this movie previously.
OK. This is the zebra fish. The zebra fish embryo.
Isn't it dark at the back? Yeah. Tom? Oh, it takes a while.
It takes a while. OK. I want to show you this movie again
that you've seen a while. You have it on your website.
And I want to show it to you now in a different way.
We're actually going to first show that during the early development of
the fish, these two cells on top initially give rise to a ball of
cells sitting on top of this yoke cell that is just a ball of cells.
And then suddenly these cells start to move during the gastrula and then
the neurula phases of development. And it's this cell movement that
builds structure. And I want to try to address with
you how this cell movement occurs and how it builds structure.
So let's look at this movie again. Here we go. Cell division,
building up the raw materials to make the embryo.
A group of cells sitting on top of the yolk. And now watch here.
It starts to move. Those cells have made the decision to move,
and they going to move towards this one dorsal side of the embryo.
And they are going to move and move some more to build the eye,
the brain and the various somites along the length of the axis.
OK. So that process, the generation of 3-dimensional
structure is a very large part of the process of building the embryo.
And it's interdigitated with making the cell types.
A muscle cell is not a muscle cell and it's not functional unless it is
a fused myotube that has the appropriate 3-dimensional
structure. So cell type and 3-dimensional
structure are very closely related. OK. So let's talk about a toolkit
that is involved in this process. And the first thing that we can
discuss is what is in this toolkit. Well, actually, you don't really
have much in the toolkit to build an organism.
I can think of one thing. What's in your toolkit to build
your 3-dimensional organism? Yes. Did I hear cells? Yeah.
I hope I heard cells. Well, there are cells. And then if you want to
look for something else to build the organism with there are cells and
then there are cells. That's what you've got.
In your toolkit you've got cells. And the challenge of the organism is
to use that singular building material to build the various and
the huge array of structures that there are. So what
about these cells?
And the first thing about these cells is that they come in two forms.
They come as so-called epithelia or epithelial sheets.
And they come as single cells which are called mesenchyme.
And these two types of cells interconvert with one another.
And it's from these two groups of
cells, that can be different cell types, but they're either sheets or
they're single cells, that one builds the organism.
So the epithelia are sheets, the mesenchyme are single cells.
And let me look at the next diagram that I drew for you.
And you have it in front of you. If you don't have the handout, does
anyone not have the handout? Could I have, Lesley, could you
[UNINTELLIGIBLE] please? Thanks a lot. OK. If you look at
the handout, you have this except for one thing I added this morning.
Here is a sheet of cells. So I'm going to do minimal board work today
because you have a lot of the information right in front of you.
You have a sheet of cells here. And this sheet of cells called an
epithelium is a sheet of cells because it is joined together very
tightly. And the cells are joined together by various junctions that
I've shown here in yellow or that I've shown in blue.
And this sheet of cells, as Professor Jacks told you,
has two part, has two sides, an apical side and a basal side.
OK? Remember from cell biology? And it touches something called the
basement membrane or the basement lamina, which is part of the
extracellular matrix that I'll mention again later.
Now, this epithelium, this sheet of cells can transform
into single cells. And these single cells have the
property of being non-adherent. And they don't attach very tightly
to the extracellular matrix, although they are in contact with it.
And in order to go from an epithelium to a mesenchymal state
there are changes in gene expression that take place,
we know, at the transcriptional level. And these result in changes
in cell adhesion and changes in the organization of the cytoskeleton.
This is a reversible process, and mesenchyme can go and turn back
into epithelium. Now, the epithelial sheet is a very
important part of the body, both as a building block for various
structures, but also as a barrier. So your skin is a barrier because
the cells in it are very tightly joined together and they form an
impermeable barrier. But that's true of essentially every
organ you have. Every organ you have is surrounded
by an epithelium, or one or more epithelia,
and these surface barriers so that stuff in the organ doesn't get out,
stuff outside the organ doesn't get in. And if there is a lesion,
a break in this epithelium it is a big deal. And that is wound.
That's what a wound is. It's a break in the epithelium.
And there is a very rapid and profound system of wound healing to
repair these epithelial sheets. Now, during development there are
many transitions from epithelium to mesenchyme. So the term that you
really need to know is abbreviated EMT. Not to be confused
with other EMTs. This refers to the epithelial
mesenchymal transition, which I did not write on your
handout so I will write it here.
The epithelial mesenchymal transition. And it's always said
that way, even if it's actually a mesenchymal epithelial transition.
One of the most profound examples of epithelial mesenchymal transitions
is during the formation of the nervous system.
Your central nervous system, as we'll discuss in a few lectures,
forms from a tube that rolls up during development.
And this tube is an epithelial sheet. As the tube is rolling up,
a group of cells, shown here in yellow, moves away,
breaks away from the tube and undergoes an epithelial to
mesenchymal transition. This group of cells is called the
neural crest cell population, and these neural crest cells then
migrate away from the neural tube and go and set up the entire
peripheral nervous system. Not eripheral nerves, peripheral
nerves. They also make all the pigment cells in the body and the
adrenal medulla. This epithelial mesenchymal
transition is not only crucial during development.
It's also believed now to be crucial for metastasis of tumors during
progression of cancer. And Professor Jacks will address
that later in the course. This is a movie demonstrating the
migration of the neural crest cells out from the neural tube which is in
the middle here, this white tube. And these little
dots migrating out are shown over a period of about 12 hours.
The single cells migrating out from the neural tube as they have
undergone that epithelial mesenchymal transition.
All right. So I wasn't wrong in facetiously saying what's in your
toolkit is cells. That's what's in your toolkit.
But clearly the cells are slightly different from one another or
profoundly different from one another in their disposition.
And we'll talk about what mesenchyme and what epithelial can
do in a moment. The other thing that's different or
the other thing that's important that makes cells actually very good
for building is that they are plastic. So let's go through a few
things that they have going for them. I want to talk about adhesion.
I'm going to mention junctions, I'm going to mention cell sorting,
and I'm going to mention the extracellular matrix.
And some of this stuff you've had before so I'm going to go through it
quickly. This is a diagram that's on your handout,
it's from your book, to indicate that there are many ways
epithelial sheets are stuck together that involve very tight apposition
of the cell membranes, or in the case of tight junctions or
slightly less tight apposition in the case of these things called
desmosomes. You can go back and remind yourselves.
We mentioned these previously. The most important thing is you
understand that cells are joined together.
In this micrograph, to demonstrate how tightly cells are
joined together, this is a sheet of cells where the
nuclei are stained green and the red is staining for a particular
specific protein that is found in a kind of junction called a tight
junction. And these tight junctions outline the cells.
In other words, the cells are glued together very tightly.
Here's another one. Cell sorting. There is another kind
of cell adhesion interaction that's very important for cell adhesion.
I've indicated here something called cadherins.
Cadherins are interesting. They're calcium-dependent adhesion
molecules. And if you have sharp eyes you'll see up here something
that's got a beta and then there's a word here catenin.
Remember beta-catenin from dorsal-ventral axis formation?
This is the same beta-catenin. Not only is it a transcription
factor, it is also involved in cell adhesion. There's an interesting
complication of biology for you, but I don't want to dwell on that.
Cadherins are essential for sticking cells together.
These are pictures of frog embryos. This is a normal embryo that's been
cut open and the cells remain a tight mass. However,
if you take an embryo and you inject it with inhibitors of cadherin
function the cells become completely loose from one another and you can
actually see the outlines of the cells because they are no
longer stuck together. Now, this is fascinating and very
important for the animal because it turns out there are lots of
different adhesion molecules. And different cells types sort out
according to the adhesion molecules they are expressing on their cell
surfaces. So this is a rendition of a fantastic old experiment that was
done in the 1950s to demonstrate how cells sort out.
What was done, these are two frog embryos,
and a piece of the future skin or epidermis was removed from one,
and then a piece of the future neural plate, the nervous system was
removed from another. And these had different colors so
you could tell which cells were which. Now, if you take those cells
and you put them in a medium that does not contain calcium all the
cadherins and other adhesion molecules cannot work.
And these cells fall into a pile of single cells. And you can take your
pipette and mix them up in your little dish, and you get this salt
and pepper arrangement of the two kinds of cells.
And then you can add a little bit of calcium back to the medium,
and the cells will form this big ball of cells.
And it's salt and pepper again. The two neural plate cells and the
epidermal cells are mixed up. But if you go away and have dinner
or have a good night's sleep and come back the next day and look at
your ball of cells, you see that amazingly the cells
have sorted themselves out. The epidermal cells have gone back
together and the neural cells have gone back together with one another.
And you can do this with cells from almost any organ.
You can mix cells from two organs particularly when they're embryonic
organs. You can mix them together, but also in the adults to some
extent. And these cells from different
organs will sort out. And they sort out because of
specific adhesive molecules that they have. And the term that one
uses for this is homotypic binding where cells will interact with one
another through membrane-bound receptors. Again,
I'm assuming you remember this is a lipid bilayer with a protein
sticking through. Two receptors,
proteins sticking through the lipid bilayer interacting with one another
in calcium-dependent or independent ways. And there may be more than
one receptor that mediates cell-specific interactions.
But these interactions keep different cells separate from one
another and facilitate development and building structure.
OK. Here's another one. The extracellular matrix.
What is the extracellular matrix? We mentioned this at the beginning
of the course. Professor Jacks threw it at you in
cell biology. The extracellular matrix refers to the stuff on which
the cells sit, so cells in your body are not
sitting on nothing. An epithelium is not just floating
free or a tube is not floating free in liquid. It is actually
surrounded by a bunch of proteins and carbohydrates that are secreted
by other cells and form the extracellular matrix.
Or, in the case of epithelia, the basement membrane. OK? That's
what the extracellular matrix is called. It's highly organized in
the case of the epithelium. And it consists of proteins and
various things called proteoglycans which are sugars bound to proteins.
Now, one of the proteins in the extracellular matrix is collagen.
And collagen is the most abundant protein in the Animal Kingdom. OK?
It comprises a very high percentage of your body mass.
And if collagen, if there are mutations in collagen many things
can go wrong. For example, there is a disorder called
osteogenesis imperfecta where your bones don't form properly.
That's a mutation in one of the collagen genes.
There are a lot of collagen genes, and the protein mass of collagen is
enormous. Now, the cells are sitting on the
extracellular matrix. But there is also a fantastic,
and then that's good, that's helpful for the cells in a support sense.
It gives them some support. But the extracellular matrix does a lot
more than that. It actually communicates to the
cells. And it does so by means of adapter proteins.
You have this as a handout, if you didn't realize. This is one
of your handout diagrams. OK? These adaptor proteins have
the property of being transmembrane or at least integral membrane
proteins that are attached to the cells.
But they also attach to proteins in the extracellular matrix.
And this allows them to sense what the extracellular matrix contains
and to transmit that information to the cells. Because the thing,
of course, about living organisms and the 3-dimensional structures in
them is that the process of both building the structure and
maintaining the structure is a dynamic one.
It's not equivalent to building Building 10 or Stata.
It's not equivalent to putting components together and getting a
structure. You get the structure, and the structure is maintained
because the structure senses how it's doing, whether it's intact or
not. If one of the walls in this building fell down,
it would fall down until someone repaired it. If one of the tubes in
your body gets a hole in it, your body will sense it and try to
repair it. That same thing is true when the
epithelia, and I'll tell you in a moment, when mesenchymal cells are
actually doing their building process, they are sensing what is
around them. And they do it through these adapter proteins.
The adaptor proteins are receptors, by definition, they're binding to
something in the ECM. And a large number of them comprise
of class of proteins called integrins, as an example of a name.
All right. So let's move on here and let's mention --
-- the cytoskeleton as being
required for shape and movement.
We've mentioned the cytoskeleton before. I'll talk about it more in
a moment. This is a diagram from your book that you had before.
The cell is not a floppy bag of liquid. It contains many filaments
that keep it rigid, that allow it to have particular
shapes and that allow it to change its shape. We're going to be
talking most about microfilaments which comprise polymers of actin,
the protein actin. There are also intermediate filaments and
microtubules which we mentioned last lecture when we talked about
the sperm flagellum. This is a micrograph of tube stained,
of cells stained for the microtubule network. The nucleus is yellow.
Stained for one of the major proteins in the microtubules,
which is tubulin. And you can see this very extensive meshwork
throughout these cells. And the same is true for actin.
The same is true for intermediate filaments. So cells are very well
supported by these filaments. OK. And, finally, let me mention cell
division and cell death as being part of the toolkit that allows one
to build structure. And I'll show you one experiment
which involves the inhibition of cell death in the embryo.
This is a mouse, normal mouse embryo, and this is a mouse embryo
in which a protein was removed. This protein is called caspase-9.
And caspases are involved in killing cells during development,
as Professor Jacks mentioned to you. In this mutant animal,
one of the things you should notice is that the brain is hugely
overgrown. And this is an indication of the amount of cell
death that has to happen during normal brain development.
OK? And the flip side I'm not going to dwell on.
You need to get not only the normal amount of cell proliferation,
but it needs to be in the right place. All right.
So let's talk about the behavior of cells and how this works together to
give 3-dimensional structures. And I want to talk very briefly
about, actually, not so briefly, about the behavior
of single cells --
-- and the property that is most important to them,
which is the ability to move. So the point about an epithelial
mesenchymal transition is that what comes out of it are single cells.
And this is important in metastasis and in normal development.
The difference between this epithelium and these single cells is
that the single cells are free to move because they are not attached
to a sheet. And that is how cells get from one place to another in the
body. OK? So let's dwell on this in more detail.
So how do cells know where they're going? Well, this is a very
interesting question. They know where they're going
because they're told to go somewhere. So I told you the neural crest
migrating out of the neural tube. This is a group of cells from an
organism called dictyostelium. And these are single cells.
They've been fluorescently labeled. And up here in the corner someone
has put invisibly a drop of the second messenger cyclic AMP.
And you can see these cells that had been milling around randomly start
to realize this and move as a consorted group towards the source
of cyclic AMP. OK? And this is one of the ways a
dictyostelium organizes itself. The cells chemotax, they follow a
particular chemical stimulus. How do they move? How do cells
move? Well, they move because the
reorganize their cytoskeleton, particularly their actin
cytoskeleton in a concerted way. So I've drawn for you a kind of
animated cartoon. You have the whole thing in front
of you. And I'll animate it and see how it goes. We start off here with
our green cell. This is a mesenchymal cell.
And it is sitting on some kind of extracellular matrix,
which I've shown as kind of organized, but actually it may not
be. But that doesn't matter. It is loosely attached to this
extracellular matrix. It's got these receptors sticking
out of its membrane that can sense extracellular matrix,
but only at the rear of the cell is it actually attached by these red
triangles to the ECM. OK? That's what my red triangles
indicate. Now, the cell suddenly as it's sitting
there or moving randomly along comes across these black circles --
-- which are some kind of molecule that can interact with these
receptors and tell it that it ought to go in a particular direction.
So how does it do this? Well, the first thing it does it to elicit
this interaction with a substrate or with these black molecules in the
substrate. This is a receptor ligand interaction.
It's exactly the same kind of interaction that you talked about in
cell biology, that we talked about in the formation,
earlier formation lectures, cell receptor ligand interaction.
And it does the same thing that other receptor ligand interactions
do. It activates some kind of signal transduction process,
but this signal transduction process has the property of telling the
actin filaments in this local region of the cell that they
should polymerize. OK? So this is cell signaling in a
very local region of the cell. And I'll show you the movie in a
moment. You'll see it's extraordinary.
So here are the actin filaments forming in this region of the cell.
And I've called this the front of the cell. OK? It's an arbitrary
term. But the front of the cell is arbitrary but it's going to refer to
the direction that the cell is moving. So there's actin
polymerization of the front. And the cell also sends out some
protrusions called filopodia or lamellipodia. And it sends out
these protrusions to try to sense where it should move next.
Now, once it's done that, once it's made these polymerized actin
filaments it contracts. However, it doesn't go anywhere
because it's still attached at the rear. So it's kind of like pushing
down on your accelerator when your hand break is still engaged.
OK? You don't really go very far. So the next thing it has to do is
to lose the adhesion at the rear. I couldn't get this to work but it
will work in a moment. OK. And it also depolymerizes the
actin at the rear. So there. There we go.
There's loss of the adhesion and there's loss of the actin at the
rear. And now it's in a position, it's disengaged the hand break and
it can move forward, so it goes forward. OK.
This is my cartoon. This is the real thing.
This is a cell where actin has been labeled fluorescently.
And I want you to watch two things. It's on a repeat loop so we'll
watch it a number of times. At the part of the cell that is
sticking out, you should be able to see these bright cables or these
bright lines. These are cables of polymerized actin.
OK? And the cell is sticking out these protrusions and
polymerizing actin. And it's moving down in this plane
of the board, or the screen as it's making these polymers.
OK? So this is the front of the cell. And then if you look at the
back of the cell, let's wait for the loop again to
start, and you will see, here's the back of the cell with
initially polymerized actin. And if you watch individual lines
you will see them go away as the actin is depolymerized and the cell
loses its adhesiveness at the back. OK?
So this process involves selective adhesion, loss of adhesion mediated
by actin polymerization through the cytoskeleton. There's a signal
transduction pathway involved. It involves things called GTPases
that you've talked about previously. And it's really an extraordinary
process. All right. But let's move on and let's talk
about cell sheets.
So cell migration is crucial to get from one place to another,
but you cannot really build much with single cells because they're
single cells. They don't really give you any kind of integral
structure, any kind of cohesive structure. And so you have to turn
these single cells back into sheets if you're going to use them then for
building. So what do cell sheets do? Ah, OK. So let me go through a
couple of things that cell sheets do.
Cell sheets can get longer. They can change the number of
layers that they have. They can change the number of
length, the number of layers that they have, and they can bend.
Here are some cartoons that I drew for you. Here's a sheet of cells.
And you can lengthen it in two ways. You can stretch it out so that the
surface area to volume ratio changes.
And you get a longer, thinner sheet of cells.
You can also take this initially bilayered sheet of cells and fit the
cells in between one another by a process called intercalation,
or the term that's usually used is convergent extension.
And that lengthens a sheet of cells. And the reason that a small embryo
becomes longer is because of this process of intercalation where the
embryonic cells interdigitate with one another and make the whole
embryo stretch out. You can also change the number of
cell layers to get a more complex tissue, a more complex structure.
You can break two cell layers into two individual layers.
That's called delamination. Or you can turn two cell layers
into one cell layer by another process of intercalation so that you
get a single sheet from what was initially a two cell thick sheet.
Again, this is in front of you and I don't want you to dwell on it.
Let's talk about bending cell sheets. This is really interesting
because one of the things that comes out of bending cell sheets are tubes.
Now, tubes are pervasive. There is not an organ in your body
that does not have some kind of tubular structure.
And if you think about it, if I were to give you a pile of
cells and say, here, build me a tube.
You could probably come up with several ways that you build a tube.
And, in fact, the organism has come up with several ways.
One of the ways is to roll an epithelial sheet into a tube.
So here is the sheet. And you roll it up to form a tube.
This is how your neural tube is made. And also part of this is that
the cells change shape as they are rolling up.
So if you have a sheet of cells that is either cuboidal or columnar,
to get that sheet of cells to bend you have to make wedge-shaped cells
out of them. You have to give them a constriction,
a so-called apical constriction. So you go from a sheet of columnar
cells to a bench sheet of cells because you have apically
constricted those particular cells. And you can do that through an actin
process or through our old friend beta-catenin here acting as a cell
adhesion molecule. All right. So here's the process
of rolling up the neural tube in frogs. It's rolling up by apical
constriction and by the epithelial sheets bending.
OK? You've seen this movie previously, and it is on your
website so you can look at it again. Here's another way. You can
balloon out an epithelium. You can imagine an epithelium,
and you make a little, you blow it and you get a little balloon.
And then you blow it some more, you pull it some more and you can
turn an indentation or a very small vesicle into a long tube.
OK? And I'll show you. This is not the best diagram,
I've got to work on this one, but the idea is that you pull this.
Dr. Gardel is agreeing with me, I've got to work on
this diagram. OK. But you pull out this sheet of cells
into a long tube. I'll talk more about this when we
talk about lung formation in a moment. Here's another way.
You can take your single mesenchymal cells that have migrated
to wherever they're going and you can get them to condense into an
epithelial sheet. And this epithelial sheet can then
form a tube. This is exactly what happens in formation of the blood
vessels. Your blood vessels have got lots of layers of epithelia.
The first one to form is the endothelium. It's the inner most
layer of the blood vessel. And it forms by condensation of
mesenchymal cells into an epithelial sheet. OK. Excellent.
So I want to end by giving you a specific example and talking about
the genes involved in a particular aspect of 3-dimensional structure
generation. And the example I've chosen is formation of the lung and
the tubules in the lung. So your lungs are made up of
multiple tubules that have got little sacks on the end of them,
alveoli that are the places where oxygen is exchanged between the air
you breath in and between the blood vessels that feed into or that
surround all the cells of the lung. Formation of the lung tubules is
very interesting because mathematically,
if you look at it, you can really define it by a
Mandelbrot set where you have some kind of reiterative
branching process. And if you look at the lung tubules
and count them and look at the number of branching,
you can figure out that you have to go through 20 branching events to
get to the set of lung tubules that is in an adult human lung.
OK? If you look at the structure, the structures during the process
start with the trachea and this thing called the bronchial bud --
-- which divides into two to give two primary bronchi.
Then each of those divide more, branch more and so on. So these are
multi-cellular, these are epithelial sheets that are
branching reiteratively during lung formation. Here's a movie of the
process. You can get this to take place in a Petri dish with the
appropriate signals added. And so here is the lung tubule
branching reiteratively over a period of about a day or
two in tissue culture. OK? So this is a 3-dimensional
tissue engineering, or at least you can get this aspect
of 3-dimensional tissue structure to form in tissue culture.
All right. So what genes control this? As you know,
as I've told you over and over and over, everything is controlled by
genes and the interaction of genes. And what do we know about lung
formation? Well, we know most about lung formation
from insects. In insects the evolutionary relic or
the evolutionary precursor or the evolutionary equivalent is the
system of trachea. These are a system of tubes that
branch out from the spiracles, which are holes that lead to the
outside. And then tubes come off these spiracles and branch in the
insect and actually carry air directly to the tissues.
There's no circulatory system equivalent as there
is in our cells. But the way these tubules branch and
form is very equivalent to ourselves. And we know that this involves a
receptor signaling system that you are familiar with,
which is a receptor tyrosine kinase signaling system.
And the particular one I'll tell you about is the fibroblast growth
factor signaling system. You remember that growth factors
bind to receptors, and these growth factors can be in
the extracellular matrix. They usually are.
That's one of the places where growth factors are.
They bind to a receptor. And then there is a cascade of
signal transduction that, in this case, involves kinases.
And eventually the final kinase moves to the nucleus where it does
stuff to transcription factors and you change the transcriptional
activity of a cell and you get stuff happening. OK.
So this is a diagram of what happens in the fruit fly where we
know most about the process. The epithelium that's going to give
rise to the trachea undergoes cell division, and at the same time it
balloons out to form this thing called the primary tubule.
You have this diagram in front of you. It forms the primary tubule.
After a bit, this primary tubules branches to give secondary tubules.
And later on it branches again to give these tertiary tubules.
There are a number of mutants in drosophila that affect each of these
processes, the primary tubule formation secondary or tertiary
tubule formation. And these are members of the
fibroblast growth factor signaling system. In the primary tubule the
FGF receptor is expressed, and the cells receive FGF which
tells them to divide and to divide in a particular orientation to give
you this primary tubule as it grows out.
OK? And there's a ligand sitting around in the extracellular matrix
that tells the cells to do this. Now, what's really cool is that at
the point where the secondary tubule, I'm going to take 20 seconds,
so I'd appreciate it if you'd listen. At the tip of the tubule an FGF
inhibitor called Sprouty is made. This inhibits cell division of
these cells right at the tip, but the cells on either side
continue to divide. And so you get the branching,
the secondary branching taking place. And I'm going to stop there.
And those of you, don't forget, come along to Stata this afternoon
if you need a review.