(soft music)
- [Narrator] We are the paradoxical ape, bipedal, naked,
large brained, long the master of fire, tools, and language,
but still trying to understand ourselves,
aware that death is inevitable, yet filled with optimism.
We grow up slowly.
We hand down knowledge.
We empathize and deceive.
We shape the future from our
shared understanding of the past.
Carta brings together experts from diverse disciplines
to exchange insights on who we are and how we got here,
an exploration made possible by
the generosity of humans like you.
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- So, I'm going to extend some
of the points that Rusty raised,
but in particular, in the context of this structure,
which is the cerebral cortex.
So, I think most would be familiar,
this large folded structure here
at the top is the cerebral cortex.
It makes up about three quarters
of the volume of the human central nervous system
and I think most people in this room
know functionally how important it is.
It is, as I say, this sort of executive
and integrative center of the central nervous system.
And it's a huge amount of real estate,
it does important things, so it's
affected by a variety of diseases
that have their etiology, or their impact,
at different stages in age, starting from
conditions with a very early onset in childhood,
like autism, and right through to diseases of later life
like dementia or stroke and those kind of things.
So, what I'm gonna talk about today
is how do we start dealing with questions
about how do you control the development
of the cerebral cortex, particularly in terms of size,
as of sort of a simple concept.
And as I'm sure you've heard in previous Carta symposiums,
and you'll hear this afternoon,
there's a lot of debate about sort of cognitive capacity
or cognitive ability and its anatomical underpinnings
and there is bigger brains make you smarter and so on,
but there's some degree of grievance around the idea
that certainly more neurons give you more processing power
at its most basic rate.
Complexity is probably very important as well,
in terms of elements that one can build circuits from
and what you do with them, so raw real estate
is also quite important.
And that becomes very obvious when you start looking
at these sort of three cherry picked primates here.
We've also got humans, chimp, and rhesus macaque
and what we're looking at here is the number of neurons
in each cerebral cortex, where you can see
that humans have roughly three fold as many
compared to chimpanzee, even though, in terms
of actual developmental time, so you know,
I could ask how long do humans take to develop,
easy one for us to all answer, nine months.
The chimp is roughly the same, but you still end up
with a brain which has a cortex
with about a third of the number of neurons.
Then, macaque, which is shortened again,
but has you know, ten fold fewer neurons.
So, there's a very basic development ability question,
is how do different organisms generate
different numbers of neurons within their cerebral cortex?
And at what level is it controlled?
And by understanding that question of control,
can we start thinking about how you might then
evolve these differences between different species
and it plugs into the kind of genetics
and that kind of work that you'll
be hearing about later in the afternoon.
So, to think about this, you've
got to think about developmental biology,
which is how do you actually make things
in the cortex in the first place.
And this is a picture from a mouse brain,
where this red thing, this is a
slice through the mouse brain,
this will be the outside, this will be the inside.
What all these little red blobs are
is these are what we would call progenitor cell.
For the sake of simplicity, I'll
call them cortical stem cells.
So, these guys aren't neurons.
This is before you have any neurons in your brain,
or in this case, the mouse brain.
This is a slice this way through
one half of your cerebral cortex.
So, the skin would be out here, for example.
There's a hole in the center of the brain here.
And these guys are lined up in a row,
and what happens is these cortical stem cells
go on, and this is done in black,
to generate cortical neurons that are
arranged in layers over time.
So, you do this inside or develop them
to make the deep layers where you make the upper levels
and so on, and this is a very stereotyped program
that plays out in every single mouse in the same way.
And in a mouse, it takes about six days
to generate all of these cells.
So, that's fine, so you've got
one bib population of stem cells.
There's a very interesting biology about them,
how they diversify, but at heart,
these guys make all of this cast of neurons.
So, when you're thinking about how you build a brain,
you've gotta think about how they do this,
but a key concept within this is what is the output
of each one of those cortical stem cells?
And there's a principal in development here
where we talk about a clone.
Now, this isn't Dolly the sheep type clone,
this is a different clone, just to
make your life slightly more complicated.
At the very basic idea, what is in a mind experiment,
if you could label one of those red cells
on the previous slide with an indelible marker,
so every time it divides, it's daughter cells
inherit the indelible marker and then
develop and play out over time.
What you'd end up doing is labeling every single cell
it made, just passively by letting it roll.
So, that's the idea.
So, you label it all.
So, if we're doing this in the cortex, we take,
we know what the start of cortical development looks like,
we've got these progenitor cells lined up.
We call this an epithelium, basically,
and we know what it looks like at the end of development
where you've got these layers of neurons
and we've got little labels that let us tell them apart.
The mind experiment, and the real experiment,
is you then label one of these guys with an indelible marker
and let time play out.
And you do this and you look at
thousands and thousands of these.
What do you find as something very simple?
If you do this before you make any neurons,
you end up with purple spots that cross all the layers
and make all of the cells.
And what that tells you is you've got this
fundamental principle where each cortical stem cell
will make a clone of cells that populates
all of the different neuronal types.
And so what that tells you then,
is that your clone is this sort of atomic unit of building,
a structure within the nervous system.
So, let's do the cortex example I've given here,
it's actually true in your spinal cord,
your eye, and so on, and what that does
is it gives you an idea of thinking about this problem
of at a kind of comparative development label,
how do you make a bigger cortex in, say,
a human compared to a macaque or a chimp?
And it's actually very simple.
So, if you think about it in clonal terms,
either have more clones, so increase the number
of stem cells before you ever even make the structure,
or actually change what each stem cell does
in terms of what the output of its clone is.
So, make the sort of production from that clone larger
and it's a way of ending up with more neurons.
An example I've done on here, this is mouse,
and my mice are, for want of a better term,
relatively simple in this respect,
which is, they have phases.
This is your starting stem cell,
it proliferates and then it has this linear program.
It expands itself, then it starts making neurons,
during which, it depletes itself,
'cause you're eating up stem cells to make neurons,
and then they switch to make these supporting, or
these second cell type in the nervous system, the astrocyte,
which is actually functionally very important.
And it's a nice, linear program.
So, one of the big questions in the field is
when you start getting to primates,
how complicated does this get?
Do you just take the simple template
and just make more of them?
And you just generate more real estate that way,
or do you diversify just to make it much more complicated?
And how would this work?
And if you can understand that,
then you can start plugging in
the kind of molecular genetic stuff,
like, well how does this work within all this?
And the question of how do you study this,
and labeling single cortical stem cells
in the human might get you in trouble
in most jurisdictions at this point.
So, we did something similar to what Rusty spoke of,
where in this case, we were very much
focused on the cerebral cortex.
So, we started with human, and we took
human embryonic stem cells, or these
induced progenitor stem cells that Rusty mentioned,
and what we essentially worked out how to do,
using principles of developmental biology
is make the cortical, just the cortical neural stem cell
and they formed into pretty rosettes within this.
And what we found is, we could put them into the dish
and they would replay this generating the layers of neurons
on exactly a timescale that they do in normal development,
but in utero, which was a bit of a surprise.
And they actually go on and they make
these beautifully connected networks.
So, these are neurons which are firing away happily,
these human neurons.
So, this was a bit of a surprise to us
that they observed rules even though they were isolated
from the sort of whole embryo.
So, we then extended this and said,
what happens if we look at these different species?
And so, this was worked on by
Tomoki Otani when he was in the lab, and this is actually
collaboration with Carol Barchetta, who you heard
mentioned by Rusty, and actually, Eliza Courno,
at the Primate Research Institute up in Seattle.
What essentially we did was work out how to make pure cortex
from these cerebral cortexes from these different species
in the lab and this is John Gerden, by the way,
down at the bottom, for those of you who don't know,
but we don't actually have John's IPSL's.
We think there's a market for them,
but a rather dodgy market for them, but anyway.
So, this is chimp, and these are actually
two different species of macaque,
because we could get access to them.
So, we could make the cortical stem cells
and they could replay development
and what we found was each species,
just as it does in sort in utero,
has a developmental time scale,
which it observes in the lab, by which I mean,
the green cells here, so the bottom line
of each one of these graphs here is time
and the green are those deep layers of neurons
and the red is these later born neurons,
so every species makes green before red,
but what happens is the macaque makes red
much sooner than the human does and the chimp.
And actually, the human and the chimp
look remarkably similar at the resolution
that we can look at with these kind of assets.
So, in fact, the rest of what I'll talk about,
we just look at human and macaque,
because actually human and chimp look very similar,
which is actually kind of reassuring,
'cause you'd want them to look very similar, given
what you've heard about in terms of genome similarities.
So, every species has a schedule
and I should say the schedule is remarkably similar
to what happens in utero.
And we extended this to all sorts
of other analysis, which I won't go into detail with here,
including when do they make neurons,
when do they make different classes of neurons,
and the kind of electrophysiology you heard about,
when do they mature, when do they form networks.
So, what this provides us now,
is a platform to start asking
the mechanistic questions I mentioned,
which is, well how does the stem cell
that actually generates the neurons
within each species behave in terms of making a cortex?
And what that involved doing is, in the lab version
of the experiment I just described,
where we label just one stem cell
in a population of tens of thousands
and actually watch that over time
and then look at lots of individual stem cells.
And to do that, we use a trick where we take a virus,
which is essentially, we've sort of damaged,
so this virus can infect a cell,
but then it can never get back out again,
so it brings the marker, the marker then stays,
so these are what we call replication of common viruses.
This is a rosette made up of,
each one of these blobs is a stem cell.
We labeled one stem cell.
And then we follow this thing dividing over say,
10 days within the lab, and we can reconstruct the clone.
And one of the challenges here is, as we mentioned,
primate development is very long and very slow.
So, to give you some kind of benchmark,
mice will take over the world, and the reason
mice will take over the world is they do
all of their development in less than 21 days
and they'll generate litters of, well,
you know, those of you who've been unfortunate enough
to have your garage overrun by mice,
as opposed to humans, and we know how long humans take.
So, what happens is, these, and I mentioned that
mice will make the cortical neurons at six days,
humans will do it over four, five, six months,
and so you get an idea that these clones become enormous.
So, we have to look at them in blocks of 10 days,
so sort of the first 10 days, the second 10 days, and so on
and the first thing we can do is start measuring, you know,
what do they make and how do they make it?
And as biologists, we tend to think biology's a mess.
There are kind of rules, but they're a bit dodgy,
and it's a bit tricky to make sense of it.
It's all about extracting order form chaos
and physicists think we're a bunch of Luddites
and that we just can't think straight
and there are clearly physicists in the audience,
I heard some snickering.
And a colleague and friend of mine and can be true
as a physicist called Ben Simon says actually,
"You biologists, actually if you take the complex data,
you do some very important principles underlying them."
And said, "Right, prove it."
And so, he did.
So, he took all this kind of data
and he ripped them apart and rather than
me boring you for 10 minutes with all the details,
I'll give you the punchline,
which is what I told him to tell me, too,
which is that essentially, when we start looking at,
for example, this is how many stem cells
are within a clone over time, and this is,
in the first 10 days, for example, 20 days, gray,
the next 10 days, black is the next 10 days.
And you see actually in each window of 10 days,
humans all look the same, and what's happening over time is
within the clone, they're actually expanding the number
of stem cells in the clone while making neurons,
so they're kind of, the whole thing's growing exponentially.
Macaque's start off looking very like human
and then they crash, and what that really means is that
essentially, the macaques stop making neurons
much sooner than the humans do.
And it's a very simple thing.
What it means is that the humans
make these big, long extended clones,
which allow them to make far more neurons
over a much longer period of time,
so they'll make more neurons per stem cell
and they do it kinda clever ways,
stuff that I wouldn't get into today.
Now, that's all fine and it's very descriptive,
and it raises the second question,
which is well fine, how does this work?
Now, there's a very broad distinction we like to make,
which is, how much of this biology
is controlled intrinsically?
Whatever that means in terms of mechanism.
And how much is controlled externally?
Signaling environment, you could boil it down to,
you know, this sort of nature nurture,
who do you know, what do you know,
but you get the basic principle.
I once heard that one referred to
the European versus the American model,
but I couldn't work out which one
was meant to be certainly not complementary,
but anyway, so in this case, what we can do,
is we can essentially do transplantation.
Now, clearly, I'm not gonna get a license
to transplant macaque cells into humans.
And probably not the other way around either,
but what we can do, is we can do transplantation in the lab,
where we can label up, say, the human stem cell
and the macaque stem cell and then move,
say one and 1,000 of those into the other environment.
So, essentially, you're exposing them
to the other species' environment
and asking, now what will they do at its simplest.
And we can then roll them out over time and do all our maths
and essentially, all I'm showing you here
is that nothing changes, so I give you the punchline,
which is the number of neurons a stem cell makes
is actually intrinsic to the species.
It's got nothing to do with the environment.
At the resolution of this assay,
with all sorts of caveats around it
with what we're measuring, by which we mean,
if I move macaque into human, macaque doesn't care.
It'll still make what it wants to make,
when it should make it.
And similarly, with the human,
if I move human into macaque, it'll still generate
a certain number of cells.
So, that's quite striking, but it's about numbers.
There's a second readout, which is very important,
which I've rather glossed over,
which is this, what neurons do you make
and when do you make them?
So, this idea's switching from the early born neurons
to the later born neurons I mentioned.
And this was a think I mentioned, so we make layers,
we mentioned this, but also, the humans here
switch to making these red guys
much later than the macaque do, for example,
and we can look at this, as well, within these systems,
and again, what we find is that,
if you look at these pie charts,
this pie chart looks similar, identical to this pie chart.
What we're doing here is putting human into human,
or human in macaque or vice versa, macaque into human,
or macaque into macaque and essentially,
the species don't care.
They're going to do what they're going to do.
They're sort of like my 11 year old son, you know.
In the sense that, it doesn't matter what environment,
there's a certain amount of pigheadedness.
So, what this means is, and I don't know where
they get it from, I blame his mother.
Anyway, so what we've learned from these kind of studies
is that the primates do development certainly differently
and I should say something, which I haven't the time
to go into today, they also do it differently
to small mammals like mice.
So, this idea that you're early on, you're expanding
your stem cells while making your own stuff,
that's quite different compared to small mammals.
So, there's sort of a big mammal problem
in addition to the primate problem.
And this is something which, as a field,
we haven't really grappled with yet.
You know, there are such huge differences
between the kind of tools we use in the lab
and the model organisms we study in the lab, like mice,
in term of their biology.
And primates, some of them aren't to do with being primates,
they're to do with being large, slowly developing mammals
more than anything else.
But even within this difference,
what we find is that humans then are different
to the smaller brained primates.
And that essentially, these differences
I mentioned are cell autonomist.
And so what that then leaves us that
and this is where we are now,
is this provides us a platform to ask
what are these mechanisms?
How do we control what a stem cell does and how it does it?
And our goal is we can identify
what those differences are, what those sort of
cell intrinsic controls are, and clearly, yes,
we all think about genetics and there is
an element to that, but you know,
we have to be quite broad minded about
what the kind of cell intrinsic clocks are.
The looking at those differences will give us insight
into evolutionary differences,
by sort of thinking backwards.
I should say there's another side to this, too,
which is, the mechanism that controls
differences between species of brain size
are also very important in two clinical contexts.
One is control of brain size developmentally,
which has a very strong impact
on various intellectual disabilities
and the other side is obviously
different types of brain cancers,
because it's the same biology there as well.
So, I'll just finish there
and just to say, I mentioned this,
this was led by Tomoki Otani who was in the lab
not too recently and is now with McKenzie in Tokyo
and is carried on by two other people within the lab
that I've mentioned here, and just to say,
none of this work would've been possible
without Ben, Rusty, and Carol, so thank you very much.
(applause)
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