I'm J.B Thomas from the Salk Institute.
And in this lecture I'll be talking
about axon guidance in Drosophila.
I'll review the impact that studies in
Drosophila have had on our understanding
of how axons are guided to
their synaptic target cells.
Thus forming the circuits that
make up the nervous system.
And in doing so I hope to give some idea
of how Drosophila has been capitalized on
as a genetic model system
to advance the field.
The axon guidance field traces its roots
to the great anatomist Ramon y Cajal
who in the late 19th century first
hypothesized that the basic unit
of the nervous system
is the single neuron.
And this led him to think about how
neurons might be assembled during
And the question he posed in
one of his early works is as
much the guiding question of the field
today as it was over 100 years ago.
He wrote, "wherein lies that marvelous
power which enables the nerve fibers from
very distant cells to make contact
directly with certain other nerve cells,
without going astray or
taking a roundabout course?" In essence,
he was asking how the nervous
system wires itself up.
And as he illustrated in many drawings,
Cajal went on to propose that a way
a neuron grows is via the growth
cone at the tip of its axon.
And that the growth cone is endowed with
the ability to sense chemical signals.
And work over the past two decades,
much of it on Drosophila has helped to
elucidate the molecular basis
effects on guidance and
continues to confirm how truly
visionary Cajal's work was.
In terms of the Drosophila nervous system,
I'd like to start by showing a section
through an adult fly's brain.
Just to give you an indication of how
complicated a tissue it actually is.
This section was stained using a technique
similar to that used by Cajal over
a century ago.
There are over 100,000
neurons in the brain,
only some of which of course
are visible in this section, and
their synaptically connected
with great precision.
The neuronal cell bodies
lie near the periphery.
They're bilaterally symmetric
around the midline.
Essentially everything else in the section
is composed of axonal projections.
And at first glance it
looks a bit disorganized.
But upon further analysis,
you can see that there's a very fine and
regular structure in which neurons project
their axons along specific pathways to
reach in synapse with their targets.
For example, this hypothetical pink
neuron might project this axe on across
the midline from one side to the other,
in order to synapse with its target cells.
And the homologous cell on the other
side would do the same in mirror image.
But what I'll be talking about is how
during development axons are guided to
their specific target destinations,
allowing them to synapse with
their appropriate partners.
We'll be examining these events in the
embryo where there are far fewer cells in
the architecture is much simpler
than it is here in the adult.