80 | FEBRUARY 2004 | VOLUME 5 www.nature.com/reviews/neuro
H I G H L I G H T S
The olfactory systems of insects and vertebrates are
remarkably similar anatomically,so the fruit fly
Drosophila melanogaster is a valuable model for
investigating the underlying organizational principles.
As reported in Development,Jefferis and colleagues
have used the Drosophila system to examine how a
spatial map is established in an olfactory relay centre in
the brain.
In insects and vertebrates,axons from olfactory
receptor neurons (ORNs) that express the same
olfactory receptor converge on structures known as
glomeruli.In Drosophila,these structures reside in the
antennal lobe.The ORN axons synapse with the
dendrites of projection neurons (PNs),which relay
information to higher olfactory centres in the brain.
The PN dendrites segregate,according to birth order
and lineage,into different glomeruli.
How is the glomerular map established during
development? Previous findings favoured the idea that
the antennal lobe is initially patterned by ORN axons,
which then act as targets for the incoming PN dendrites.
However,by labelling clones of PNs and tracking the
early development of their dendrites,Jefferis et al.
showed that the PN dendrites in fact segregate into
rudimentary patterns resembling future glomeruli
before the ORN axons arrive at the antennal lobe.
What is the source of the signals that drive the
segregation of PN dendrites? One candidate that the
authors considered was the larval olfactory system,
which shows a similar glomerular organization to the
adult system.However,contact between the adult and
larval antennal lobes is minimal during the stage when
PN dendrites are invading the adult lobe,so it seems
unlikely that residual larval neurons impose a pattern
on these dendrites.
The glomeruli also contain the processes of glia and
interneurons,and Jefferis etal. investigated whether
these processes could provide positional information.
No glial processes were detectable in the antennal lobe
while the prototypic glomerular map was emerging,
although glia could still exert a patterning influence
outside the lobe.The involvement of interneurons is
uncertain,as there are no reliable reagents to detect
them.The authors suggest that segregation is at least
partly driven by interactions among the PN dendrites
themselves;dendrites with the same molecular signature
might be induced to cluster through mutual attraction.
So,these findings indicate that PN dendrites
establish the initial glomerular map in the antennal
lobe.Although this represents a significant shift from
previous models that emphasized the role of ORNs,it is
not incompatible with the idea that ORN axons are also
prepatterned.Also,the map is probably refined through
interactions between ORN axons and PN dendrites.
The next challenge will be to identify the signals that
pattern these two sets of neuronal projections.
Heather Wood
References and links
ORIGINAL RESEARCH PAPER Jefferis, G. S. X. E. et al. Developmental
origin of wiring specificity in the olfactory system of Drosophila. Development
131, 117­130 (2004)
FURTHER READING Strausfeld, N. J. & Hildebrand, J. G. Olfactory systems:
common design, uncommon origins? Curr. Opin. Neurobiol. 9, 634­639
(1999) | Buck, L. B. The molecular architecture of odor and pheromone
sensing in mammals. Cell 100, 611­618 (2000)
WEB SITES
Liqun Luo's laboratory: http://www.stanford.edu/group/luolab/
Encyclopedia of Life Sciences: http://www.els.net/
Olfaction
A projected olfactory map
DEVE LOPM E NT
Shadow play
According to the
Independent (UK, 15
December 2003), Peter Pan
had every reason for being a
crazy mixed-up kid: "nothing
to do with refusing to grow
up, according to scientists at
Trento University, but a
matter of losing his shadow".
New findings reported in
Nature Neuroscience show
that we are rather attached
to our shadows, and even
perceive them as extensions
of our bodies.
As the Times (UK, 15
December) reports,
"shadows help the brain's
visual system to work out
how objects are moving, and
can influence the sense of
touch". The researchers
asked their subjects to judge
whether an electrical
stimulus was being applied
to their index finger or
thumb. If flashing red lights
were placed close to the
shadow that was cast by the
stimulated hand, the
subjects took longer to
decide which digit was being
stimulated. If the hand did
not cast a shadow, or if the
shape of the shadow was
altered with a glove, the
lights proved to be less of a
distraction.
This is reminiscent of
situations where the brain is
tricked into thinking that an
object is part of the body.
"It was known that people
form a mental connection
with a false limb or tools if
they are trained to associate
manipulation of that object
with a sensation, for example
if their own hidden hand is
stroked whenever a visible
false hand is touched" (New
Scientist, 14 December).
These findings might also
explain our feelings of
unease when someone or
something encroaches on
our shadow. Vision
researcher Margaret
Livingstone says, "we all
have, as children,
experienced a reluctance to
have others step on our
shadows. I have a graduate
student in my lab right now
who still feels that way"
(New Scientist).
Heather Wood
IN THE NEWS
2004 NaturePublishing Group