Biology Reference
In-Depth Information
DE
NSE ARRAYS OF WAYPOINTS MAKE PATHWA
YS
When waypoints are arrayed densely enough, they form a continuous pathway for guid-
ance of cells or growth cones from source to destination. An example of such a pathway is
important in guiding axons that will carry olfactory information to their targets deep in the
brain. Mitral cells in the olfactory bulb of the brain, the part concerned with the sensation of
smell, project axons to connect with the amygdala, part of the limbic system that is connected
with emotion and sexual arousal. The axons follow a rather indirect route, bundling together
along the side of the telencephalon (fore-brain) before making a sharp turn towards the amyg-
dala. While in the telencephalon, the mitral axons form a bundle distinct enough to be given an
anatomical name, the lateral olfactory tract. If the telencephalon, which includes the olfactory
bulb and the amygdala, is removed from an embryonic mouse and placed in organ culture,
mitral cells still send their axons precisely through the correct region of the telencephalon to
form a lateral olfactory tract. The axons grow across a specific set of neurons already present
in the telecephalon, all of which express a distinct antigen apparently unique to that area,
which is detectable by the monoclonal antibody 'mAblot1'.
31
These neurons forma dense array
that marks out the whole lateral olfactory tract even before mitral axons have arrived. If these
neurons are ablated in organ culture, mitral axons fail to follow the lateral olfactory tract;
indeed they fail to go anywhere.
31
This is true even though the amygdala, the ultimate targets,
will still be present in cultured telencephalon. If just some of the mABlot1-binding neurons are
ablated, creating an interrupted chain of cells, mitral axons extend normally as far as the break
in the chain, and then stall. This suggests that, for mitral axons, hopping between waypoints is
not enough and that they require a continuous pathway of positive guidance cues.
THE MAN
Y WAYPOINTS OF THE VERTEBRATE VISUAL
SYSTEM
The visual system of vertebrates depends on light being detected by retinal cells at the
back of a simple eye, and information about the levels of light being relayed, after some local
processing and analysis at the back of the eye, to appropriate regions of the brain. The system
requires a high precision of connections, so that the spatial arrangement of photoreceptors in
the retina is represented in a similar spatial arrangement of the cells that receive information
from them, in the brain. In mice, each eye produces about 50,000 axons, each of which has to
find its appropriate target; in humans, there are over 1,000,000.
32
These axons are produced
by retinal ganglion cells, which collect information from photoreceptors and pass it on to the
brain. The path between retina and brain is complex and involves many intermediate cues.
Retinal ganglion cells do not all differentiate at once but are produced in a centrifugal
sequence as the retina develops. The first retinal ganglion cells, and therefore the axons
that first form the optic nerve, represent the centre of the retina. More peripheral parts of
the retina are represented by retinal ganglion cells that differentiate later, as a 'wave' of
maturation sweeps out from the centre. This growth pattern effectively re-codes spatial infor-
mation relating to the radial axis of the retina as differences in the time domain. Axons from
peripheral retinal ganglion cells arrive at the optic nerve later and therefore travel on the
outside of the axon bundle already formed.
33
In zebrafish, at least, the later axons depend
