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A
B
A
Commissural
interneuron
Chick spinal cord
B
TAG1
NrCAM
Floorplate
C
D
Control
Antibodies to TAG1 or NrCAM
Fas I
Fas II
FIGURE 5.24 CAM changing. A. Two panels showing FasI (top)
and FasII (bottom) distribution in the embryonic CNS of Drosophila
as revealed with specific antibodies. B. Axons express different
CAMs on different segments. A commissural axon in an embryonic
Drosophila CNS. This axon expresses FasII in the longitudinal
pathway to help it fasciculate with other FasII expressing axons in
this pathway, switches to FasI while it is in the commissure and fas-
ciculating with other FasI expressing axons, and then switches back
again to FasII once it has reached the other side. (After Zinn et al.,
1988; Lin et al., 1994; Goodman, 1996)
FIGURE 5.25 Crossing the midline of the vertebrate spinal
cord. A. A commissural interneuron sends an axon toward the floor-
plate. B. At higher magnification and with antibodies, we see
that the growth cone of the commissural neurons expresses the
heterophilic CAM, TAG1, while the floorplate cells express a
heterophilic CAM partner, NrCAM. C. In a normal animal, these
commissural axons cross the midline, while in (D) if either TAG1 or
NrCAM function is perturbed with antibodies, the axons do not
cross. (After Stoeckli et al., 1997)
and the result is that commissural interneurons are
much less likely to cross the floorplate. Instead, they
remain ipsilateral (Stoeckli et al., 1997) (Figure 5.25).
obtained by co-culturing pieces of olfactory bulb with
septum, a medial structure of the forebrain. Axons
from the bulb run laterally in the forebrain appearing
to grow away from the septum. When an explant of
bulb tissue is placed near the explant of septal tissue,
the olfactory axons emerge from the side of the explant
opposite the septum (Pini, 1993) (Figure 5.26C,D).
These data provide indications that a chemorepulsive
mechanism could play a role in axon guidance.
When two explants are co-cultured, there is usually
an intermingling of axons, but in the case of retinal and
sympathetic co-cultures, it is apparent that the axons
avoid one another. Time-lapse video films made of
growth cones from one explant as they approach the
axons of the other show that these growth cones col-
lapse when they make contact with the foreign axon.
They lose their filopodia, retract, and become tem-
porarily paralyzed (Kapfhammer and Raper, 1987a)
(Figure 5.27). Often, after a few minutes, a new growth
cone is formed which advances once more until it
again encounters the unlike axon in its path and again
collapses. These studies demonstrate that just the
briefest contact from a single filopodium is all that is
necessary to elicit this aversive behavior, strongly sug-
gesting that growth cones sense a repulsive signal on
REPULSIVE GUIDANCE
In addition to adhesion molecules and extracellular
matrix molecules that promote axon growth, there
are factors that do just the opposite. These are the
inhibitory or repulsive factors (Kolodkin, 1996). Tissue
culture experiments have indicated that neural tissue
produces substances that repel some axons. In these
sorts of experiments, the trajectories of axons are
observed when they are cultured in the presence of
tissues they normally avoid. For example, the axons of
dorsal root ganglia (DRG) innervate the dorsal spinal
cord and generally do not enter the ventral regions of
the spinal cord. When these DRGs are grown in tissue
culture alongside pieces of dorsal and ventral spinal
cord, most DRG axons preferentially invade the dorsal
cord and avoid the ventral cord even if they have
to grow in a circuitous fashion around it (Peterson
and Crain, 1981) (Figure 5.26A,B). Similar results are
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