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Dosal root
ganglion
DRG
A
B
Dosal
root
D
Ventral
root
V
Dorsal
spinal
cord
Ventral
spinal
cord
DRG
Floor plate
C
D
Olfactory
bulb
Olfactory
bulb
Septum
E
F
Epidermis
DRG
Dermo-
myotome
Floor plate
Notochord
DRG
Dermo-
myotome
Notochord
FIGURE 5.26 Repulsive guidance. A. The central projections of most DRG axons do not enter the ventral
horn of the spinal cord, but rather make synapses in the dorsal horn. B. When cultured together, DRG neurons
avoid ventral spinal cord explants to grow to dorsal targets. C. The telencephalon shows olfactory tract fibers
originating from the olfactory bulb traveling in the lateral region, far away from the medial septum. D. When
cultured together, olfactory bulb axons travel away from the septum indicating the existence of a diffusible
chemorepellent. E. Surround repulsion. DRG axons outside the spinal cord elongate in a bipolar fashion
between the dermomyotome and the ventral spinal cord and notochord. Many surrounding tissues, includ-
ing the epidermis, the dermomyotome, the floorplate, and the notochord, secrete diffusible repellents.
F. When placed in a collagen gel between a piece of notochord and dermomyotome, DRG axons extend
in a bipolar fashion, similar to their pattern in vivo. (After Peterson and Crain, 1981; Pini, 1993; Keynes
et al., 1997)
the surface of the other axon. By pairing different types
of explants in such cultures and observing the growth
cone interactions, a variety of different collapsing
activities effective on different types of growth cones
were discovered, showing that there is a rich hetero-
geneity of repulsive interactions between neurons
(Kapfhammer and Raper, 1987b).
Attempts to purify collapsing factors biochemically
were aided by a bioassay in which reconstituted mem-
brane vesicles were added to cultures of axons growing
on laminin substrates. When vesicles enriched in col-
lapsing activity from the CNS were added to cultures,
they caused the immediate collapse and paralysis of all
the sensory ganglion cell growth cones on the plate. A
100kd glycoprotein that could cause growth cone col-
lapse, initially called Collapsin, was eventually puri-
fied sufficiently to obtain a partial protein sequence
(Luo et al., 1993). Through use of the sequence data,
the gene was obtained. Collapsin turned out to be a
member of a large molecular family, which was then
renamed the semaphorin family owing to the role that
these molecules have in signaling. Collapsin became
known as Sema3A. The first member of the semaphorin
family to be identified in grasshoppers, SemaI, (Kolod-
 
 
 
 
 
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