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neurons into motor columns is accomplished in
response to a gradient of FGF8 (high FGF8 posteriorly
to low FGF8 anteriorly) secreted by the paraxial meso-
derm. This gradient establishes domains of different
Hox genes (Dasen et al., 2003). The anterior Hox genes
inhibit the expression of the posterior Hox genes,
and vice versa, so that sharp borders are established.
The boundaries between these domains establish the
boundaries of the different motor columns. We can
appreciate that this logic is strikingly similar to that
governing the positioning of the motor neurons in the
ventral region of the spinal cord. Thus, exposure to
gradients in both axes leads to the differential expres-
sion of homeobox genes that through cross-repression
establish sharp borders and different motor columns
in the ventral spinal cord (Figure 4.24).
These motor columns can be further divided into
pools that innervate specific muscles. In zebrafish, each
spinal segment has just three primary motor neurons:
RoP, MiP, and CaP (for rostral, middle, and caudal
primary, respectively; Figure 4.25). CaP innervates
ventral muscle, RoP innervates lateral muscle, and MiP
innervates dorsal muscle. If these motor neurons are
transplanted to different positions a few hours before
they begin axonogenesis, they seem to switch fate: for
example, CaP transplanted into the RoP position can
innervate lateral instead of ventral muscle (Eisen,
1991). These results suggest that the position of the cell
soma specifies the axonal projection of the different
primary neurons.
Motor pools that innervate individual muscles are
distinguished by the expression of distinct members of
Time
A
C
RoP MiP CaP
Spinal
cord
Dorsal
Notochord
Center
Muscle
segment
Ventral
Notochord
Somite
D
Floorplate
RoP CaP CaP
Motor
Neurons
B
E
CaP CaP
Notochord
Somitic
Muscles
FIGURE 4.25 Position determines primary motor neuron identity in zebrafish. A. Zebrafish embryo at
about 1 day old. B. Schematic cross section showing the location of the primary motor neuron and the somites
that give rise to the axial musculature. C. The rostral (RoP), middle (MiP), and caudal (CaP) primary motor
neurons of a single segment develop over a course of about 24 hours. D. If CaP is transplanted to the MiP
position before axonogenesis begins, it develops a MiP axonal projection. E. However, if the transplant is
done several hours later after the axons have begun to grow, the axonal fates are fixed.
 
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