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Fig. 2. Combinatorial action of LIM-homeodomain and Hox transcription factors
dictate Drosophila and vertebrate motor axon guidance. Motor
neurons (MNs) in Drosophila and vertebrates can be identified by the
routes that they take and the muscle fields that they innervate. (A) In
Drosophila, most MNs exit from the ventral nerve cord along two major
nerve routes, the segmental nerve (SN) and intersegmental nerve (ISN), from
which they defasciculate to innervate discreet populations of muscles
(represented by numbers 1-29). The MNs express different combinations of
transcription factors that appear to dictate which muscle fields they
innervate, as shown in the key. (B) In vertebrate spinal cord, somatic
MNs are arranged in columns that project to common targets and can be
distinguished by the combinatorial expression of LIM-homeodomain transcription
factors (see key). The medial motor column (MMC; blue) projects axons to axial
muscle, whereas, at the brachial and lumbar levels, the lateral motor column
(LMC; red and green) projects to the limb. On reaching the limb, the LMC
subdivides such that the medial (m) division (red) projects to the ventral
limb, whereas the lateral (l) division (green) projects to the
Scapulohumeralis (Sca) muscle of the dorsal limb. These divisions are further
subdivided into pools of MNs that innervate particular muscle groups. At
brachial levels, the LMC is subdivided by the expression of Hox5 and
Hoxc8, which appear to control the projection pattern of LMC axons
into distinct motor pools in the Pec (pectoralis), anterior latimuss dorsi
(ALD) and flexor carpi ulnaris (FCU) muscles. (A) Modified with permission
from Landgraf and Thor (Landgraf and Thor,
2006) and (B) modified with permission from Kania et al.
(Kania et al., 2000).
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