Muscle–Tendon Interactions in the Absence of Bones: Lessons from the Fruit Fly, Drosophila - Structural Interfaces and Attachments in Biology - page 118

Fig. 6.3 Muscle cell
morphology during migration
and following attachment to
the tendon cell. A single
muscle (muscle 12) was
labeled with membrane
bound GFP (CD8-GFP) and
also stained for integrin ( red ).
( a ) At stage 14 the muscle is
migrating towards its tendon
cell and exhibits a polarized
shape. ( b ) At stage 16 the
muscle has been attached to
tendon cells at its both ends.
Its morphology has been
changed and integrin ( red
line ) is highly detected at the
interface between the muscle
and the tendon
systems may reflect a high degree of similarity between invertebrates and
vertebrates. Below, I discuss what is known regarding muscle targeting and MTJ
formation in Drosophila .
At stage 12-13 of embryonic development, following fusion of the myoblasts to
distinct founder cells, the myotubes acquire characteristic polarity, in which the
edges of the cells are directed to either anterior-posterior or dorsal-ventral positions
[ 1 ]. By detecting individual GFP-labeled myotubes, the migration path of muscle
cells towards their targeted tendon cells may be followed (Fig. 6.3 ). In this manner,
it is possible to distinguish between mutations affecting the migration per se, or
mutations affecting muscle attachment to tendons. Although both defects result in
a phenotype characterized by dissociation between muscles and tendons and the
rounding of the muscle cells, defects in muscle migration appear at earlier devel-
opmental stages [ 21 ].
A unique aspect of muscle migration toward its target tendon cells is its
concomitant bipolar extension towards two tendon cells located at its two opposite
ends. Imaging of live single muscle cells during their migration suggests a simulta-
neous bi-directional extension of the muscle cell [ 1 ] (Fig. 6.1 ). How do the muscle
ends respond in opposite directions to guiding signals? It is possible that the initial
extension of the two muscle ends occurs due to the intrinsic polarity of this cell type
independent of any external signals. Only when the two muscle ends are distant
enough from each other, might they respond to short-range signals provided by the
tendon cells located at the segment border. Such a scenario might take place during
vertebrate muscle migration where, in a manner similar to Drosophila , the muscles
are connected at their two ends to attachment sites.
Several signaling pathways involved in axon guidance were described that
mediate muscle guidance toward tendon cells, as well. These include the Slit/