Muscle–Tendon Interactions in the Absence of Bones: Lessons from the Fruit Fly, Drosophila - Structural Interfaces and Attachments in Biology - page 117

Fig. 6.2 Muscle-tendon recognition. Following the arrival of muscles to their corresponding
tendon cells, the muscle signals to the tendon through EGFR activation by means of the
neuregulin-like secreted ligand Vein to initiate tendon differentiation and elevate SrB expression.
SrB further induces the expression of Slit and LRT, the latter is required to arrest muscle
migration. Tendon precursors that do not bind muscles loose SrB expression and become ectoderm
cells
The muscle-dependent signal required for differentiation of tendon progenitors
into fully mature tendon cells is provided by Vein, a neuregulin-like secreted ligand
of the EGF-receptor pathway. Following muscle binding, Vein accumulation at the
muscle-tendon junction site specifically activates the EGFR-receptor pathway in
the muscle-bound tendon progenitor, driving it to differentiate to a mature tendon
cell (Fig. 6.2 )[ 20 ].
Thus, the initial determination of Drosophila tendon progenitor cells in the
ectoderm takes place sequentially (Figs. 6.1 and 6.2 ). The initial weak signal is
initiated by segment polarity genes. Then, the signal is strengthened as a result of
StripeB activity, which is positively auto-regulated, but also maintained at low
levels as a result of the posttranscriptional inhibitory activity of HOW(L). In this
manner, the tendon progenitor cells produce the necessary signals for attracting
muscle cells towards the attachment sites; however, the cells are not fully
committed to a tendon fate, and their final differentiation is still dependent on
future interaction with muscles.
6.3 Targeting and Anchoring of Muscles to Tendons and the
Formation of the Myotendinous Junction in Drosophila
Whereas the initial determination of muscles and tendons appears to be autonomous
in Drosophila , targeting of muscles to tendons requires cross-talk between these
two tissue types. Muscle migration towards tendons depends on several factors
including the initial polarity of the muscle cell, local signals available during
muscle migration, target recognition, and signals involved in the migration arrest.
Due to the limited amount of information available on these processes in both
vertebrates and invertebrates, it is too early to speculate as to the degree of
conservation between these processes in the two systems. However, the gradual
construction of the MTJ mediated primarily by integrin-dependent adhesion in both