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Fig. 7. Model for Gli2 and Gli3 function in somite myogenesis. Dorsoventral and
mediolateral patterning of the somite is dictated in part by the opposing
action of a ventral gradient of Hedgehog signalling (in red) and a lateral
gradient of Bmp4 signalling (in blue). In the medial somite, the combined
activity of Gli2A and Gli3A, converted from Gli3R into Gli3A by Shh, leads to
the activation of Myf5 in DML cells and the activation of
Pax1 in sclerotomal cells. Myf5 is also a target of Gli2A
and Gli3A in the epaxial myotome, although a repressor function of Gli3 is
revealed in Gli2–/–Gli3+/–
embryos. In the lateral somite, characterised by low levels of Shh signalling
and high levels of Bmp4 signalling, Myod1 and Lbx1
expression is controlled by Gli3R and Smad. We suggest that Gli3R could
directly control Lbx1, Myf5 and Myod1 levels, and/or could
interfere with Bmp4 signalling via an interaction with Smad in the control of
Lbx1, Myf5 and Myod1 levels in the VLL and hypaxial myotome.
Additional Gli/Shh-independent mechanisms of Myf5 activation are
active in both the medial and lateral somite, possibly mediated by Wnt and/or
FGF signalling.
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