Craniofacial Muscles

102 M. Lewis et al.
BB (PDGFBB), epidermal growth factor [EGF], IL-6 (Robertson et al. 1993 ; Hawke
and Garry 2001 ) ) and involvement with cell adhesion molecules may not only rescue
MPCs from apoptosis, but also play similar roles for erythroblasts and neuronal
cells (Meucci et al. 2000 ; Chazaud et al. 2003 ) .
Within skeletal muscle, fi broblasts are surrounded by the peri- and endomysium
and are attached to the ECM via integrins (Mackey et al. 2008 ) enabling the cells to
register and respond to any mechanical stimuli transmitted through the ECM: this
ability has been suggested to be essential for correct fi broblast functioning (Sarasa-
Renedo and Chiquet 2005 ) . The endomysial collagen is primarily produced by
fi broblasts, and it has been demonstrated in vitro that fi broblasts produce factors
that may modify MPC response (Sinanan et al. 2008 ) .
In summary, the muscles of mastication, and especially the masseter muscle, differ
from non-cranial muscles in several important ways. Firstly, the range and variation
of MyHC protein isoforms, and in particular, the persistence of developmental isoforms
which can be considered part of the normal adult MyHC isoform population.
Secondly, the presence and location of certain metalloproteinases, especially MMP-9,
adjacent to myo fi bres in normal, healthy craniofacial muscle tissue is seen as
opposed to its presence only in response to disease or injury, and thirdly, differences
exist in the regenerative capability with a suggestion that the masseter muscle may
be less effective at this process compared to somite-derived muscle.
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