Craniofacial Muscles

6 Masticatory Muscle Structure and Function
97
increased vertical facial proportions shifts to a greater proportion of type IIA (fast)
MyHC after jaw surgery (Harzer et al. 2007 ; Maricic et al. 2008 ; Oukhai et al.
2011 ) . The functional adaptability of the masticatory muscles seems to be key to the
future surgical stability of severe craniofacial deformities.
6.4 Biochemistry
Cell–cell and cell–extracellular matrix (ECM) interactions lead to the upregulation
of speci fi c transcription factors and genes essential in the normal development and
maintenance of skeletal muscle (Maley et al. 1995 ; Melo et al. 1996 ; Grounds et al.
1998 ) . The three-dimensional ECM consists of the interstitial connective tissue and
basement membrane in intimate contact with satellite cells and myo fi bres.
The basement membrane, regulating cell polarity and separating tissue types, is
composed of mainly collagen IV, laminin, entactin, and heparan sulphate proteoglycans
(HSPGs) (Sanes et al. 1986 ) , whereas the interstitial ECM between myo fi bres
is composed of mainly collagen I, fi bronectin, and HSPGs (Cornelison 2008 ) .
Collagen provides tensile strength and the proteoglycans (PGs) create space for the
tissue and allow for diffusion; additionally, the ECM behaves as a storage depot for
cytokines and growth factors. The main ECM components form four groups—
collagenous glycoproteins, non-collagenous glycoproteins, proteoglycans, and elastin
(Lewis et al. 2001 ). The ECM differs between muscle groups, and all members play
an essential, co-ordinated, often synergistic role in functionality. The binding of
growth factors and their proteolytic fragments in the ECM has been demonstrated to
exert a number of important in fl uences, such as direct mitogenic effects (Foster et al.
1987 ) . Importantly, the majority of biologically active ECM molecules exhibit multiple
active binding sites with the capacity to bind different ligands and bring about
different activities. Structural integrity of the muscle tissue is crucial to normal function,
and the ECM, with its vast array of molecules, lends itself well to this role.
The adhesion molecules present on myogenic precursor cells (MPCs) consist of
fi ve groups, of which three, the ADAMs (a disintegrin and metalloproteinase
domain), cadherins (M-, N- and R-Cadherin), and immunoglobulin superfamily
(e.g., neural cell adhesion molecule 1 (NCAM-1) and vascular cell adhesion molecule
1 [VCAM-1]), are involved in control of direct cell–cell adhesion (Lewis et al.
2001 ). The dystrophin–dystroglycan complex and integrins, on the other hand, play
an important role in cell–ECM adhesion. The matrix metalloproteinases (MMPs),
secreted into the ECM as latent proenzymes, are essential to ECM maintenance—
for example, the gelatinases (MMP-2 and -9) degrade the major components of the
ECM, namely collagen IV and laminin (Lewis et al. 2001 ). Activation occurs by
proteolysis and is inhibited in a 1:1 manner by the tissue inhibitors of the metalloproteinases
(TIMPs) (Birkedal-Hansen 1995 ) with any disturbances in the
MMP:TIMP ratio manifesting clinically as disturbances in wound healing (Carmeli
et al. 2004 ; Guillen-Marti et al. 2009 ) . The molecules support muscle regeneration
by the MPCs in disease and injury.