Craniofacial Muscles

6 Masticatory Muscle Structure and Function
99
It has been found that the ratio of satellite cell nuclei to total nuclei number
varies between masticatory and somatic muscle groups with the latter demonstrating
a higher ratio (Renault et al. 2002 ) . This is despite the presence of a greater
mean number of nuclei per fi bre within the masseter muscle. Importantly, the ratio
of satellite cells decreases in both groups with age; however, it has been suggested
that generally the masseter muscle regenerates less effectively following a traumatic
injury than somatic muscle (Pavlath et al. 1998 ) . The contrast in regenerative
capability may be explained by the different embryological origins of craniofacial
and somatic skeletal muscle and variations within the myoblast populations.
Numerous animal models have been used to examine the structural and
functional characteristics of regenerating skeletal muscle (Carlson and Gutmann
1975a, b ; Faulkner et al. 1980 ) . Regardless of the nature, severity, and extent of the
injury, the process of muscle regeneration is comparable, although the outcome and
timetable of the reparative process may vary (Lee et al. 2000 ) . Some in vivo studies
have used differential labelling of satellite cells and myonuclei with 3H-thymidine
to demonstrate the satellite cell as the sole source of new MPCs (Schmalbruch
1976 ) . Recent studies have demonstrated that the population of satellite cells in
muscles is quite heterogeneous, both molecularly and functionally (Collins et al.
2005 ; Biressi and Rando 2010 ; Rossi et al. 2010 ) . In response to injury, satellite
cells become activated and proliferate to form a pool of MPCs (Lee et al. 2000 ) .
Some of the MPCs differentiate to provide a source of nuclei to damaged myo fi bres
or alternatively fuse together to form new multinucleated myo fi bres (Moss and
LeBlond 1971 ; Snow 1978 ; Campion 1984 ) , whereas some daughter cells replenish the
satellite cell population (Barof fi o et al. 1995, 1996 ; Yoshida et al. 1998 ; Beauchamp
et al. 1999 ) . The migration of MPCs through the basal lamina of myo fi bres both
within (Phillips et al. 1990 ) and between (Watt et al. 1994 ) skeletal muscles and
formation of the connective tissue network by the proliferating MPCs is key to successful
repair and regeneration (Hughes and Blau 1990 ; Li and Huard 2002 ; Hill
et al. 2003 ) .
Satellite cells were originally thought to be of somitic origin: classic quail-chick
chimera experiments revealed the association of satellite cell nuclei derived from
implanted quail somite with host chick myo fi bres (Armand et al. 1983 ) . However,
more recent studies have suggested a non-somitic origin of satellite cells (Ferrari
et al. 1998 ; De Angelis et al. 1999 ; Hawke and Garry 2001 ; Asakura and Rudnicki
2002 ; LaBarge and Blau 2002 ; Polesskaya et al. 2003 ) . For craniofacial muscles,
these cells are derived from the same source of craniofacial mesenchyme as the
muscles themselves (Harel et al. 2009 ) . Further, it has been proposed that MPCs
may still retain the plasticity to transcend the lineage boundary when exposed to
certain in vitro and in vivo environmental cues (Jackson et al. 1999 ; Lee et al. 2000 ;
Geiger et al. 2002 ; Wada et al. 2002 ; Cao et al. 2003 ; Zheng et al. 2007 ) , although
some evidence exists that they may arise from circulating blood-borne or other stem
cell populations. Another source of stem cells (muscle-derived stem cells, MDSCs)
is present in adult skeletal muscle with the ability to give rise to different cell types,
including MPCs (Royer et al. 2002 ; Tamaki et al. 2002 ; Wada et al. 2002 ) . Further
studies need to be undertaken to determine whether these cells truly are satellite cell