Craniofacial Muscles

326 L.K. McLoon and F.H. Andrade
17.2 Embryology
One of the most striking differences between limb and craniofacial muscles relates
to the genetic programs that control their early embryologic development. As
described in Chap. 2 , while somite-derived skeletal muscles depend on early expression
of Pax3 to develop (Tajbakhsh et al. 1997 ) , non-somite-derived craniofacial
muscles develop normally in the absence of Pax3. Over the last decade, the genetic
networks that control differentiation of craniofacial muscles have been illuminated,
and are quite divergent from each other as well as from limb skeletal muscles
(Table 17.1 ; for review, see Chap. 2 ; Sambasivan et al. 2011 ; Bothe et al. 2011 ) .
The muscles derived from the pharyngeal arches have some overlapping early
gene control, with Tbx1 being important in the formation of muscles derived from
pharyngeal arches 2 and 3, and partially controlling masticatory muscle formation.
The extraocular muscles represent the most distinct of this group of muscles, and
depend on Pitx2 expression for their formation (Diehl et al. 2006 ) . The importance
of these early genetic differences is underlined by examination of a Pitx2 conditional
knockout, where over time the extraocular muscles from these mice show
decreased expression of slow-tonic and EOM-speci fi c myosin heavy chain (MyHC)
isoforms, as well as decreased numbers of en grappe neuromuscular junctions (Zhou
et al. 2009, 2011 ) , resulting in extraocular muscles that have increased similarity to
limb muscle.
17.3 Fiber Types and Contractile Properties
Limb and body skeletal muscles express four main MyHC isoforms, types IIA, IIB,
IIX, and type I; while these can be co-expressed in a variety of patterns, they still
only express these four isoforms. As we have seen, all the craniofacial muscles
included in this book, as well as muscles within the soft palate and associated with
the ear (Ståhl and Lindman 2000 ; Jung et al. 2004 ) , are predominantly fast twitch
muscles. With the possible exception of the muscles of facial expression, the craniofacial
muscles express “unusual myosins.” These include the alpha-cardiac and
the fetal MyHC isoforms (Stål et al. 1994 ; Korfage et al. 2005 ) . In addition individual
subsets of craniofacial muscles express muscle-speci fi c myosins such as the
Table 17.1 Development
Muscle type Genetic control Mesodermal source
Limb muscles Pax3 Somitic mesoderm
Tongue Pax3 Somitic mesoderm
Laryngeal muscles Tbx1 Pharyngeal arch 3
Facial muscles Tbx1 Pharyngeal arch 2
Masticatory muscles Tcf21 (partial loss,Tbx1), Pitx2 Pharyngeal arch 1
Extraocular muscles Pitx2 Non-segmented cranial mesoderm