Craniofacial Muscles

12 I. Harel and E. Tzahor
2.2 Head Muscles
In contrast to our understanding of how skeletal muscle is formed in the trunk, much
less is known about the tissues and molecules that induce the formation of the head
musculature. This chapter summarizes studies of the origins, composition, signaling,
genetics, and evolution of distinct craniofacial muscles. Cellular and molecular
parallels are drawn between cardiac and pharyngeal arch muscle developmental
programs, and argue for the tissues’ common evolutionary origins. It is clear that the
developmental paths that lead to the formation of skeletal muscles in the head
appear to be distinct from those operating in the trunk. Considerable cellular and
genetic variations also exist among the different craniofacial muscle groups.
Approximately 60 muscles are present in the vertebrate head, which, rather than
serving for locomotion, move the eyes, control the cranial openings and facial
expression, facilitate food uptake and, in humans, speech (Noden 1983a ; Noden and
Francis-West 2006 ; Wachtler and Jacob 1986 ) (Fig. 2.1a, b ). These head muscles
encompass the extraocular muscles (EOM), the muscles of mastication that open
and close the jaw apparatus (derived from pharyngeal arch 1; PA1) and the muscles
of facial expression (derived from pharyngeal arch 2; PA2). The muscles of the third
pharyngeal arch (also known as branchial arches) operate the pharynx and larynx.
A number of these head muscles, including the hypobranchial muscles, the tongue
muscles, and the muscles of the posterior PAs, develop from the somites (Fig. 2.1a, b ).
While head muscles exhibit the same tissue architecture as muscles in the trunk,
their development is remarkably distinct.
Fig. 2.1 The anatomy and mesodermal origins of craniofacial muscles in the mouse. ( a ) A transverse
section of an E16.5 embryonic head stained for the muscle marker MyHC ( red ). DAPI staining
( gray ) is seen in the background. Distinct marked muscles are detailed in ( b ) with respect to their
muscle subgroups and their origins. ( b ) An anatomical cartoon of adult mouse head highlighting
the craniofacial muscles shown also in ( a )