Craniofacial Muscles

2 Head Muscle Development
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2.3 Head Muscles Are Heterogeneous in Terms of Their
Mesodermal Origins
Head muscles are highly heterogeneous in their structure, function, anatomical
position, and developmental origins. In contrast to the segmented paraxial mesoderm
(the somites) in the trunk, the head mesoderm lacks any sign of segmentation
(Noden and Trainor 2005 ) . Myoblasts that form head muscles arise within several
precursor populations, including prechordal, paraxial, and splanchnic (lateral)
mesoderm, and migrate into regions where the connective tissue progenitors may be
either ectodermal (neural crest) or mesodermal in origin (Noden and Trainor 2005 ) .
In the past, craniofacial development was widely viewed within the context of the
neural crest cells, leading to the misconception (often seen in textbooks) that the
head musculature originates from neural crest cells. In fact, all head muscles derive
from mesodermal cells (Couly et al. 1992 ; Harel et al. 2009 ; Noden 1983a ) .
Head mesoderm precursors undergo gastrulation in the primitive streak, prior to
those of trunk mesoderm (Kinder et al. 1999 ; Psychoyos and Stern 1996 ) . Pharyngeal
muscles are derived from the pharyngeal arch mesodermal core, which constitutes a
subset of head mesoderm surrounding the pharynx. The pharyngeal mesoderm (PM)
is divided into two subdomains: the loosely connected mesenchymal paraxial mesoderm,
located on both sides of the neural tube and notochord (Fig. 2.2a , b), and the
medial splanchnic mesoderm, which is maintained as epithelial tissue, although
there seems to be no clear division between these two mesodermal populations
(Fig. 2.2 , Tzahor and Evans 2011 ) .
During embryonic ventral folding, the lateral splanchnic mesoderm is located on
the ventral side, beneath the fl oor of the pharynx (Fig. 2.2b ). Both paraxial and
splanchnic mesodermal cells converge to form the mesodermal core within the pharyngeal
arches (Nathan et al. 2008 ) (Fig. 2.2c , d). Hence, the PM includes both
paraxial and lateral splanchnic mesoderm cells that surround the pharynx (Fig. 2.2 ,
green). Taken together, PM cells contribute to the cores of the PAs (Fig. 2.2d ) and
give rise to signi fi cant areas of the heart and pharyngeal muscles. Moreover, PM
cells are found in close proximity to the pharyngeal endoderm, ectoderm, and neural
crest cells, all of which tightly in fl uence pharyngeal muscle development
(Fig. 2.2d ) (summarized below).
In addition to their contribution to the pharyngeal muscles, PM cells give rise to
cardiac progenitors (Grifone and Kelly 2007 ; Tzahor 2009 ; Tzahor and Evans 2011 ) .
Studies in both chick and mouse embryos have shown that cardiac progenitor cells
populating the cardiac out fl ow tract and right ventricle, collectively referred to as
the anterior heart fi eld (Kelly et al. 2001 ; Mjaatvedt et al. 2001 ; Waldo et al. 2001 ) ,
are progressively added by PM cells during heart looping stages. In the mouse, the
anterior heart fi eld is a subset of the second heart fi eld, which contributes to the outflow
tract and right ventricle, and will later contribute a majority of cells to the atria.
Thus, a subset of PM cells constitutes the second heart fi eld, in contrast to the
more lateral splanchnic mesoderm, known as the fi rst heart fi eld (Fig. 2.2a, d, red),