Craniofacial development requires the orchestrated integration of multiple tissue interactions. Defining the spatial relationship and the interactions between neural crest cells and muscle cells and their derivatives during jaw development is an important step towards understanding craniofacial malformations.
Jaws originate from the bilateral first branchial arches. The first arch gives rise to the maxillary and mandibular prominences and subsequently to musculo-skeletal structures of the upper and lower jaws 
. More caudally, the other branchial arches will provide the neck and throat components. Branchial arches are composed of pharyngeal endoderm, surface ectoderm, and two mesenchymal cell populations, originating from the neural crest and from cranial mesoderm, respectively. The ectodermal and endodermal components envelope the two mesenchymal cell types. Mapping of the cephalic neural folds, using quail chick chimeras, retroviral and DiI injections have shown that neural crest cells filling the branchial arches give rise to all the skeletal elements, connective tissues and tendons of the jaw, while the mesodermal core gives rise to myogenic cells of the jaw muscles 
Although previous fate mapping experiments have identified the majority of derivatives of neural crest cells and cranial mesoderm in the jaw, the spatial relationships and the interactions over time between both cell types are not completely understood. Neural crest cells colonising the first branchial arches originate from the posterior mesencephalon to rhombomere 3 
. Neural crest cells have been described as migrating in between the overlying surface ectoderm and the cephalic mesoderm (containing the myogenic progenitors), effectively separating these two tissues. Then, cephalic mesodermal cells and neural crest cells expand ventrally at the same time into the future branchial arch region. It has been described that neural crest cells envelop but initially do not penetrate the centrally located muscle plate of the branchial arches. Subsequently, coincident with muscle segregation, each muscle plate becomes infiltrated by neural crest cells, which may provide the muscle connective tissue of muscles, reviewed in 
. Consequently, throughout their migration and subsequent organisation, neural crest cells are in close contact with the myogenic precursor cells during arch development. These extended interfaces between both cell populations have being suspected to be important for cell interactions during arch development and subsequent jaw morphogenesis.
Muscle formation relies on intrinsic program and extrinsic cues. The genetic program controlling head muscle specification is distinct from that underlying trunk and limb myogenesis, reviewed in 
. Moreover, specific genetic programs drive the specification of head muscles, highlighting a genetic heterogeneity underlying head muscle development. MyoR
regulate an initial step in the specification of a specific subset of branchiomeric muscles, the major muscles of mastication, derived from the first branchial arch 
. The absence of Tbx1
function in Tbx1−/−
mutant mice, severely perturbs branchiomeric muscle development, while extra-ocular and body muscles are unaffected 
is expressed before MyoR
and is required for their expression in the first branchial arch, although a second study has revealed Pitx2
. Although there is an emerging network of transcription factors involved in branchial arch muscle formation, we still do not have a global picture of the intrinsic developmental program responsible for the specification of different head muscles. In addition to the intrinsic genetic program, classical experiments in avian embryos have shown that head muscle formation is also dependent on extrinsic tissues 
. It has long been suspected that cranial neural crest cells influence head muscle formation 
. However, the early steps of facial muscle specification have been shown to be independent of the presence of neural crest cells, since ablation of cranial neural crest cells in chick and amphibian embryos does not block initiation of the myogenic program in the branchial arches 
. All muscle genes reflecting early steps of the myogenic program, including Capsulin
are expressed in the branchial arches following neural crest cell ablation in chick embryos, although their expression domains are altered 
. The muscle differentiation program based on myosin expression is also initiated in crest-ablated arches; indicating that neural crest cells are not necessary to initiate muscle differentiation 
. However, in the absence of neural crest cells, jaw muscles were found to be severely reduced showing the requirement of neural crest cells for normal muscle organisation after the onset of muscle specification and differentiation 
. These experiments indicate an early neural crest cell-independent phase and then a later -dependent phase for branchiomeric muscle formation.
Branchiomeric muscle formation has been studied in the absence of neural crest cells 
, but we do not know how neural crest cells behave in the absence of muscle. Tendons are one of the neural crest cell-derived tissues likely to interact with forming jaw muscles. Tendons did not attract much attention from developmental biologists probably due to the lack of early markers. The discovery that the gene encoding the bHLH transcription factor Scleraxis is expressed in embryonic tendons in body and limb muscles provided a major step forward 
. Genetic ablation of Scleraxis
in the mouse leads to defective differentiation of limb muscle tendons 
. Although no head phenotype has been reported to date in the Scleraxis
mutant mice 
is expressed in the branchial arches and head tendons of mouse embryos 
In the present study, we have reinvestigated the relationship between neural crest cells and myogenic cells (both mesodermal progenitors and differentiated muscle cells) during development of the first branchial arch. Using molecular and embryological markers, we observe an unexpected intermingling between myogenic cells and neural crest cells at various stages of development. Furthermore, we have show that all tendons in the embryonic head are of neural crest cell origin and express Scleraxis. By analysing tendon formation in absence of muscle using a genetic mouse model displaying loss of branchiomeric muscles, we were able to demonstrate that tendons initiate their development independently of muscles but that muscle is required for further tendon development.