File talk:Chicken HH20 MyoR expression 01.jpg

Comparison of MyoR and MyoD expression domains during chick first branchial arch development. (A) Lateral view of a HH20 chick embryo hybridized with a MyoR probe. HH20 (B,C), HH22 (D,E) HH24 (F,G) and HH26 (H,I) embryos were frontally sectioned at the head level. The plane of section is indicated in the panel A. Adjacent sections from each stage were hybridized with DIG-labelled antisense probes for either MyoR (B,D,F,H) or MyoD (C,E,G,I). MyoR delineates the myogenic core of the first branchial arch (B,D,F) and is subsequently expressed in all branchiomeric muscles (H). MyoD transcripts are first detected in a lateral sub-region of the MyoR domain at HH20 (B,C, arrows) and HH22 (D,E, arrows), then spread progressively from lateral to medial regions to overlap with the MyoR domain in branchiomeric muscles (E,G,I). (D–G) Arrows point to the lateral domains of the core, while arrowheads show the medial domain of the core. (H,I) Arrows indicate the branchiomeric muscles expressing both MyoR (H) and MyoD (I). (A) Arrowheads indicate the hypaxial lips at the interlimb level, expressing MyoR. BA1, first branchial arch, di, diencephalon; e, eye; fl, forelimb; hl, hindlimb; mb, mandibular arch; mes, mesenchephalon; MR, medial rectus; mx, maxillary arch; ph, pharynx; tel, telencephalon; VO, ventral oblique; VR, ventral rectus. doi:10.1371/journal.pone.0004381.g001

PLoS One. 2009;4(2):e4381. doi: 10.1371/journal.pone.0004381. Epub 2009 Feb 9. Relationship between neural crest cells and cranial mesoderm during head muscle development. Grenier J1, Teillet MA, Grifone R, Kelly RG, Duprez D. Author information Abstract BACKGROUND: In vertebrates, the skeletal elements of the jaw, together with the connective tissues and tendons, originate from neural crest cells, while the associated muscles derive mainly from cranial mesoderm. Previous studies have shown that neural crest cells migrate in close association with cranial mesoderm and then circumscribe but do not penetrate the core of muscle precursor cells of the branchial arches at early stages of development, thus defining a sharp boundary between neural crest cells and mesodermal muscle progenitor cells. Tendons constitute one of the neural crest derivatives likely to interact with muscle formation. However, head tendon formation has not been studied, nor have tendon and muscle interactions in the head. METHODOLOGY/PRINCIPAL FINDINGS: Reinvestigation of the relationship between cranial neural crest cells and muscle precursor cells during development of the first branchial arch, using quail/chick chimeras and molecular markers revealed several novel features concerning the interface between neural crest cells and mesoderm. We observed that neural crest cells migrate into the cephalic mesoderm containing myogenic precursor cells, leading to the presence of neural crest cells inside the mesodermal core of the first branchial arch. We have also established that all the forming tendons associated with branchiomeric and eye muscles are of neural crest origin and express the Scleraxis marker in chick and mouse embryos. Moreover, analysis of Scleraxis expression in the absence of branchiomeric muscles in Tbx1(-/-) mutant mice, showed that muscles are not necessary for the initiation of tendon formation but are required for further tendon development. CONCLUSIONS/SIGNIFICANCE: This results show that neural crest cells and muscle progenitor cells are more extensively mixed than previously believed during arch development. In addition, our results show that interactions between muscles and tendons during craniofacial development are similar to those observed in the limb, despite the distinct embryological origin of these cell types in the head. PMID 19198652