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Cranial Paraxial Mesoderm
In addition to the hypaxial and epaxial musculature, there is a third source of skeletal muscle—the cranial paraxial mesoderm. While the paraxial mesoderm of the trunk is segmented into somites, there is no obvious segmentation of the cranial paraxial mesoderm. Moreover, there is no division of the cranial paraxial mesoderm into sclerotome, myotome, and dermatome.
Despite this lack of segmentation, the cranial paraxial mesoderm will form the muscles of the face and some bones of the caudal skull (Noden, 1983; Couly et al., 1993). Using DiI-labeled cells, Hacker and Guthrie (1998) showed that the cranial paraxial mesoderm cells migrate from the region surrounding the neural tube into the center of the branchial arches. (The neural crest cells migrate to the periphery of these arches.) Mesoderm surrounding rhombomeres 1–4 enter the center of the first branchial arch, while mesoderm from rhombomeres 3–6 enter the core of the second arch. Mesoderm flanking rhombomeres 5 and 6 enter into the third arch. The existence and independence of this third source of muscles was shown by the double knockout of the pax3 and myf5 genes in mice (Hacker and Guthrie, 1998). In this double mutant, both the epaxial and hypaxial musculature fail to form. However, the head musculature from the cranial paraxial mesoderm remains.
Changes in cranial paraxial mesoderm can be seen in the faces of people with Down syndrome and other trisomies (Bersu and Ramnirez-Castro, 1977; Bersu, 1980). In Down syndrome, three of the variations—1) the presence of an extra facial muscle, 2) multiple vertebral arteries, and 3) the presence of dilatations and nerve rootlets associated with the spinal accessory and 1st cervical nerves—may be the result of a failure of regression of otherwise transient embryonic structures. In fetuses who died of Trisomy 18 syndrome, there are several anomalies of facial musculature, including hypoplastic occipitofrontalis, auricular and nasal muscles, as well as extensive fusion of the muscles around the corner of the mouth. There was also a supernumerary muscle band that extended from the region near the corner of the mouth to the occipital attachment of trapezius.
The cranial paraxial mesoderm represents a third, and very important, source of skelatal muscles.
Bersu, E. T. 1980. Anatomical analysis of the developmental effects of aneuploidy in man: the Down syndrome. Amer. J. Med. Genet. 5: 399–420.
Bersu, E. T. and Ramirez-Castro, J. L. 1977. Anatomical analysis of the developmental effects of aneuploidy in man—the 18-trisomy syndrome: I. Anomalies of the head and neck. Amer. J. Med. Genet. 1: 173–93.
Couly, G. F., Coltey, P. M., and Le Douarin, N. M. 1993. The triple origin of skull in higher vertebrates: a study in quail-chick chimeras. Development 117: 409–429.
Hacker, A. and Guthrie, S. 1998. A distinct developmental programme for the cranial paraxial mesoderm in the chick embryo. Development 125: 3461–3472.
Noden, D. M. 1983. The embryonic origins of avian cephalic and cervical muscles and associated connective tissues. Amer. J. Anat. 168: 257–267.
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