ForĮxample, CNC cells migrating in stream 2 (hyoid) are the most anterior neural crest cells toĮxpress Hox genes. This is thought to coordinate A-P patterning of the hindbrain with the arches that it innervates (reviewed in ref. Nested expression of homeobox (Hox and Otx) genes ( fig. In all vertebrates, segmental differences between streams of CNC are conferred through the Hindbrain rhombomeres (r2-r7) give rise to streams of CNC in (more.) Camera lucida drawing of the head of a zebrafish embryo at 24 hours postfertilization, lateral view. Pathways of migration and patterns of homeodomain gene expression in the skeletogenic CNC. As discussed below, this has implications relating to signals received early in development during CNC migration. Suggesting that the mandibular stream has a more significant contribution to the skull than has been appreciated. Reveal a contribution of the most anterior (mandibular) stream to the ventral neurocranium 14, 15 Mandibular (stream 1), hyoid (stream 2), and five branchial (stream 3) arches. While viscerocranial precursors emigrate in three distinct streams from the hindbrain into the Neurocranial precursors emerge from midbrain levels and migrate between the eyes to form the palatal shelves, ![]() 11 - 13 CNC cells delaminate from the ectoderm overlying the dorsal neural tube and migrate as separate streams into the pharyngeal arches ( fig. Well as to all of the pharyngeal arches of the “viscerocranium”, 8 - 10 and these results have been confirmed in other species. Showed that CNC contributes directly to the “neurocranium”, which surrounds the brain, as Lately, new debates on the origin and significance of this “ectomesenchymal” population have emerged, suggesting that cranial skeletal elements arise from a migratory cell population distinct from the CNC. They contradicted the germ layer theory in which skeletal tissues were thought to be exclusively 4 - 6 These results were treated with skepticism at the time, since Specification and Migration of the Skeletogenic CNCĪ CNC origin for the skull was first suggested near the end of the 19th century by ablationĮxperiments in amphibians. Provides the spatial information required for the development of the final skeletal pattern. Including the surface ectoderm and the pharyngeal endoderm. Tube, position within the arch following migration and proximity to local signaling centres These include the site of origin of the CNC in the neural Several factors influence the fate of CNC within the arches. 3 The pharyngeal skeleton forms within a reiterated series of arches that surround the anterior foregut. Larger head segments that are established in the embryo. Such changes are best understood if the skeletal elements are considered as units within Subtle differences in patterning of CNC in the embryo can result in significant morphological ![]() Remarkable changes in function and shape of vertebrate cranial skeletal elements reveal how 2 Similarly, the mammalian homologue of the hyoid bone that primitively supports the jaw in fish, instead forms the stapes of the mammalian middle ear. These CNC-derived cartilages and bonesĪre modified in different vertebrate lineages, such that in mammals the branchial elementsįorm laryngeal bones in the throat, in contrast to the primitive function in fish where they Skeletons which are derived from mesoderm. Much of the skull and all of the pharyngeal skeleton, including jaws, hyoid and gill structures,Īlso have a unique embryonic origin from CNC, unlike the more posterior axial and appendicular ![]() The skull and jaws were key innovations in vertebrate evolution, vital for a predatory lifestyle.
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