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Figure 2. Components and patterning of the zebrafish pharyngeal skeleton. (A) Neural crest cell and mesodermal components of the zebrafish. Elements within a segment are shaded corresponding to: mandibular, 1 (neural crest; mesoderm); hyoid, 2 (neural crest; mesoderm); branchials, 3-7 (neural crest; mesoderm). Schematic representation of early development, mid-somitogenesis, when head segments are first forming in a generalized vertebrate embryo. Neural crest cells migrate ventrally into pharyngeal arches along three separate migratory streams (1-3). By late embryonic stages, each pharyngeal segment is further subdivided into dorsal and ventral regions, with neural crest lying in a cylinder surrounding mesodermal cores and wrapped in epithelia. By larval stages in the zebrafish, each arch contains segment-specific cartilages derived from neural crest and muscles derived from mesoderm. (B) Schematized, larval pattern of zebrafish pharyngeal cartilages. Segment- and cell-type specification of zebrafish neural crest cells. Anterior-posterior origins of pharyngeal neural crest and mesoderm and medio-lateral organization in the mandibular arch (B). Elements within a segment are shaded as marked: mandibular (neural crest; mesoderm); hyoid (neural crest; mesoderm); and branchial (neural crest; mesoderm). Schematic representation of an early zebrafish embryo, mid-somitogenesis, as neural crest begins to migrate. The hindbrain is segmented into rhombomeres (r2-r7). Neural crest cells migrate to join particular regions of mesoderm in particular arches, depending on their segmental origins adjacent to the hindbrain. Schematic of transverse section (in the plane indicated), showing locations of lateral neural crest that forms neuronal and glial fates and medial crest cells that form cartilage and pigment.
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