The endoderm plays an important role in patterning the segmented pharyngeal region in zebrafish (Danio rerio)

The development of the vertebrate head is a highly complex process involvin g tissues derived from all three germ layers. The endoderm forms pharyngeal pouches, the paraxial mesoderm gives rise to endothelia and muscles, and t he neural crest cells, which originate from the embryonic midbrain and hind brain, migrate ventrally to form cartilage, connective tissue, sensory neur ons, and pigment cells. All three tissues form segmental structures: the hi ndbrain compartmentalizes into rhombomeres, the mesoderm into somitomeres, and the endoderm into serial gill slits. It is not known whether the differ ent segmented tissues in the head develop by the same molecular mechanism o r whether different pathways are employed. It is also possible that one tis sue imposes segmentation on the others. Most recent studies have emphasized the importance of neural crest cells in patterning the head. Neural crest cells colonize the segmentally arranged arches according to their original position in the brain and convey positional information from the hindbrain into the periphery. During the screen for mutations that affect embryonic d evelopment of zebrafish, one mutant, called van gogh (vgo), in which segmen tation of the pharyngeal region is absent, was isolated. In vgo, even thoug h hindbrain segmentation is unaffected, the pharyngeal endoderm does not fo rm reiterated pouches and surrounding mesoderm is not patterned correctly. Accordingly, migrating neural crest cells initially form distinct streams b ut fuse when they reach the arches. This failure to populate distinct phary ngeal arches is likely due to the lack of pharyngeal pouches. The results o f our analysis suggest that the segmentation of the endoderm occurs without signaling from neural crest cells but that tissue interactions between the mesendoderm and the neural crest cells are required for the segmental appe arance of the neural crest-derived cartilages in the pharyngeal arches. The lack of distinct patches of neural crest cells in the pharyngeal region is also seen in mutants of one-eyed pinhead and casanova, which are character ized by a lack of endoderm, as well as defects in mesodermal structures, pr oviding evidence for the important role of the endoderm and mesoderm in gov erning head segmentation. (C) 2000 Academic Press.