The developing chick hindbrain is transiently divided into a series of repeating units or rhombomeres. Recent work has shown that an alternating periodicity exists both in the cell surface properties of rhombomeres and in the segmental origin of hindbrain neural crest cells. Experiments in which rhombomeres from different axial levels were confronted in the absence of an interrhombomere boundary showed that odd-numbered segments 3 and 5 combined without generating a boundary, as did even-numbered segments 2, 4 and 6. When rhombomeres originating from adjacent positions, or three rhombomeres distant from one another were combined, a new boundary was regenerated. Mapping of the migration pathways of neural crest cells showed that odd-numbered and even-numbered rhombomeres share properties with respect to the production of neural crest cells. In the hindbrain region the neural crest is segregated into streams. Neural crest cells migrating from rhombomeres 1 and 2, rhombomere 4 and rhombomere 6 respectively populate distinct cranial nerve ganglia and branchial arches. In contrast, rhombomeres 3 and 5 are free of neural crest cells.

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