Development of segmentation in zebrafish.

Recent findings on the nature and origin of segmentation in zebrafish, Brachydanio rerio, are reviewed. Segmented peripheral tissues include the trunk and tail myotomes, that are derived from somitic mesoderm, and the pharyngeal arches that are derived from head mesoderm in addition to other sources. Two major regions of the central nervous system, the spinal cord and hindbrain, are also segmentally organized, as deduced from the distribution of identified neurones in both regions and by formation of neuromeres in the hindbrain that contain single sets of these neurones. Neural and mesodermal segments in the same body region can be related to one another by their patterns of motor innervation. This relationship is simple for the spinal/myotomal segments and complex for the hindbrain/pharyngeal arch segments. Development of the segments is also complex. Mesodermal and ectodermal progenitors have separate embryonic origins and indeterminate cell lineages, and the embryonic cells migrate extensively before reaching their definitive segmental positions. Results of heat-shock experiments suggest that development of myotomal and spinal segments are regulated coordinately in postgastrula embryos. Segmental patterning may be a relatively late feature of zebrafish embryonic development.