Somite development in zebrafish

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• The highly controlled, reiterative nature of somite formation in zebrafish is also characteristic of somite formation in other vertebrate species, and a number of models have been proposed to account for the precise regulation of this process.

• The zebrafish has long been recognized as an ideal organism for cellular and histological studies of somite patterning.

• genetic screens for zebrafish mutants defective in somitogenesis have identified over 50 genes that are necessary for normal somite development.

• Although there is underlying segmentation in the trunk of all vertebrates, the segmented nature of fish remains among the most obvious in the adult animal. Even a cursory examination of the filets in a seafood store reveals that the fish trunk is composed of reiterated blocks of muscle, cartilage, and bone. These serially homologous mesodermal segments are derived from somites that form during embryogenesis

• This molecular clock would translate a smooth maturational or positional gradient present in the psm into a spatially periodic pattern, allowing somitogenesis to occur at regular intervals in successive, uniformly sized blocks of cells

• notochord precursors begin to express signaling molecules such as Sonic hedgehog

• notochord and prechordal plate, and they exert profound patterning influences on surrounding tissues