Developmental Biology Select

Embryogenesis requires the careful orchestration of complex cellular differentiation events that are precisely tuned by an elaborate array of signaling networks. These networks establish patterning of the embryo body plan and direct the eventual formation of specific body structures. This issue's Developmental Biology Select highlights recent studies that delve into the mechanisms underlying how cellular differentiation patterns are generated, maintained, or modulated to sculpt the developing embryo. Segmentation is a characteristic of most metazoan body plans. In vertebrates, this phenomenon is apparent in the formation of the vertebrae from precursors called somites. Intriguingly, the actual number of vertebrae differs dramatically between vertebrate species, ranging from a mere 10 or fewer in frogs to over 300 in snakes. But which factors ensure that a precise number of vertebrae are formed in a given species? By comparing somite formation in species with vastly different numbers of vertebrae (corn snake, zebrafish, chicken, and mouse), Gomez et al. (2008) now uncover evidence that a conserved ''clock and wavefront'' mechanism provides a way to determine somite number. In the clock and wavefront model, an oscillating molecular clock drives the periodic activation of Notch, Wnt, and fibroblast growth factor (FGF). This pulse of signaling regulates the rhythmic sequential budding of somites from the anterior region of the presomitic mesoderm at the front of a Wnt/FGF signaling gradient (the wavefront). Using the expression of the transcription factor MSGN1 as a marker of the Wnt/FGF gradient, the authors compared the area of MSGN1 expression to the size of the presomitic mesoderm in embryos of all four species throughout embryonic development. Consistent with the prediction from the clock and wavefront model that somite size corresponds to the distance traveled by the wavefront during one oscillation period, Gomez et al. observed that the anterior edge of the MSGN1 expression region receded by the size of one somite length during one period of somite formation. The authors then calculated the clock oscillation period by measuring the number of somites at different stages and compared this to the overall development rate of each species to obtain a segmentation rate. Surprisingly, Gomez et al. find that in corn snakes where the final vertebrae count is 315, this segmentation rate is four times faster than the rate in the other three species where the vertebrae number ranges from 31 to 65. This exciting study suggests that it may be the segmentation rate that governs …