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The vertebrate segmentation clock is a gene expression oscillator controlling rhythmic segmentation of the vertebral column during embryonic development. The period of oscillations becomes longer as cells are displaced along the posterior to anterior axis, which results in traveling waves of clock gene expression sweeping in the unsegmented tissue. Although various hypotheses necessitating the inclusion of additional regulatory genes into the core clock network at different spatial locations have been proposed, the mechanism underlying traveling waves has remained elusive. Here, we combined molecular-level computational modeling and quantitative experimentation to solve this puzzle.

Our model predicts the existence of an increasing gradient of gene expression time delays along the posterior to anterior direction to recapitulate spatiotemporal profiles of the traveling segmentation clock waves in different genetic backgrounds in zebrafish. We validated this prediction by measuring an increased time delay of oscillatory Her1 protein production along the unsegmented tissue. Our results refuted the need for spatial expansion of the core feedback loop to explain the occurrence of traveling waves. Razor vst crack download. Spatial regulation of gene expression time delays is a novel way of creating dynamic patterns; this is the first report demonstrating such a control mechanism in any tissue and future investigations will explore the presence of analogous examples in other biological systems.

INTRODUCTION Vertebral segments differentiate from their embryonic rudiments called somite segments. Segmentation of somites, namely somitogenesis, occurs at a species-specific pace during embryonic development. Somitogenesis is controlled by a gene expression oscillator called the vertebrate segmentation clock that ‘ticks’ in cells located in the unsegmented presomitic mesoderm (PSM). The period of the segmentation clock in cells located at the posterior end of the PSM is equivalent to the period of somite formation. The period of oscillation increases as cells are passively displaced closer to the anterior end of the PSM. The oscillations finally halt as cells bud off as a somite segment at the anterior end of the tissue.

Cells exiting the PSM are arrested in different states of the oscillation cycle, defining which portion of a somite they will form and which set of genes they will express during differentiation (). The spatiotemporal dynamics of oscillations create a phase difference among cells along the PSM.

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Anterior PSM cells are retarded in the oscillator phase relative to posterior PSM cells; the further anterior the cells are located, the greater the retardation is. This phase difference results in the formation of traveling wave-like gene expression patterns along the PSM (A), which can be visualized by in situ hybridization for oscillating mRNAs or real-time imaging of oscillating proteins. The different phases of the oscillator cycle are mapped out in space as stripes of expression of the oscillating genes. The number of expression stripes reflects the number of oscillation cycles by which the anterior cells are lagging behind posterior cells (; ).

The spatial period profile of the segmentation clock was measured in zebrafish by utilizing fluorescent in situ hybridization data (). We have further demonstrated that the period profile is conserved between zebrafish and corn snake (). The traveling segmentation clock waves.