Estimation of Clock Rate ω(x) and Period T(x) as a Function of Distance x along the PSM (A) Flat mount of a seven-somite embryo stained by ISH for her1 (purple, NBT/BCIP detection chemistry) to show the oscillation pattern and for myoD (red, Fast Red detection chemistry) as a marker of formed somites. Spatial wavelength as a function of position, S(x), is measured as the interval between one her1 peak and the next, or one trough and the next, where x is the midpoint of that interval. S0 is the width of one formed somite. Position x = 0 corresponds to the tail end of the notochord, and position x = L corresponds to the anterior end of the PSM. (B and C) Clock frequency ω(x) and period T(x) as a function of distance x along the PSM. Each data point corresponds to one measurement of spatial wavelength S(x). The graph represents data from ten different, randomly chosen specimens of the type shown in (A) fixed at the 7–15-somite stages. The line (a hyperbola) is a least-squares fit to the data. To combine data from different specimens, distances are scaled relative to the length of the PSM. Clock rate and period are calculated for each data point according to the Equation 2 shown in the text. We use a simple linear approximation for u(x), the speed of forward movement of the cells along the anteroposterior axis relative to the tail end of the notochord: by definition, at x = 0, u(x) = 0, and at x/L = 1, u(x) = 1 (in somite lengths per somite cycle), so we assume u(x) = x/L at intermediate values of x.
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