Reproduction of the somitogenesis clock pattern

The pattern of oscillatory gene expression, as first described by (Palmeirim et al., 1997), can be readily reproduced with the Lunatic fringe secretion model: the posterior half of the PSM can oscillate in near-synchrony, with each wave of expression travelling anteriorly (see Figures 2 to 4, and Movie 1). It is not straightforward to model the shrinkage the wave of expression as it travels; rather, a small domain of roughly constant size travels anteriorly from the region of near-synchrony.

Note the difference in intensities of Lunatic fringe expression between the anterior and posterior parts of the PSM, which is compatible with experimental data, and does not require a specific mechanism as in the cell-autonomous case.

The way the system works seems to be the following: the coupling in the model seems to reduce the oscillation period, as L-fng provided by neighbours prompts earlier firing (secreted L-fng acts both cell-autonomously and on neighbouring cells, and it is thus possible for an isolated cell to show oscillating expression of clock genes). The stronger coupling in the posterior PSM has two effects. Firstly, its cells tend to fire earlier than those in the anterior PSM (with the right initial conditions). Secondly, cells in the posterior PSM fire more in synchrony than those in the anterior PSM. A wave originated in the posterior PSM travels to the anterior PSM, but more slowly, because of the reduced coupling. It is crucial that L-fng is involved in a positive feedback circuit (with Notch signalling), so that a cell which is firing recruits its neighbours.

Figure 2: Phase 1 in the Lunatic fringe secretion model: a wave of expression initiates in the posterior half of the PSM. Abscissae correspond to cell number in the PSM (anterior is to the left, posterior to the right), and ordinates to L-fringe expression levels (in arbitrary units). At the anterior end, 12 somites have already formed, and their oscillation phase is frozen.

Figure 3: Phase 2 in the Lunatic fringe secretion model: the wave of expression now has a much more restricted extent and is travelling anteriorly. Abscissae correspond to cell number in the PSM (anterior is to the left, posterior to the right), and ordinates to L-fringe expression levels (in arbitrary units).

Figure 4: Phase 3 in the Lunatic fringe secretion model: the wave of expression reaches the anterior border of the PSM. Abscissae correspond to cell number in the PSM (anterior is to the left, posterior to the right), and ordinates to L-fringe expression levels (in arbitrary units).

Movie 1: Simulation of the L-fng secretion model.

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