We thank @chloe_jollivet, @nKiaru, @ORSPF, Clément Helsens, Luis G. Morelli and Koichiro Uriu for their great work, @GValentinEPFL and members of the Oates lab for their feedback and @ViventisMicro for their technical support. 15/15

2. The segmentation clock forms a densely connected network. We show here that, in addition to driving oscillations of dpErk activity (Simsek et al. 2023) and inhibiting the expression of ripply1/2 (Yabe et al. 2023), Hes/Her genes also drive oscillations of Tbx6. 14/15

1. The pacemaker (Hes/Her genes) is separated from the effector (Tbx6) that directly patterns the tissue. We speculate that this design allows the effector to be spatiotemporally modulated by external factors, while shielding the pacemaker from these external perturbations. 13/15

Together, our findings imply that Tbx6 acts as a genetic clutch, converting Her/Hes pacemaker waves into Mesp stripes and bring light to two design principles of the segmentation clock: 12/15

Cellular oscillations and waves of Tbx6 expression are abolished when the segmentation clock is disrupted (in her1;her7 double mutants), indicating that the Hes/Her genes drive oscillations of Tbx6 expression. 11/15

As Tbx6 is required for Mesp expression, these results strongly suggest that the last oscillation of Tbx6 instructs cells to take a Mesp-positive cell fate. 10/15

At the tissue-level, waves of Tbx6 expression arrest at positions prefiguring the Mesp striped pattern. Notably, the first wave of Tbx6 arrests at the cellular position of the first segment boundary, thereby demarcating the embryonic transition from head to trunk mesoderm. 9/15

In contrast to Her1 oscillations, Tbx6 oscillations provide a clear signature distinguishing Mesp-positive and Mesp-negative cells: Mesp-positive cells (orange) display one more Tbx6 oscillations (arrows) than Mesp-negative cells (blue). 8/15

To our surprise, we observed tissue-level waves of Tbx6 protein expression travelling in the pre-somitic mesoderm, as shown on this kymograph. Cell tracking revealed that these waves are underlain by cellular oscillations in Tbx6 expression (see below). 7/15

We hypothesized that this function might be carried by Tbx6, an essential gene for somitogenesis in both mice and zebrafish. @chloe_jollivet thus developed a novel transgenic for Tbx6 (yellow), that we naturally imaged on our Viventis microscope. (Mesp is still in magenta) 6/15

We back-tracked Mesp-positive and Mesp-negative cells forming a somite boundary and compared their Her1 signals. Strikingly, Her1 oscillations do not distinguish between Mesp-positive and Mesp-negative cells forming a given somite boundary. 5/15

Bonus point: We took this opportunity to characterize the onset of the zebrafish segmentation clock. Here is a kymograph and a schematic showing the first waves of Her1-YFP expression and their location within the embryo from mid-epiboly to early-somitogenesis. 4/15

To tackle that question, we imaged zebrafish embryos, simultaneously carrying Mesp (magenta) and Her1 (yellow) transgenes, at single-cell resolution with our @ViventisMicro light-sheet microscope. 3/15

The segmentation clock manifests as waves of Hes/Her gene expression travelling anteriorly in the presomitic mesoderm. How these kinematic waves are converted into the striped Mesp gene expression pattern that prefigures morphological somite boundaries remains elusive. 2/15

How does the segmentation clock instruct cells to form a somite boundary? Read our new pre-print “Clock driven waves of Tbx6 expression prefigure somite boundaries”, by @Olivier_Venzin and colleagues, to find out. https://t.co/23LwXR3aUT Summary thread below: 1/15

But, what are the cells doing? Our step by step guide to following single cells in vivo is now out (in collaboration with @nKiaru) 🔬

Our collaborative work with @mssakar on a robotic microsurgery platform to investigate the mechanics of zebrafish embryo elongation.

Excited to build on the footsteps of hugely #interdisciplinary mentors @StephanGrill @mpicbg @oateslab @epflsv as I make the transition to @TIFRScience as a group leader in September. 1/n https://t.co/PTeHNL7GHE https://t.co/ksiPrg6zIR

Many thanks to the amazing colleagues @OatesLab, @LeBIOP for phenomenal support & the incredible fish facility @EPFL_en Special thanks to @EbisuyaMiki @XavierTrepat for the news and views piece on the article https://t.co/0rDSinBfuX

The proposed adjustment mechanism does not depend on whether the somite interior is a fluid, or a yield stress material as long as the somite is in a uniformly flowing state