Compliance with growth hormone treatment--are they getting it?

ABSTRACT Rhythmic spatial gene expression patterns termed the segmentation clock regulate vertebrate body axis segmentation during embryogenesis. The integrity of these patterns requires local synchronization between neighboring cells by Delta-Notch signaling and its inhibition results in defective segment boundaries. The oscillating tissue deforms substantially throughout development, but whether such tissue-scale morphogenesis complements local synchronization during pattern generation and segment formation is not understood. Here, we investigate pattern recovery in the zebrafish segmentation clock by washing out a Notch inhibitor, allowing resynchronization at different developmental stages, and analyzing the recovery of normal segments. Although from previous work no defects are expected after recovery, we find that washing out at early stages causes a distinctive intermingling of normal and defective segments, suggesting unexpectedly large fluctuations of synchrony before complete recovery. To investigate this recovery behavior, we develop a new model of the segmentation clock combining key ingredients motivated by prior experimental observations: coupling between neighboring oscillators, a frequency profile, a gradient of cell mixing, tissue length change, and cell advection pattern. This model captures the experimental observation of intermingled normal and defective segments through the formation of persistent phase vortices of the genetic oscillators. Experimentally observed recovery patterns at different developmental stages are predicted by temporal changes of tissue-level properties, such as tissue length and cell advection pattern in the model. These results suggest that segmental pattern recovery occurs at two scales: local pattern formation and transport of these patterns through tissue morphogenesis, highlighting a generic mechanism of pattern dynamics within developing tissues. SIGNIFICANCE Interacting genetic oscillators can generate a coherent rhythm and a tissue-level pattern from an initially desynchronized state. Using experiment and theory we study resynchronization and pattern recovery of the zebrafish segmentation clock, which makes the embryonic body segments. Experimental perturbation of intercellular signaling with an inhibitor results in intermingled normal and defective segments. According to theory, this behavior may be caused by persistent local vortices scattered in the tissue during pattern recovery. Full pattern recovery follows dynamic global properties, such as tissue length and advection pattern, in contrast to other genetic oscillators in a static tissue such as circadian clocks. Our work highlights how dynamics of tissue level properties may couple to biochemical pattern formation in tissues and developing embryos.