surgical-size-reduction-zebrafish-for-study-embryonic-pattern

Surgical Size Reduction of Zebrafish for the Study of Embryonic Pattern Scaling

In the developmental process, embryos exhibit a remarkable ability to match their body pattern to their body size; their body proportion is maintained ...

Harvard Medical School

A small number of signaling pathways are used ubiquitously throughout development. How can enough information be carried through such a small number of channels? Reporting in Cell, Nandagopal et al. (2018) reveal how different Notch ligands can regulate different targets through the same receptor using differences in signaling dynamics.

Dynamic Encoding in the Notch Pathway.

Balancing the rate of differentiation and proliferation in developing tissues is essential to produce organs of robust size and composition. Although many molecular regulators have been established, how these connect to physical and geometrical aspects of tissue architecture is poorly understood. Here, using high-resolution timelapse imaging, we find that changes to cell geometry associated with dense tissue packing play a significant role in regulating differentiation rate in the zebrafish neural tube. Specifically, progenitors that are displaced away from the apical surface due to crowding, tend to differentiate in a Notch-dependent manner. Using simulations we show that interplay between progenitor density, cell shape and changes in differentiation rate could naturally result in negative-feedback control on progenitor cell number. Given these results, we suggest a model whereby differentiation rate is regulated by density dependent effects on cell geometry to: (1) correct variability in cell number; and (2) balance the rates of proliferation and differentiation over development to 'fill' the available space.

Feedback between tissue packing and neurogenesis in the zebrafish neural tube.

Little is known about how the sizes of animal tissues are controlled. A prominent example is somite size, which varies widely both within an individual and across species. Despite intense study of the segmentation clock governing the timing of somite generation, how it relates to somite size is poorly understood. Here, we examine somite scaling and find that somite size at specification scales with the length of the presomitic mesoderm (PSM) despite considerable variation in PSM length across developmental stages and in surgically size-reduced embryos. Measurement of clock period, axis elongation speed and clock gene expression patterns demonstrate that existing models fail to explain scaling. We posit a 'clock and scaled gradient' model, in which somite boundaries are set by a dynamically scaling signaling gradient across the PSM. Our model not only explains existing data, but also makes a unique prediction that we confirm experimentally - the formation of periodic 'echoes' in somite size following perturbation of the size of one somite. Our findings demonstrate that gradient scaling plays a central role in both progression and size control of somitogenesis.

Article	Year
Dynamic Encoding in the Notch Pathway. Developmental cell\| PubMed ID: 29486192	2018
Feedback between tissue packing and neurogenesis in the zebrafish neural tube. Development (Cambridge, England)\| PubMed ID: 29678815	2018
Size-reduced embryos reveal a gradient scaling-based mechanism for zebrafish somite formation. Development (Cambridge, England)\| PubMed ID: 29769221	2018

Sean G. Megason

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