Abstract
Segmentation is the partitioning of the body axis into a series of repeating units or segments. This widespread body plan is found in annelids, arthropods, and chordates, showing it to be a successful developmental strategy for growing and generating diverse morphology and anatomy. Segmentation has been extensively studied over the years. Forty years ago, Cooke and Zeeman published the Clock and Wavefront model, creating a theoretical framework of how developing cells could acquire and keep temporal and spatial information in order to generate a segmented pattern. Twenty years later, in 1997, Palmeirim and co-workers found the first clock gene whose oscillatory expression pattern fitted within Cooke and Zeeman’s model. Currently, in 2017, new experimental techniques, such as new ex vivo experimental models, real-time imaging of gene expression, live single cell tracking, and simplified transgenics approaches, are revealing some of the fine details of the molecular processes underlying the inner workings of the segmentation mechanisms, bringing new insights into this fundamental process. Here we review and discuss new emerging views that further our understanding of the vertebrate segmentation clock, with a particular emphasis on recent publications that challenge and/or complement the currently accepted Clock and Wavefront model.
Highlights
Development is the process by which multicellular organisms start out as just one cell, multiply into millions of others that biochemically communicate with each other, rearrange their positioning, and even die in a programmed way
Embryo segmentation represents a interesting phase of development, where a collection of physical, chemical, and biochemical events are successfully orchestrated in space and time in order to develop a fully grown organism
This process occurs in the embryo by subdivision of the presomitic mesoderm (PSM) into metameric structures termed somites, other segmented systems exist in vertebrates
Summary
Development is the process by which multicellular organisms start out as just one cell, multiply into millions of others that biochemically communicate with each other, rearrange their positioning, and even die in a programmed way. This provides a possible explanation for how the embryo maintains stable segment proportions despite overall changes in size, e.g. due to normal embryo growth or as observed when, after experimental reduction of embryo size, the total number of segments remains unaltered and the segments become proportionally smaller[44,45] Using their newly developed ex vivo experimental setting composed by a quasi-monolayer of mouse primary PSM cells (mPSM) combined with real-time gene expression imaging, the authors observed that (i) the period of oscillation in the central mPSM remains constant regardless of PSM length, (ii) the velocities of kinematic waves change linearly with overall mPSM length (larger samples display proportionally faster kinematic waves, indicating that oscillatory activity adapts to match the spatial context in which it occurs), and (iii) the phase-gradient slope is predictive of segment size. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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