Spatiotemporal Dynamics in Zebrafish Skin Development

Abstract

Understanding the molecular mechanisms controlling tissue growth and patterning remains a fundamental open question in developmental biology. To investigate the dynamic biochemical signals regulating tissue growth, we have developed a system for live imaging zebrafish scales during development. Zebrafish scales are bony skin appendages which are positioned on the exterior of the fish and have a flat, circular shape that makes them conducive for live imaging. Scales develop sequentially in a spreading wave along the anterior-posterior and dorsal-ventral axes until the entire fish is covered in a hexagonal array. While some of the molecular pathways regulating this process have been identified, exactly how this wave of scale development is regulated remains unsolved. We have found that scale development requires communication between cells and thus likely proceeds through an active wave mechanism of positive feedback loops. Introducing a cut in the skin to interrupt cell-cell communication caused a delay in scale development until the cut healed. This indicates that new scales only form in response to a signal from neighboring cells. These cuts also block NF-κB activity, which acts downstream of Ectodysplasin A signaling to activate pathways required for scale development, including Fgf20a. A wave of NF-κB activity precedes scale initiation and is required for proper scale development. Experiments decoupling the propagation of the wave from dermal placode formation and osteoblast differentiation demonstrate that the Eda/NF-kB activity wavefront times the sequential patterning of scales. Moreover, this decoupling resulted in defects in scale size and significant deviations in the hexagonal patterning of scales. Thus, our results demonstrate that a biochemical traveling wave coordinates scale initiation and proper hexagonal patterning across the fish body.