| Abstract: | Hydra are small freshwater animals capable of whole-body regeneration from excised tissue fragments, making them a valuable model for studying animal morphogenesis. Hydra tissues feature a spatially and temporally varying calcium ion concentration field that has been shown to play a significant role in mature animals and during regeneration. In addition, Hydra exhibit supracellular arrays of parallel muscle fibers. During regeneration, the nematic field describing muscle fiber orientation was shown to be slowly varying and aligned with the emerging body axis of the regenerating animal. To study how the calcium field evolves during regeneration and how these dynamics correlate with the emergence of the body plan, we use live imaging to follow the spatiotemporal dynamics of the calcium distribution superimposed on the muscle fiber organization. We find that the calcium distribution is graded along the future body axis with a bias toward the future head side. We also find that the calcium excitation-area distribution exhibits scale-free statistics together with system-spanning events. Both the head-directed calcium concentration gradient and the scale-free to system-size spatial statistics appear early in the regeneration process and remain stable throughout the entire process. Understanding how calcium dynamics are correlated with the structure of the developing muscle fiber array during Hydra regeneration can help gain insight into the development of other tissues that combine excitable calcium dynamics and a nematically organized muscle fiber array, such as the developing heart. |