How hydras know where to regrow their heads

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Hydras, petite pond polyps known for their seemingly eternal youth, exemplify the art of bouncing back. The animals’ cellular scaffolding, or cytoskeleton, can regrow from a slice of tissue that’s just 5 percent of its full body size. Researchers thought that molecular signals told cells where and how to rebuild, but new evidence suggests there are other forces at play.

Physicist Anton Livshits and colleagues at the Technion-Israel Institute of Technology genetically engineered Hydra vulgaris specimens so stretchy protein fibers called actins, which form the cytoskeleton, lit up under a microscope. Then, they sliced and diced to look for mechanical patterns in the regeneration process.

Beheaded hydras appear to inherit skeletal patterns from their past adult forms, the researchers found. Actin fibers in pieces of hydra exert mechanical force that lines up new cells and guides the growth of the animal’s head and tentacles. Manipulating the alignment of actin fibers results in hydras with multiple heads. Both mechanical and molecular forces may mold hydras in regeneration, the team reports February 7 in Cell Reports.

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When researchers anchored rings of hydra tissue to a wire (right), they found that the added mechanical stability made a hydra grow normally along one body axis, and thus grow one head. Without this stability, the actin scaffolding was more disrupted and the animal grew two heads (left).

Video: A. Livshits et al/Cell Reports 2017