The hydra, from the jellyfish family, is less than half an inch long. Yet it has long intrigued scientists with its ability to regrow itself completely from decimated parts. Now, Prof. Kinneret Keren from the Faculty of Physics with colleague Prof. Erez Braun and students Anton Livshits, Lital Shani-Zerbib, and Yonit Maroudas-Sacks, have revealed the role of the cytoskeleton in this process.
Publishing in Cell Reports, the research team described how hydras have a structural memory that helps shape their new body plan according to a pattern inherited by the animal’s skeleton. Until now, it was believed that only chemical signals informed the hydra’s reformation.
Regenerating hydras use a network of tough, stringy protein fibers, called the cytoskeleton, to align their cells. When pieces are cut or torn from hydras, the cytoskeletal pattern survives and becomes part of the new animal. The pattern generates a small but potent amount of mechanical force that shows cells where to line up. This mechanical force can serve as a form of “memory” that stores information about the layout of animal bodies. “You have to think of it as part of the process of defining the pattern and not just an outcome”, says Keren.
When pieces of hydra begin the regeneration process, the scraps of hydra fold into little balls, and the cytoskeleton has to find a balance between maintaining the old shape and adapting to new conditions. “If you take a strip or a square fragment and turn it into a sphere, the fibers have to change or stretch a lot to do that,” explains Keren. However,
some portions retain their pattern. As the little hydra tissue ball stretches into a tube and grows a tentacle-ringed mouth, the new body parts follow the template set by the cytoskeleton in fragments from the original hydra.
The cytoskeleton is like a system of taut wires that helps the hydra keep its shape and function. The main cytoskeletal structure in adult hydra is an array of aligned fibers that span the entire organism.
Damage to this cytoskeleton will disrupt the formation of new hydras, the researchers found. In one experiment, the researchers cut the original hydra into rings which folded into balls that contained multiple domains of aligned fibers. Those ring-shaped pieces grew into two-headed hydras. However, anchoring the hydra rings to stiff wires resulted in healthy one-headed hydras, suggesting that mechanical feedbacks promote order in the developing animal.
Hydras are much simpler than most of their cousins in the animal kingdom, but the basic pattern of aligned cytoskeletal fibers is common in many organs, including human muscles, heart, and guts, says Keren. Studying hydra regeneration may lead to a better understanding of how mechanics integrate with biochemical signals to shape tissues and organs in other species. “The actomyosin cytoskeleton is the main force generator across the animal kingdom,” says Keren. “This is universal.”