How a scrap of hydra regrows a whole animal
The hydra is a favorite subject of middle school science. It is a fearsomelooking tentacled predator, but it is tiny — less than a half inch — and lives in ponds. You can collect or buy hydras and the tiny crustaceans they eat, then watch the capture under a dissecting microscope. Hydras, like jellyfish, have stinging cells in their tentacles.
They usually reproduce by budding. If you cut them up in pieces, the odds are good that a piece will become a new hydra, sometimes a hydra with two heads. But if you want to get children interested in science, that is hardly a drawback.
Adult, professional scientists are interested in them, too, particularly the way a hydra snippet grows up. For example, one of the great discoveries of modern science is the way that genes control how a few cells develop into a highly complex organism. Fruit flies, zebrafish and humans share some of the same genes that direct this process.
Hydras are much simpler, made of just a few layers of cells. But they still respond to chemical signals sent out by genes as they grow into a tubelike body and tentacle-encircled maw.
However, scientists in Israel, who cut up a lot of hydras in the process of their experiments, found that there are structures in even a small bit of hydra that also guide growth.
A hydra body has a kind of scaffolding made of protein fibers that act like muscles and help the organism keep its shape. The way these actin fibers are arranged helps determine how the hydra grows, even in a hydra scrap.
Kinneret Keren, a physicist at the TechnionIsrael Institute of Technology who wrote about the research with her colleagues in the journal Cell Reports, said this is not simply a matter of growing along the length of the fibers, because the first thing a bit of hydra does in regenerating a full animal is fold into a sphere. Somehow — and this is a subject for future research — the aligned fibers tell the growing ball of cells which direction to grow.
Some scraps grow better than others. A ring cut horizontally through the body of the hydra often ends up with a confused alignment of the fibers when it folds into a ball. The result may be a hydra with two heads.
Not an insurmountable problem for a hydra. “They’re fine,” Keren said. She plans to continue work on the hydra. The little animal is a good target for research, she said.
“They’re complicated enough, but simple enough.”