Why do Hydra end up with just a single head?

The ability of Hydra to regenerate or maintain a single head results from a subtle balance between activating and inhibiting activities.


Hydra is single polyps, generally 1–15mm in length, with a whorl of tentacles at the top of a column and an adherent base at the bottom. This freshwater organism can potentially regenerate any part of its body to rebuild an entire individual.

Any piece of the body with a few thousands of cells can regenerate the entire animal, which measures around one centimeter. The head organizer performs two opposite activities, one activating, which causes the head to differentiate, and the other inhibiting, which prevents the formation of supernumerary heads.

UNIGE scientists now have discovered the personality of the inhibitor, a protein called Sp5, and deciphered the dialogue between these two antagonistic activities, which keeps up a single-headed adult body and compose a suitable regenerative response. It suggests that this component has been rationed all through evolution, both in Hydra and in people. Sp5 could thusly be an incredible contender to be tried as an inhibitor of human tumors in which the activator pathway is the motor of proliferation.

Brigitte Galliot, a professor at the Department of Genetics and Evolution of the UNIGE Faculty of Science said, “Regeneration of the head relies on the transformation of the stump into a tissue called the head organizing center, which has developmental properties and, like an architect, directs the construction of the future head.”

The head coordinator takes away two inverse activities, one enacting and the other inhibiting. The first induces the separation of stem cells into specialized head cells. The activator is a growth factor called Wnt3, whose activity permits the inception of a three-dimensional cell differentiation program that empowers the development of the head. Consequently, without Wnt3, the head recovery program can’t continue. The inhibitory movement, delivered under the control of the activator activity, keeps the arrangement of supernumerary heads.

Galliot said, “These two antagonistic activities establish a dialogue between them, but we knew neither the identity of the inhibitor nor the nature of this dialogue.”

Matthias Vogg, a researcher at the Department of Genetics and Evolution of the UNIGE Faculty of Science said, “Using the results of a study conducted by a German team on the planarian flatworm, we developed a gene screening strategy to identify this inhibitor. We started with 124 candidates who met specific criteria to single out a unique winner, who met all of them. It is a gene that codes for a protein called Sp5.”

Scientists then demonstrated that Sp5 binds to the regulatory region of the gene that codes for Wnt3, blocks its expression and thus the formation of the head.

The seven heads of the freshwater Hydra

How does the dialogue between the activator pathway and the inhibitor work?

Brigitte Galliot explained, “We have quantified the expression of the genes encoding Wnt3 and Sp5 in different parts of the body of intact or amputated Hydra, and discovered that a regulatory loop between the two activities is established according to the location and quantity of each gene expressed.”

“Thus, in intact animals, the growth factor Wnt3 will be mainly present at the tip of the head, while Sp5 will be primarily active in the surrounding area, to prevent the appearance of other heads.”

At the point when specialists hinder the expression of Sp5, Hydra polyps, intact or regenerating build up numerous heads, all superbly practical, their tentacles apprehending the food to lead it to the mouth.”

Matthias Vogg said, “We also replicated these results from Hydra polyps whose cells had been completely dissociated from each other, then reaggregated and left in culture: multi-headed Hydra re-formed completely in four to five days.”

The study is published in the journal Nature Communications.


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