Hydra, a polyp that defies ageing
Hydra is a polyp that lives in freshwater ponds and lakes of temperate areas. This polyp is a member of the ancestral Cnidarian clade, which is the sister group of Bilaterian, where us, humans, belong. The life cycle of Hydra switch between sexual and asexual reproduction. Under normal condition, Hydra’s default mode of reproduction is asexual budding of newly developing small polyps from its lower body part. However, in order to overcome the harsh winter season, it can also undergo sexual reproduction by producing sperm cells and eggs. After fertilization, the blastula enters a larvae stage, which will finally grow into a polyp.
Credit: Schaible, R., Ringelhan, F., Kramer, B., & Scheuerlein, A. (n.d.). Hydra: Evolutionary and Biological Mechanisms for Non-Senescence. In (pp. 238-254)
Despite its simple body plan with only two cell layers (ectoderm and endoderm), Hydra exhibits an enormous tissue dynamic. Indeed, half of the cells that composed Hydra are adult stem cells. The three stem cell lineages permanently self-renew by proliferation in the central part of the body, and also permanently differentiate in the terminal head and foot areas. As a result, all cells of the polyp are replaced within three to four weeks. The interstitial stem cell system is among the best-studied stem cell lineages of invertebrates.
Hydra’s extensive stem cell activity and plasticity is the basis for some astonishing features: the polyps show an unparalleled capacity for regeneration. Small tissue fragment regenerate intact animals within days. Even dissociated cell suspensions self-organize and re-aggregate into completely normal polyps again within days. Their tissues and organs such as the nervous system are not distinguishable from normal specimens.
Furthermore, Hydra exhibit an unlimited life span and are regarded as virtually immortal under conditions of asexual growth. Obviously, they are able to suppress common cellular and molecular mechanisms of ageing. Therefore, Hydra is an excellent model for stem cell biology, regeneration and aging research.