In Cladonema pacificum, a jellyfish that can regrow its tentacles, aging destroys the cells responsible for regeneration: tentacles regrown by older individuals cannot capture or consume prey
In Cladonema pacificum, a jellyfish that can regrow its tentacles, aging destroys the cells responsible for regeneration: tentacles regrown by older individuals cannot capture or consume prey
On October 1, biologists at the University of Tokyo published data in Aging Cell on aging in Cladonema pacificum, a hydrozoan jellyfish capable of regrowing its tentacles. In jellyfish younger than 40 days, an amputated tentacle fully regrows within a week and catches prey again. In jellyfish older than 60 days, the new tentacle is almost never able to capture prey, and not a single one managed to swallow it. The reason lies in the wound zone: in older individuals, the number and activity of cells forming the blastema, the cluster of progenitor cells that builds new tissue, drops sharply.
Cladonema pacificum is a hydrozoan jellyfish belonging to the cnidarians, a phylum that includes jellyfish, hydras, and corals. In its adult stage it retains a strong capacity to regrow damaged tentacles, but it lives only a few months and ages noticeably over that span: the bell shrinks, the tentacles shorten, and at their base the number of cells carrying γH2AX, a protein marker of DNA damage, increases.
Jellyfish are known for their ability to circumvent aging: Turritopsis dohrnii collapses into a cyst under stress, and its cells undergo transdifferentiation, changing type and re-forming the polyp stage. Cladonema represents the opposite and more typical case: it does not reverse its life cycle and ages like most animals. What sets it apart is the strength of its tentacle regeneration, which also deteriorates with age.
At the cellular level, older jellyfish simultaneously lose stinging cells that sting and hold prey, neurons at the base of the tentacle, and the stem cells that replenish the tissue. The number of these stem cells and the rate at which they divide decline even without injury: baseline tissue renewal weakens on its own, independently of wound healing.
The biologists amputated tentacles and compared regrowth at different ages. In young individuals the wound closes within two days, and within a week the regrown tentacle catches and swallows prey as effectively as before: every young jellyfish captured and swallowed a brine shrimp (Artemia) with its regrown tentacle. In jellyfish older than 60 days, the wound in most cases does not fully close even after three days, and by the end of the week most regrown tentacles could not capture prey, and none could swallow it.
Within the blastema itself, the number of dividing cells and of cells bearing stemness markers, the proteins Piwi and Nanos1, drops sharply. In older jellyfish these candidate cells do not disappear, but a smaller fraction of them carries stemness markers. The authors describe this as exhaustion: the cells lose the ability to enter the active, dividing state that blastema formation requires. This collapse of the blastema is a symptom of a broader decline already underway in the tissues long before injury; the wound merely reveals it at full scale.
Hydra, a close relative of Cladonema, shows almost no aging in the laboratory as long as it reproduces by budding, which makes it unsuitable as a model for studying the limits of cnidarian regeneration. Cladonema, free-swimming and sexually reproducing, ages measurably while retaining tentacle regeneration, which is why it was chosen for this study. In Hydra oligactis, the transition to sexual reproduction accelerates death and tissue loss: hydra's "immortality" depends on asexual reproduction and vanishes under sexual reproduction.
The authors sum up their finding in one sentence:
regenerative ability alone is not enough to counteract aging.