Every seven days, a tiny marine organism called Botryllus schlosseri takes its brain apart and builds a new one. The process runs like a conveyor belt that never stops: the old nervous system vanishes and a fresh one grows in its place. Now scientists have peered inside that conveyor belt for the first time and seen which cells keep the renewal going.
The ascidian's brain, it turns out, is not a uniform mass but a structured territory. The precursor cells that give rise to neurons sit around it, not inside it. These cells divide actively only in the early stages of the seven-day cycle, then go quiet. The researchers isolated 470 individual cells and read their genetic program to figure out who was who.
But the biggest surprise came when the cells were tagged with a fluorescent dye and transplanted into another colony. The small precursor cells set off on a journey: they migrated into developing buds, into the brain of the next generation, and accumulated at an injured gland. So there is a navigation system that steers these cells to wherever they are needed.
In old colonies, nearly 19 years old, the proportions of cellular programs shift. This resembles the aging of blood stem cells in mammals, except here it happens in the brain. Ascidian neurons live no longer than a week, so age-related changes pile up not in them but in the precursor pool itself.
The work shows for the first time that the adult brain of a tunicate is not just a ganglion but an organized system with distinct niches for renewal. Every week the conveyor belt takes itself apart and puts itself back together, and now we know which parts it uses.
Every adult neuron of Botryllus schlosseri lives no longer than seven days. Over ten years, a colony goes through more than 500 complete cycles of brain regeneration, and not a single neuron survives into the following week.
The idea of cyclically replacing organs echoes Frank Herbert's Dune, where axolotl tanks grow new bodies, but in Botryllus it happens naturally and without outside intervention.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.