Every seven days, the tiny sea squirt Botryllus schlosseri takes its brain apart and builds a new one. It works like a conveyor belt that never stops. For the first time, scientists have looked at the individual cells on that conveyor belt and figured out who does what.
It turns out that progenitor cells live around the sea squirt's brain. Think of spare parts in a warehouse: when they are needed, they turn into nerve cells. These cells divide actively only at the start of the weekly cycle. The researchers isolated 470 cells and read their genetic program to work out what types exist.
The scientists also tagged cells with a glowing dye and transplanted them into another colony. The progenitor cells found their own way: they swam to developing buds, to the new brain and to the injured spot. So they carry an inner compass that tells them where to go.
In old colonies that are nearly 19 years old, the balance of cells shifts. Sea squirt neurons live no longer than a week, so aging accumulates not in them but in the progenitor cells themselves. This resembles how blood stem cells age in mammals.
Now scientists know which cells control the weekly rebuilding of the brain. The next step is to understand how these cells find their way, and whether this mechanism could be used to help the mammalian brain recover from injury.
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.