Marine animals called sea squirts can regenerate their entire bodies from nothing but a tiny fragment of a blood vessel. Their secret is a special population of stem cells floating in their blood.
The finding could help us understand how such regenerative abilities evolved. In the long run, it could also help doctors regenerate damaged or lost tissues or even limbs in our own bodies.
Sea squirts are oval or cylindrical animals a few centimetres across. They belong to a larger group of animals called the chordates, most of which are backboned vertebrates, such as fish, birds and humans. That means sea squirts are our closest living non-backboned invertebrate relatives.
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In 2007, researchers found that one sea squirt, Botrylloides leachii, can regrow its body from a fragment of a blood vessel. 鈥淏ut no one had done any in-depth studies showing which cells give rise to the new body that is being made,鈥� says Susannah Kassmer at the University of California, Santa Barbara.
She and her colleagues studied the cells in separated fragments of blood vessels as they regenerated, and found a population of cells that expressed genes known to be involved in this process. Destroying the cells halted regeneration, while adding just one restarted it.
Kassmer calls the cells 鈥淧rimordial Blasts鈥�. They are stem cells, meaning they can grow into a variety of different tissues. It isn鈥檛 yet clear if they are pluripotent, meaning they can form any kind of cell, but Kassmer says it is likely that at least some of them are.
Many more primitive invertebrates, such as certain jellyfish and flatworms, can regenerate their bodies from fragments. But vertebrates cannot: at most, they can regrow a lost limb, as some salamanders do.
The most likely explanation, says Kassmer, is that more primitive animals used to have this ability and then it somehow got lost as vertebrates evolved. 鈥淭he biggest question in the field is to figure this out and understand how we might be able to start regeneration in a human when we鈥檙e injured.鈥�
In line with this, Kassmer found that the Primordial Blast cells expressed a gene called pou3. This is similar to a vertebrate gene called pou5 that is important in pluripotent stem cells. The team compared all the genes in the family to find out how they were related. 鈥淚t looked like pou5 from humans and pou3 in invertebrates might have evolved from the same ancestral gene,鈥� says Kassmer.
Reference: , DOI: 10.1101/647578
