Figuring out how an odd, gutless worm regrows its head (or tail)

Enlarge / Some of the gutless worms (orange) cover a coral. (credit: Samuel Chow )
In the movies, regeneration is the stuff of superheroes like Deadpool, who regrew the lower half of his body through some seriously awkward transitional scenes. Here in reality, regeneration is run of the mill, with lizards and amphibians regrowing limbs and tails while various worms are able to regrow half their entire body. How they manage this has been the subject of extensive study, and we have a fair idea of some of the genes and processes involved. But it's fair to say we don't have a strong idea of how the whole process is coordinated and directed to form all of the needed tissues.
A step in that direction comes from a recent study that takes a strange angle on regeneration. To understand the process, the authors sequenced the genome of a worm that can regenerate into two full organisms after being cut in half. But the worm also happens to be part of a group that contains the closest living relatives of bilateral animals—those with a left and right side. As such, it could provide a fascinating perspective on our own evolution, but it's something the researchers choose to ignore in this paper.
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Most of the animals we're familiar with are bilaterals, which have a left and right side. That includes some creatures (like sea urchins) where the two sides aren't all that obvious. These bilateral animals also start out early in their development as three layers of cells: an outer layer that forms the skin and neural tissue; a central one that forms internal structures like muscles and bone; and an inner layer that goes on to form the lining of the gut.
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