To regenerate a head, you first have to know where your tail is
Ars Technica 2025-04-18
For those of us whose memory of high school biology hasn't faded entirely, planarians will probably sound very familiar. They're generally used as an example of one of the extreme ends of regenerative capacity. While some animals like mammals have a limited ability to regenerate lost tissues, planarians can be cut roughly in half and regenerate either an entire head or entire tail, depending on which part of the body you choose to keep track of.
In doing so, they have to re-establish something that is typically only needed early in an animal's development: a signaling system that helps tell cells where the animal's head and tail are. Now, a US-based team asked a question that I'd never have thought of: What happens if you cut the animal in half early in development, while the developmental head-to-tail signaling system is still active? The answer turned out to be surprisingly complex.
Heads or tails?
Planarians are small flatworms that would probably be living quiet lives somewhere if biologists hadn't discovered their ability to regenerate lots of adult tissues when damaged. The process has been well-studied by this point and involves the formation of a cluster of stem cells, called a blastema, at the site of damage. From there, many of the signals that control the formation of specialized tissues in the embryo get re-activated, directing the stem cells down the developmental pathways needed to reproduce any lost organs.