Distinguishing left from right seems like something we figure out long after we've mastered other, more important tasks — such as talking and walking. But our cells establish the difference early in development. The left–right axis of most vertebrates is set up with the help of flows created by cilia, tiny hair-like structures that coat the inside of our cells. Debate is ongoing as to whether the sensing mechanism cells use to detect flow asymmetry is chemical or mechanical in nature. And now, Rita Ferreira and colleagues have shown that flow detection can't occur via mechanosensing — at least not in the way we'd previously thought.
One hypothesis holds that the flow creates a chemical gradient, which in turn establishes the asymmetry. Another model implicates a type of sensory cilium that is able, in principle, to detect the flow mechanically. But the calculation performed by Ferreira et al. — including parameters from in vivo experiments on zebrafish — suggests that there simply aren't enough of these cilia to elicit such a response. This doesn't mean that mechanosensing is not at play, but it certainly widens the search for the culprit.
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Klopper, A. Not hairy enough. Nature Phys 13, 824 (2017). https://doi.org/10.1038/nphys4262