How Flatworms Know Which End Is Up

When you cut a planarian flatworm in two, the front piece grows a new tail and the back piece grows a new head — every single time. But how does each piece know what it's missing? In this video, I break down a landmark 2009 PNAS paper by Petersen & Reddien that upended a classic assumption in regeneration biology: that body axis restoration is driven purely by self-organizing molecular gradients. Instead, the wound itself sends a signal — triggering rapid expression of a Wnt gene called wntP-1 — and that signal kicks off a cascade that determines regeneration polarity. Link to original article: https://www.pnas.org/doi/10.1073/pnas.0906823106 #brain #neuroscience #regeneration #academia #science #biology 📌 Disclaimer & Credits This video is an independent educational summary of a published scientific paper. All research, data, and findings belong to the original authors — Christian P. Petersen and Peter W. Reddien (Whitehead Institute for Biomedical Research / MIT). This video is not affiliated with, endorsed by, or produced in collaboration with the authors or their institutions. No copyright infringement is intended.