This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: From the hairs on our arms to the microscopic sensory structures of the inner ear, many tissues in the body are built with a clear sense of direction. To build these structures throughout development, individual cells must establish which way they are facing—and coordinate that orientation with cells around them.
If they get this wrong, tissues may fail to form, form improperly or fail to function. How tissues establish this collective sense of direction from an initially unorganized state has long been a question in developmental biology. Now, researchers in the lab of Whitehead Institute member Pulin Li have recreated this process in engineered epithelial cells, which form sheets that cover the body's surfaces and line its organs.
The researchers used these cells to watch planar polarity—or the coordinated orientation of cells within such a sheet—emerge in real time. Published in the Proceedings of the National Academy of Sciences on Oct. 15, their findings show that collective cell migration provides a cue that establishes coordinated polarity across many cells. The study, led by first author Leah Wallach, a graduate student in the Li lab, in collaboration with fellow graduate student Connor Thomas, also identifies the junctional protein CELSR as a key driver of polarity initiation.
The work focuses on planar cell polarity (PCP), the asymmetric or directional organization of protein complexes within the plane of a tissue. PCP enables cells to coordinate their orientation across tissues and is essential for processes that shape the developing body and its organs. For example, sound travels across the sensitive hairs in our ears in a directional wave; if the hairs do not all grow facing the same direction, then the wave cannot flow, and the ear cannot properly register sound.
Disruption of PCP is associated with developmental disorders including spina bifida and craniorachischisis, as well as hearing loss and chronic lung disease. Scientists have identified many of the proteins involved in PCP. But one fundamental mystery remains: What initially tells cells which direction to face?
Gradients of signaling molecules such as WNT proteins have been proposed as guidance signals, whereas mechanical forces can help maintain or redirect polarity in already polarized tissues. But what initiates PCP in the first place has remained unclear. Studying that step in animals is especially difficult because polarity develops as tissues develop, grow, move and change shape, making it hard to catch the moment of initiation.
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