The membrane viscosity of a cell plays a crucial role in membrane transport and cellular functions. Until now, viscosity measurements have largely relied on lipid model membranes—artificial systems composed only of lipids—because of technical limitations. We developed a method for measuring membrane viscosity that can be applied to living cell membranes and measured the viscosity of the plasma membrane of the Caenorhabditis elegans early embryo. By applying a point force to the membrane, a vortex flow was induced on the plasma membrane. The membrane viscosity can be obtained by comparing the flow pattern with that of the hydrodynamic model.

The image on the cover of the October 7 issue of Biophysical Journal shows the vortex flow on the plasma membrane of the C. elegans early embryo. We attached fluorescent beads to the plasma membrane surface of embryos and induced membrane flows by applying point forces. The flows were recorded by using confocal time-lapse microscopy, and pseudocolor overlays of the sequential frames were used to visualize the vortex flows. The vortex flows observed in four embryos were arranged to form an Apollonian tapestry for the cover art.

We found that the membrane viscosity of a living cell is 10,000 times that of lipid-model membranes, particularly at the cellular scale. This increased viscosity may arise from complex structures unique to living cells, such as the cytoskeleton and membrane proteins that hinder membrane flow. On the basis of these findings, we proposed a new concept: in addition to the well-known short-range viscosity derived from molecular diffusion, living cells also exhibit a long-range viscosity that emerges from cell-scale (long-range) structures.

This long-range viscosity is deeply involved in cellular functions that require membrane flow across the cell, such as cell division and cell migration.

Beyond membrane biophysics, our findings are also relevant for broader fields such as cell biology, developmental biology, and biomedical sciences, because membrane flow is crucial for processes ranging from embryonic development to cancer cell invasion. This discovery represents a significant advance in the understanding of the physical properties of living cells. You can find more information on our work at：https://www.bio.phys.tohoku.ac.jp/en/index.html.

 —Yuka Sakuma, Kazunori Yamamoto, Saya Ichihara, Toshihiro Kawakatsu, Kenya Haga, Masayuki Imai, and Akatsuki Kimura