Stem cells grown on a soft substrate develop into pre-bone after water is removed.COURTESY MARCENE ROBINSON, UNIVERSITY OF BUFFALOSimply by altering the amount of water in mouse stem cells, researchers have changed the cells’ fates. Reporting in PNAS this week (September 25), the scientists found that removing water from mesenchymal stem cells—making them stiffer—aimed the cells toward becoming bone, while adding water and making the cells softer directed them to become fat.

“For the first time, we’re beginning to understand the importance of cell volume and cellular water content in the mechanical properties and physiological functions of cells,” MIT’s Ming Guo, the lead author of the study, says in a press release.

The volume of a cell can greatly influence its characteristics, the authors write in their report, including stiffness, protein transport, and chromatin density. In one set of experiments to see how cells manage volume, they grew...

Stiffness is known to affect the fate of stem cells, so the researchers undertook another set of experiments using mesenchymal stem cells to see what role water flow out of the cell plays in this relationship. On stiff substrates, which normally encourage differentiation to bone, cells could be redirected toward becoming adipocytes by reducing the osmotic pressure, thereby swelling the cells with water. Similarly, on a typically fat-promoting soft substrate, stem cells exposed to greater osmotic pressure were more likely to favor becoming bone.

“The surprising thing about these experiments is the observation that volume seems to be related to so much about the cell,” coauthor David Weitz of Harvard University says in the release. “It seems to dictate the cell stiffness as well as the cell fate.”

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