Tautologically, research articles with eye-catching titles tend to catch my eye. “How Deeply Cells Feel…”
is one such report that I’m glad did grab my attention. The report is the latest in a serious of studies concerned with cell adhesion to surfaces. Being chemists, we certainly aren’t surprised that cells respond to chemicals in their local environment, but I was unaware that the physical environment matters too. Cells actively sense the stiffness of the substrate that they are attached to by pulling on it. If the substrate is soft enough, the cells can even form wrinkles in the surface. This particular report was concerned with layered substrates and how well cells can detect the stiffness of the underlying substrate when it is covered with a thin, softer layer.
Having a rheological bent to my thoughts, I was struck by this question which does not seem to have been raised:
When the cell pulls on the substrate, is the pulling continuous or, as occurs in dynamic mechanical analysis, is it modulated in some manner?
Certainly the latter approach would be able to help the cell better understand the substrate, and given that large molecules such as proteins naturally oscillate, it is not inconceivable that the adherents of the cell would too. This can be important as in all the cases that I’ve seen, the modulus of the substrate is taken as a constant, an assumption that clearly needs to be questioned when the substrate is a softer viscoelastic gel.
So are cells the worlds smallest rheologists?
Stiffness is a relative concept depending on the timescale of the observer.
The relaxation time of the whole cell is much slower than the relaxation time of the biochemical reactions inside it. What's stiff for a reaction is probably rather soft for the cell as a whole.
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