What effects can a force as strong as gravity have on the behavior of something as small as individual cells? The paper “Hypergravity affects cell traction forces of fibroblasts,” published in the March 2 issue of Biophysical Journal, tested how fibroblast cells—cells that form connective tissue—can physically change, adapting to forces as great as almost 20 times the force of Earth’s gravity.

The study found that under forces up to 5.4 g,  cells lose some of their ability to maintain their shape, stay adhered to a surface, or exert force on their environment. Yet, when those forces are beyond 5.4 g, the cells seemed to increase the force they exerted instead.

Eckert et. al. performed their experiments using a large-diameter centrifuge at the European Space Agency’s European Space Research and Technology Centre (Figure 1A). The centrifuge’s axis-to-sample distance of several meters long made it capable of generating the high levels of gravitational forces required to test its effects on cells.

To perform the experiment, the authors cultured the cells on top of surfaces made of micro-sized upright pillars (Figure 1C). Once the cells were spinning, any forces they exerted would then “deflect” the pillars under them by nudging them from their upright positions. The study involved spinning the cells for 16 hours at a time before counting the deflected pillars to measure the forces exerted by the cells. Comparing the results after applying 5.4 g, 10 g, and 19.5 g revealed some interesting cellular behavior in response to different levels of hypergravity (Figure 3).

At 5.4 g, the cells exerted less force on the pillars. Under these conditions, the cells could be impacted by the shear strain of the gravitational forces, developing a less densely cross-linked network of actin in response. The actin network would normally provide support, shape, and movement for a cell. However, at 10 g and 19.5 g, the cells regained and even slightly increased their forces on the pillars. The results suggest that mechanical stress from higher gravitational forces could be activating a reorganization of the cellular cytoskeleton to form a more densely cross-linked actin network. A denser actin network would help maintain stiffness as well as the integrity of a cell’s shape and function. The authors suggest focusing future studies on further understanding this process of actin formation under hypergravitational forces. The authors also propose that these observations could be useful for considering the “potential health risks for fighter pilots and in planned long-haul space flights.”