Issue Date: September 23, 2013
Why Are Cells The Size They Are? Gravity May Be A Factor
Eukaryotic cells, which are defined by having a nucleus, rarely grow larger than 10 μm in diameter. Scientists know a few reasons why this is so. For example, cells with a larger volume have a harder time obtaining nutrients from their surroundings. A new study suggests another reason—gravity (Nat. Cell Biol. 2013, DOI: 10.1038/ncb2830).
Scientists had thought gravity’s effects at the cellular scale were negligible. Yet Princeton University researchers Marina Feric and Clifford P. Brangwynne observed gravity at work in egg cells from the African clawed frog Xenopus laevis. The frog eggs can reach up to 1 mm across, and they are commonly used in research. The duo made the surprising discovery in a seemingly unrelated experiment studying why organelles seem held in place within frog egg nuclei.
They injected microscale polymer beads into the eggs and tracked their motion. Small beads easily diffused throughout the nucleus, but larger ones were trapped, suggesting an elastic meshwork constrains things. They suspected the mesh contained the protein actin, known for its structural role in the cytoplasm but whose role in the nucleus is unclear. When the team treated the eggs with drugs that disrupt actin, the organelles in the nucleus not only diffused freely but settled to the bottom of the nucleus within a few minutes—clear evidence of the effect of gravity.
“We hadn’t thought about gravity at all,” Brangwynne says. “We were just doing our thing, but when we destabilized this actin network it became apparent gravity was at play.”
Calculations based on the motion of the microbeads and the organelles showed that gravity starts becoming important around the 10-μm scale.
The researchers put forth an intriguing possibility: “Cells are typically no greater than ~10 μm, because beyond this size gravity is increasingly disruptive and additional stabilization mechanisms become necessary,” according to their paper.
“This is a very important finding,” says František Baluška of the University of Bonn, in Germany, who has studied the cytoskeleton in plants and its role in responding to gravity. “Gravity is a very weak force, and it is still enigmatic how gravity is perceived by living organisms.”
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