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Why Are Cells the Size They Are? Gravity May Be a Factor
carmendrahl writes "Eukaryotic cells, which are defined by having a nucleus, rarely grow larger than 10 micrometers in diameter. Scientists know a few reasons why this is so. A new study suggests another reason — gravity. Studying egg cells from the frog Xenopus laevis, which reach as big as 1 mm across and are common research tools, Princeton researchers Marina Feric and Clifford Brangwynne noticed that the insides of the eggs' nuclei settled to the bottom when they disabled a mesh made from the cytoskeleton protein actin. They think the frog eggs evolved the mesh to counteract gravity, which according to their calculations becomes significant if cells get bigger than 10 micrometers in diameter."
If I'm reading this correctly, it seems very possible that any children born in space would grow up to look like jabba the hut, since the whole gravety issue would no longer be a problem.
And if it's not obvious (mainly for our SI-challenged readers), 10 micrometers equals 0.01 mm.
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This makes no sense. Actin is found in practically all eukaryotic cells, including those that are tiny. TFA makes no claim that "frog eggs evolved the [actin] mesh to counteract gravity".
I know micrometers can vary in size, but it seems like 10 micrometers should be around thirty inches long.
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I was under the impression that surface area played a significant role in this as well, which probably couldn't be easily discounted. The surface area of a sphere is 4 pi times the radius squared while the volume is 4/3 pi times the radius cubed. So, the greater the radius, the greater the ratio of volume to surface area. This usually doesn't scale well, as it means there is more mass to support and less means of getting the input and output needed to support it. I'm not saying that gravity doesn't contribute as well, but that's a fairly difficult barrier as well.
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Or it could be to minimise their surface area for a given volume?
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By osmosis. Imagine inflating a water balloon, if you pump a heavier fluid like water into it, even in the presence of gravity it will take on a spherical shape. Of course it will be *slightly* deformed, but that's all up to the ratio of weight:surface strength.
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The only cells that are spherical are floating. And not all floating cells are spherical (yeast, for example). The cells of most multicellular organisms take on a shape by adhering to each other to to an extracellular matrix, and they generate internal tension by pulling on the adhesions. When you disaggregate the tissue, the individual cells still try to maintain that tension, but with nothing to pull against tend to pull the cell into a little ball.
Does this mean there is also a "goldilocks size" to a planet for advanced life, even any life to evolve? Can it be at the correct distance from its star, have water, and still be essentially dead? Can a planet look like the Earth but because it is five times the size, be sterile?
E Proelio Veritas.
Scientists have been doing stem cell (mostly plant stem cells, but also some mammalian etc.) growth experiments on the ISS for some years (IIRC six flights so far). Results are interesting. Among other things, perhaps the two most interesting results have been as follows.
In microgravity, cell growth is not limited to 2D. For example, that $250,000 hamburger was made by growing hundreds or thousands of one-cell-thick strips on petri dishes. In space, that is no longer the case. So stem cells can be grown one or two orders of magnitude faster, limited only by the need to get nutrients delivered to each cell and wastes removed.
Some mammalian cells that are very difficult or so far impossible to grow down here on Earth have been shown to grow pretty well up there in microgravity, including some human tissue types.
While some form of life on Earth has encountered and adapted to almost every other environmental condition (temperature, light, pH, etc.), so far as we know no living systems have ever had to deal with microgravity. So when grown in space, the cells basically 'freak out', not knowing what to do, and apparently try turning all of their genes to see what works. This seems to make them more amenable to influence by the environment, such as by adjusting temperature outside the norm for the species. Zero Gravity Solutions, a biotech company, is preparing further experiments on the ISS to explore this and related questions. (disclosure: I have a small investment in ZeroGSI.)
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Couldn't a cell be neutrally buoyant in water negating the effects of gravity? I thought it was something like surface tension keeping these things small, it would also make sense that this mesh would counteract surface tension.
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