cell biology
The average animal cell is 10 µm across - why? Why aren’t they bigger?
Gravity explains why cells are microscopic
T
Giant Xenopus egg cells manage to support
actin mesh that contained them. They injected a
thousands of membrane-less compartments
tiny magnetic bead into the nucleus and turned on
inside the nucleus. Most cellular compartments
a magnet to add a known force. They found that
are surrounded by membranes, but some aren’t.
the actin mesh in the nucleus is softer than jelly
he traditional explanation has been
They’re like beer without a bottle or sheep without
but, like jelly, with some prodding it returns to
that if cells were any bigger it would be difficult to
a fence. There’s nothing to really support them or
its original shape. Actin continues to hold up the
get enough nutrients and energy to support them.
keep them separate. They don’t seem to be affected
nuclear bodies against gravity like pieces of fruit in
However, in a paper in Nature Cell Biology late last
by gravity either; otherwise, they would eventually
a jelly mould but, under rising force, the nuclear
year, Princeton bioengineers Marina Feric and Cliff
all pool together at the bottom of the nucleus. Yet
jelly undergoes sheer thickening, a non-Newtonian
Brangwynne described their probing of cellular inner
they don’t.
property where a liquid becomes more viscous, a
space, the cell nucleus and their discovery that gravity could limit cell size.
One type of these membrane-less compartments
property that probably protects the nucleus. At a
are the nuclear bodies. They are liquid-like drops,
high enough force though, the actin mesh breaks
The researchers studied the mechanics of the
made up of RNA and proteins. Inside the nucleus,
and can no longer hold up the nuclear bodies.
cell nucleus using eggs from the African clawed frog,
they act like droplets of vinegar in oil, that is,
This suggests that actin’s mechanical properties
Xenopus laevis. (Bio factoid: scientists who work in
whenever they get close together, they fuse and sink.
are finely tuned to resist the force of gravity but also
Xenopus call each other ‘froggers’.)
And yet unlike the vinegar in salad dressing, all the
allow flexibility and rigidity of the cell nucleus to
As cells go, Xenopus eggs are enormous. The
little nuclear bodies in the nucleus don’t clump into
support life.
Xenopus egg nucleus alone is bigger than most of
one big pool at the bottom. Feric and Brangwynne
American Society for Cell Biology
the cells found in humans. If the average human cell
previously found that an actin mesh was preventing
www.ascb.org
is the size of an average American house, frog eggs
the nuclear bodies from clumping by keeping them
are the size of the Empire State Building! Feric and
small. But how strong was the mesh?
Brangwynne want to know more about the mechanics of these skyscraper-sized cells.
40 | LAB+LIFE SCIENTIST - March 2015
Feric and Brangwynne wanted to measure gravity’s pull on nuclear bodies and against the
Image: Xenopus laevis 02 by Brian Gratwicke - Flickr: Xenopus laevis. Licensed under CC BY 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/ File:Xenopus_laevis_02.jpg#mediaviewer/File:Xenopus_ laevis_02.jpg
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