Original version With this story appeared in Quanta Magazine.
Drink a glass of wine and you will notice that the liquid constantly flows down the saturated side of the glass. In 1855 James Thomson, brother of Lord Kelvin, explained In Philosophical magazine that these “tears” or “legs” of wine result from the difference in surface tension between alcohol and water. “This fact explains some very interesting conclusions,” wrote Thomson. He did not realize that the same effect, later called the Marangoni effect, could also affect the development of embryos.
In March, a group of biophysicists in France reported that the Marangoni effect is responsible for the key moment at which a homogeneous patch of cells elongates and develops a head-tail axis – which is the first defining characteristic of the organism it will become.
The discovery is part of a trend that defies the norms in biology. Typically, biologists attempt to characterize growth, development, and other biological processes as a result of chemical stimuli triggered by genetic instructions. However, this picture often seemed incomplete. Scientists now increasingly appreciate the role of mechanical forces in biology: forces that push and pull tissues in response to their material properties, controlling growth and development in ways that genes cannot.
Current imaging and measurement techniques have opened scientists’ eyes to these forces, flooding the field with data that encourage mechanical interpretations. “What has changed over the last decades is really the ability to watch what’s happening live and see the mechanics in terms of cell movement, cell rearrangement and tissue growth,” he said Pierre-François Lenne from the University of Aix Marseille, one of the researchers behind the recent study.
The shift toward mechanical explanations has revived interest in pregenetic models of biology. For example, in 1917 the Scottish biologist, mathematician and classics scholar D’Arcy Thompson published About growth and formwhich highlighted the similarities between shapes found among living organisms and shapes appearing in non-living matter. Thompson wrote this book as an antidote to what he believed was an excessive tendency to explain everything in terms of Darwinian natural selection. His thesis that physics also shapes us is coming back into fashion.
“The hypothesis is that physics and mechanics can help us understand biology at the tissue scale,” he said Alexandre Kablaphysicist and engineer at the University of Cambridge.
The task now is to understand the causal interplay in which genes and physics somehow work together to sculpt organisms.
Grow with the flow
Mechanical models of embryo and tissue growth are not up-to-date, but biologists have long lacked ways to test these ideas. Just seeing the embryos is hard; they are miniature and distracting, reflecting airy in all directions like frosted glass. However, up-to-date microscopy and image analysis techniques have opened a brighter window for development.
Lenne and his colleagues used some of the up-to-date techniques to observe cell movement inside mouse gastruloids: bundles of stem cells that, as they grow, mimic the early stages of embryonic growth.