Cells in retina found to behave like soap bubbles

Soap bubbles delight children and the young at heart, but they also have been objects of scientific study for centuries. Operating under the laws of physics, bubbles always try to minimize their surface area, even when many bubbles are aggregated together.

Now two Northwestern University scientists have demonstrated that the tendency to minimize surface area is not limited to soap bubbles but extends to living things as well. In a paper published Oct. 7 in the journal Nature, they show that cells within the retina take on shapes and pack together like soap bubbles, ultimately forming a pattern that is repeated again and again across the eye. Gaining insight into these patterns can help researchers understand the interplay between genetics and physics in cell formation.

“The cells we studied, those found in the retina of the fruit fly, adopt mathematically predictable shapes and configurations,” said Richard W. Carthew, professor of biochemistry, molecular biology and cell biology and a co-author on the paper. “Like bubbles, life has co-opted a physical tendency for surfaces to be minimized and has harnessed it to design intricate cellular patterns within complex structures such as the eye.”

Similar to the colored dots in a Georges Seurat painting, though on a three-dimensional scale, the cell is the indivisible unit that gives shape to something larger and recognizable — a butterfly, a maple tree, a human being. How is this amazing diversity of species created?

“It is like designing the pieces of a jigsaw puzzle so that they fit together seamlessly,” said Carthew. “Understanding how cells fit together in space is an underappreciated area of science that has started to gain serious momentum in the last decade. Cells are different shapes and pack together in different ways depending on where they are located in a living thing and what their function is.”

In investigating the physical basis of biological patterning in the retina, Carthew and co-author Takashi Hayashi, a post-doctoral fellow at Northwestern, looked at normal retinal cells where four cells group together to form an aperture that is circular in shape. They found that they did so in exactly the same pattern as a group of four soap bubbles. Then, they varied the number of cells in each aperture and looked at how the cells fit together. Again, the cell configurations correlated perfectly to those of bubbles of the same number. When an aperture had one to five cells each resulted in one configuration. If an aperture had six cells, three different configurations were possible, but always the same three.

“By looking at one exquisitely structured tissue in one species, we discovered how the cells order themselves,” said Carthew, who with Hayashi has been studying the form and function of the retina for years. “This experiment illustrates the importance of mathematics and physics in biology and points to a general principle of patterning found in a wide range of living things.”