Animal skin patterns, such as the stripes of zebras and the colorful spots of poison dart frogs, serve a variety of biological functions, including temperature regulation, camouflage, and warning signals. The colors that make up these patterns must be distinct and well separated to be effective. For example, as a warning sign, different colors make them very visible to other animals. Just like camouflage, well-separated colors allow animals to better integrate into their environment.
In our work recently published in Science Advances, my student Ben Alessio and I proposed a possible mechanism to explain the formation of these characteristic patterns, which could be applied to medical diagnostics and synthetic materials.
A thought experiment can help visualize the task of obtaining distinct color patterns. Imagine carefully adding a drop of blue and red food coloring to a cup of water. The droplets will spread slowly through the water as a result of the diffusion process, where molecules move from an area of higher concentration to an area of lower concentration. Over time, the water will contain equal concentrations of blue and red pigments and will turn purple. Thus, diffusion tends to create color uniformity.
Naturally, the question arises: how can distinctive color patterns form during diffusion?
Movement and borders
Mathematician Alan Turing first addressed this question in his first 1952 paper, The Chemical Basis of Morphology. Turing showed that under the right conditions, the chemical reactions involved in producing color can interact with each other in ways that prevent diffusion. This allows the colors to self-organize and create interconnected regions of different colors, now forming Turing patterns.
However, in mathematical models, the boundaries between colored regions become blurred due to diffusion. This is different from nature, where edges are usually sharp and colors are well separated.
Our team believes that the key to how animals create distinctive color patterns can be found in laboratory experiments with micrometer-sized molecules, such as the cells involved in producing animal skin colors. My work and that of other laboratories has revealed that micron-sized particles form banded structures when placed between an area of high concentration of other solutes and an area of low concentration of other solutes.
In the context of our thought experiment, changes in the concentration of blue and red pigments in water can cause other molecules in the liquid to move in certain directions. When the red dye moves to an area where it is less concentrated, nearby molecules are dragged with it. This phenomenon is called diffusiophoresis.
You benefit from the diffusion electrophoresis process when washing laundry: dirt particles are removed from clothing, like soap particles from a shirt, and spread into the water.
Draw defined boundaries
We wondered whether Turing structures made up of regions of different concentration could also move micron-sized particles. If so, will the resulting patterns of these particles be sharp and not blurry?
To answer this question, we performed computer simulations of Turing patterns (including hexagons, lines, and double dots) and found that diffusion electrophoresis made the resulting patterns much clearer in all cases. This diffusion simulation made it possible to reproduce the complex patterns on the skin of the decorated goatfish and the moray eel, which is not possible with Turing theory alone.
To further support our hypothesis, our model was able to reproduce the results of a laboratory study on how E. coli bacteria transport molecular cargo within themselves. Diffusion electrophoresis led to clearer movement patterns, confirming its role as the physical mechanism behind biological patterns.
Since the cells that produce the pigments that make up animal skin color are also micrometer-sized, our results suggest that diffusive photophoresis could play a key role in creating distinctive color patterns on a larger scale.
Learn the trick of nature
Understanding how nature's software works specifically can help researchers design artificial systems that perform similar tasks.
Laboratory experiments have shown that scientists can use diffusion electrophoresis to create membrane-free water filters and low-cost drug development tools.
Our work suggests that a combination of Turing patterning and diffusion electrophoresis conditions can also provide the basis for artificial skin patches. Similar to adaptive skin patterns in animals, when Turing patterns change (e.g., from hexagons to lines), this indicates fundamental differences in the concentrations of chemicals inside or outside the body.
Skin patches that can detect these changes can diagnose diseases and monitor a patient's health by detecting changes in biochemical markers. These skin spots can also detect changes in the concentration of harmful chemicals in the environment.
Work in advance
Our simulation focuses exclusively on spherical particles, while pigment-producing cells in the skin have different shapes. The influence of shape on the formation of complex patterns has not yet been elucidated.
In addition, melanocytes evolve in a complex biological environment. Further research is needed to understand how this environment limits movement and possibly freezes existing patterns.
In addition to animal skin models, Turing models are also essential to other processes such as embryonic development and tumor formation. Our work shows that diffusive photophoresis may play an underestimated but important role in these natural processes.
Learning how biological patterns form will help researchers take a step closer to mimicking their functions in the laboratory, a centuries-old endeavor that could benefit society.
This article is from The Conversation, an independent, nonprofit news organization bringing you data and analysis to help you make sense of our complex world.
By Ankur Gupta, University of Colorado Boulder .
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