Hair begins forming long before an animal is born. As a mammal develops inside the womb, tiny structures called hair follicles appear across the skin in patterns that are surprisingly organized.
Some animals grow evenly spaced coats, while others develop hair that follows clear directions or forms unique arrangements.
Scientists have spent decades trying to understand what decides the exact spot where each hair begins.
Simple rules for complex patterns
The question matters for more than scientific curiosity. Hair follicles, feathers, and many types of scales all begin as small embryonic structures called placodes.
Learning how placodes organize themselves could help researchers better understand how tissues form during development and how simple biological rules can create complex body patterns.
A new study points to a much simpler explanation than many researchers expected.
Scientists from the University of Geneva (UNIGE) found that hair follicles may not need an elaborate biological program that tells each one exactly where to grow.
Instead, they appear to form naturally as cells respond to chemical signals while moving through developing skin.
A long-standing idea
For years, the leading explanation for hair follicle formation in laboratory mice, Mus musculus, was known as the expansion-induction model.
According to this idea, each newly formed placode releases a chemical that prevents nearby placodes from forming.
As the embryo’s skin expands, regions eventually move beyond the reach of that signal, allowing new placodes to develop.
Although this model has been widely accepted, researchers had never shown that it could explain hair pattern formation across different mammal species.
Cells organize themselves
The research team was co-led by Athanasia Tzika and Professor Michel Milinkovitch at UNIGE’s Faculty of Science. They explored a different possibility based on chemotaxis.
Chemotaxis is the ability of cells to move toward or away from chemical signals. It is already known to help guide white blood cells to areas of inflammation.
Using a mathematical model, the scientists examined how mobile dermal cells interact with an attractive chemical signal produced by the epidermis.
They simulated skin growth in embryos and watched how placodes developed over time.
Order without a blueprint
The results showed that this simple process produced the same pattern of hair follicle formation previously explained by the older expansion-induction model in laboratory mice.
“Our findings show that the observed patterns do not require a complex system telling each placode where to form,” explained Muhamet Ibrahimi and Ebrahim Jahanbakhsh, researchers in the Department of Genetics and Evolution.
“Instead, placodes emerge spontaneously from local interactions between cells and chemical signals.”
Explaining the mouse
The researchers then asked whether the same idea could explain another animal with a very different coat pattern.
They turned to the spiny mouse, Acomys dimidiatus, a rodent whose hair follicles grow in an unusually regular and highly organized arrangement.
The traditional expansion-induction model could not account for this distinctive pattern.
The team combined their mathematical model with experimental data gathered from three-dimensional images of developing embryonic skin.
Differences across species
The researchers found that the same chemotactic process successfully recreated the spiny mouse’s hair follicle pattern.
The model successfully reproduced the pattern when the researchers incorporated the species’ unique biochemical properties and skin growth characteristics.
The findings suggest that different species may rely on the same basic biological process while producing very different results.
Small differences in how cells communicate and respond during development appear to produce these differences.
Building complex tissues
Many structures in nature appear highly ordered, yet they often arise from countless local interactions rather than from a central controller.
Biologists have seen similar self-organizing behavior in processes ranging from early embryo development to the formation of blood vessels and branching organs.
The new study adds hair follicle development to that growing list. Rather than requiring detailed instructions for every follicle, embryonic skin is capable of organizing itself.
“Our work suggests that simple cellular interactions can generate the remarkable diversity of tissue architectures observed throughout evolution,” concluded Tzika.
“The differences observed between species therefore result from the same self-organization process, but with variations in the interactions between cells and chemical signals.”
The full study was published in the journal Proceedings of the National Academy of Sciences.
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