Scientists explain why dogs have textured noses.

In the animal kingdom, complex patterns on the skin are often observed: stripes on zebras and spots on leopards. These structures are established during the embryonic development stage.

There are two approaches to describing them. The chemical approach is based on reaction-diffusion models by Alan Turing. A crucial element is the signaling centers that regulate cell growth. For instance, bird feathers, mammal hair, and snake scales develop from placodes—anatomical structures in the heads of vertebrates.

The second approach is mechanistic. According to this view, mechanical constraints in the growing tissue lead to the formation of various structures: networked, "brain-like," and folded. Many folded structures in biology, such as leukocyte membranes, airway walls, and tumors, have been described from this perspective. By observing living nature, bioengineers have replicated these folded structures in artificial organs and sensors.

Researchers led by Michel Milinkovic from the University of Geneva (Switzerland) applied the mechanistic approach to morphogenesis to investigate how the rhinarium arises.

"Finding examples of amazing patterns in nature is very easy. You just need to look around! That's exactly what we did in our new work, studying the noses of dogs, ferrets, and cows, covered with a unique network of polygonal structures," commented Professor Milinkovic regarding the publication of the article in the journal Current Biology.

The rhinarium of these animals consists of protruding polygons surrounded by epidermal folds. In dogs, it plays a crucial role in facilitating smell.

Since the tip of the nose is always moist, it captures more molecules from the air, which then move into the nasal cavity and from there into ducts in the upper palate for subsequent scent recognition. Additionally, the rhinarium is involved in thermoregulation. For example, rodents use it to cool their brains.

The researchers observed the development of the rhinarium in embryos. Optogenetics aided in visualizing the process. It was discovered that the polygonal structure of the nasal skin forms very early, with this process being largely self-organizing but relying on mechanical constraints in the form of a basic network of "rigid" blood vessels.

Using a two-part model of the rhinarium, where multilayered skin grows on a substrate with a network of vessels, the researchers initiated 3D modeling and obtained characteristic polygonal structures. Ultimately, they were able to trace the emergence of the dog's rhinarium and understand the overall mechanics.

Initially, excessive growth of the basal vascular layer causes the epidermis to bulge. Then, the protruding polygons grow along the vessels that mechanically constrain them. This results in a process that is no longer purely random.

"Our numerical modeling showed that the mechanical stresses created by extensive epidermal growth emanate from the underlying blood vessels. These are sort of nodal points. The layers of the epidermis are pushed upwards, like domes over rigid columns," explained the lead author of the study, Paul Dagenne.

According to the researchers, they have for the first time described the principle of "mechanistic positional information" to explain the formation of structures during embryonic development. This may help elucidate the formation of other biological structures in the presence of blood vessels in the future.