Geometric mechanics shapes the dog’s nose

The living world is full of remarkable shapes, some of which can be identified by their coloration patterns or 3D motifs. Zebras and cheetahs, for example, can be recognized by their stripes or patches of fur, while pineapples are characterized by their spiral organization. These fascinating patterns are generated by various morphogenetic processes, that is, the generation of shapes during embryonic development.

On the one hand, self-organized morphogenesis can be mediated by chemical reactions, as described by Alan Turing’s reaction-diffusion model, where chemicals diffuse and interact to create relatively regular patterns, such as stripes or spots on the skin of mammals and reptiles. On the other hand, some forms are the result of mechanical limitations. The convolutions of the human brain, for example, are produced by a process of differential growth: the cortex forms folds because it grows faster than the deeper layer to which it is attached.

The diversity of life

The group of Michel Milinkovitch, professor in the Department of Genetics and Evolution of the Faculty of Sciences of the UNIGE, investigates the evolution of the developmental mechanisms that produce the complexity and diversity of life. ”Finding specific examples of beautiful patterns in living organisms is easy. All we have to do is look around! Our latest study focuses on the noses of dogs, ferrets and cows, which present a unique network of polygonal structures,” explains Michel Milinkovitch.

In fact, the bare skin of the rhinarium (nose) of many mammalian species presents a polygonal network formed by furrows in the skin. By retaining moisture, these furrows keep the nose moist and, among other functions, facilitate the collection of pheromones and odorant molecules. The Geneva team collaborated with the Université Paris-Saclay, the École Nationale Vétérinaire d’Alfort (EnvA) and the Institut de Neurocièncias de San Juan de Alicante for the collection of embryo nasal samples of dogs, cows and ferrets.

3D visualization of the nose

These samples were observed using “light sheet fluorescence microscopy”, a technique that allows the visualization of biological structures in three dimensions. In all three mammal species, the researchers found that the polygonal networks of folds in the epidermis, the outer layer of the skin, appear during embryogenesis and are systematically and precisely superimposed on an underlying network of rigid blood vessels located in the dermis, the deepest layer of the skin. They also observed that epidermal cells proliferate faster than dermal cells.

Blood vessels form ‘architectural pillars’

Using this data, the scientists developed a mathematical model and performed computer simulations of tissue growth. This model takes into account the difference in growth rates between the dermis and the epidermis, their respective stiffnesses and, most importantly, the presence of blood vessels in the dermis. ”Our numerical simulations show that the mechanical stress generated by excessive epidermal growth is concentrated at the positions of the underlying vessels, which form rigid support points. The epidermal layers are then pushed outwards, forming domes, similar to arches that rise against rigid pillars,” explains Paule Dagenais, postdoctoral fellow in the Department of Genetics and Evolution of the Faculty of Sciences of the UNIGE and first author of the study.

These results show that, in the case of rhinaria, the position of the polygonal structures in the epidermis is imposed by the position of the rigid blood vessels in the dermis, which exert local constraints during epidermal growth, resulting in the formation of grooves and domes. in precise places. ”It is the first time that this mechanism, which we call ‘mechanical positional information’, has been described to explain the formation of structures during embryonic development. But we are sure that it will help to explain the formation of other biological structures associated with the presence of blood vessels”, concludes Michel Milinkovitch.