The skin on the nose of many mammals such as dogs, ferrets, and cows, shows grooves forming a multitude of polygons.
A team from the University of Geneva (UNIGE) has thoroughly analyzed how these patterns form in the embryo using imaging techniques and computer simulations. They discovered that the unequal growth of different tissue layers causes the formation of domes, which are supported by the underlying blood vessels.
Volumetric image of the nose of a dog embryo, obtained through "light sheet" fluorescence microscopy.
© Milinkovitch, Dagenais
This work describes for the first time this process of morphogenesis, which could help explain the formation of other biological structures associated with blood vessels. The findings are published in the journal
Current Biology.
Living organisms feature remarkable shapes, some of which are recognizable by their coloring or pattern repetitions. This is how we identify zebras or cheetahs by the geometric repetition of their coats or pinecones by their regular spiral organization. These fascinating patterns are generated by various morphogenesis processes, meaning the formation of shapes during embryonic development.
On the one hand, chemical self-organizing morphogenesis allows structures or patterns to emerge from chemical reactions. A particularly illustrative example is 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 shapes result from mechanical constraints. The folds in the brain, for example, appear through a differential growth process: the cortex forms folds because it expands faster than the deeper layer to which it is attached.
The diversity of life
The group led by Michel Milinkovitch, professor in the Department of Genetics and Evolution at the Faculty of Sciences of UNIGE, studies the evolution of developmental mechanisms that generate the complexity and diversity of species. "Finding examples to study nature's beautiful patterns is easy. Just look around us! Our latest study focuses mainly on a dog's nose, whose skin presents a unique network of polygonal structures," explains Michel Milinkovitch.
The bare skin of the rhinarium (nose) of many mammal species indeed features a network of polygons formed by grooves in the skin. These grooves retain physiological fluids, helping to keep the nose moist, and among other functions, facilitate the collection of odorant and pheromone molecules. The Geneva-based team collaborated with the Université Paris-Saclay, the National Veterinary School of Alfort (EnvA), and the Institute of Neurosciences of San Juan de Alicante to collect samples of rhinaria from embryos of dogs, cows, and ferrets.
3D nose visualization
These samples were observed using "light sheet" fluorescence microscopy, a technique that allows the visualization of biological structures in three dimensions.
The researchers found, in all three mammal species, that the polygonal network of folds in the epidermis—the outer skin layer—appears during embryogenesis and always overlaps a rigid, underlying network of blood vessels located in the dermis—the deep layer of the skin.
They also observed that the epidermal cells proliferate faster than the dermal cells.
Blood vessels, "architectural pillars"
With this data, a mathematical model allowed scientists to perform computer simulations of tissue growth.
These simulations take into account the difference in growth rates between the dermis and epidermis, their respective rigidity, 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 locations of the vessels that form rigid support points.
The epidermal layers are then pushed outward, forming domes—somewhat like arches rising against rigid pillars," explains Paule Dagenais, post-doctoral scholar in the Department of Genetics and Evolution at the Faculty of Sciences of UNIGE, and first author of the study.
The findings demonstrate 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 impose local constraints during the growth of the epidermis, leading to the formation of grooves and domes in precise locations.
"This 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 have no doubt that it will help to explain the formation of other biological structures associated with the presence of blood vessels," concludes Michel Milinkovitch.