Many mammals, like dogs, ferrets, and cows, have noses that display a unique pattern of grooves creating various polygons. Researchers from the University of Geneva (UNIGE) have closely examined how these patterns develop in embryos by utilizing 3D imaging techniques and computer simulations. They found that different growth rates in skin tissue layers lead to dome formation, supported mechanically by blood vessels underneath. This study offers a new perspective on the morphogenetic process and could shed light on how other biological structures related to blood vessels form. These findings have been published in Current Biology.
The natural world showcases a variety of intriguing shapes, often marked by unique coloration patterns or three-dimensional designs. For instance, zebras and cheetahs are identifiable by their distinctive stripes and spots, while pine cones are recognized for their spiral arrangement. Such captivating patterns result from different morphogenetic processes, which involve shape creation during the early stages of development.
On one hand, self-organizing morphogenesis may be influenced by chemical reactions, as illustrated by Alan Turing’s reaction-diffusion model, in which chemical substances disperse and interact to form relatively uniform patterns like those seen in mammalian and reptilian skins. Conversely, some formations arise from mechanical pressures, like the convolutions of the human brain, which form folds as the outer cortex expands more quickly than the inner layer it’s connected to.
The diversity of life
Professor Michel Milinkovitch and his team at UNIGE’s Faculty of Science explore how the mechanisms of development contribute to the diversity and complexity of life. ”Identifying specific examples of stunning patterns in living organisms is straightforward. They are all around us! Our recent study investigates the noses of dogs, ferrets, and cows, which possess a distinct array of polygonal shapes,” explains Milinkovitch.
In fact, the exposed skin of the rhinarium (nose) in numerous mammals showcases a polygonal network formed by grooves. These grooves help retain moisture, keeping the nose damp, which serves various purposes, including aiding the collection of pheromones and scent molecules. The Geneva team collaborated with Université Paris-Saclay, École Nationale Vétérinaire d’Alfort (EnvA), and the Institute of Neurosciences de San Juan de Alicante to gather rhinarium samples from embryos of dogs, cows, and ferrets.
Nose 3D visualization
Using a technique called ”light sheet fluorescence microscopy,” the samples were examined to visualize biological structures in three dimensions. Across all three mammal species, the researchers observed that during embryonic development, an arrangement of polygonal folds emerges in the epidermis – the outer skin layer – directly aligned over an underlying network of rigid blood vessels in the dermis – the deeper skin layer. Additionally, they noted that the cells in the epidermis multiply at a quicker rate than those in the dermis.
Blood vessels form ”architectural pillars”
With these findings, the team created a mathematical model and conducted computer simulations of tissue growth. This model incorporated the varying growth rates of the dermis and epidermis, their stiffness levels, and crucially, the impact of blood vessels in the dermis. ”Our simulations reveal that the mechanical pressure from heightened epidermal growth concentrates at the location of the rigid vessels, which act as solid support points. As a result, the epidermal layers are forced outward, forming domes—much like arches rising against sturdy pillars,” explains Paule Dagenais, a post-doctoral fellow at UNIGE’s Faculty of Science and the lead author of the study.
The results indicate that in the case of rhinaria, the positioning of the epidermal polygonal formations is dictated by the rigid blood vessels beneath, which create local constraints during epidermal growth. This leads to the precise formation of grooves and domes. ”This is the first time that we have described this mechanism, which we refer to as ‘mechanical positional information,’ to elucidate how structures form during early development. We believe it will also aid in understanding the formation of other biological structures related to blood vessels,” concludes Michel Milinkovitch.
