How do humans account for the variety of morphologies found in living things? Genetics is usually the first explanation that comes to mind, but it is not the only one. A multidisciplinary team from the University of Geneva has shown that the crocodile head scales originate from the mechanics of developing tissues rather than molecular genetics by fusing observations of embryonic development, sophisticated microscopy, and state-of-the-art computer modeling.
The diversity of jaw and snout scales in different crocodilian species is the result of changes in mechanical parameters. Image Credit: Michel Milinkovitch - University of Geneva, Switzerland
The evolution of mechanical parameters like skin stiffness and growth rate accounts for the diversity of these head scales in various crocodile species. These findings, published in the journal Nature, clarified the physical forces underlying the formation and evolution of living forms.
One of the biggest mysteries in science is where the complexity and diversity of morphology in living things came from. Scientists investigate a vast array of species to unravel this mystery.
To comprehend the basic processes underlying this diversity, Michel Milinkovitch's laboratory, located in the UNIGE Faculty of Science's Department of Genetics and Evolution, studies the evolution and development of vertebrate skin appendages like feathers, hair, and scales.
It is commonly believed that chemical processes involving interactions between multiple molecules brought on by gene expression control the embryonic development of these appendages.
Like a Propagating Crack
The team previously demonstrated that, in contrast to body scales, the embryonic development of crocodile head scales results from a process akin to spreading cracks in a material under mechanical stress. However, the actual nature of this physical process was still unknown.
Scientists have now figured out this mystery. First, they observed how the head scales changed throughout the Nile crocodile embryo's development, which takes about 90 days. Skin folds start to show around day 51, but the skin covering the jaws stays smooth until day 48.
Uneven polygonal scales, including large, elongated scales on the top of the snout and smaller units on the sides of the jaws, are then formed by the spreading and connecting of these folds.
Humans can anticipate that an animal's skin will buckle and fold if its growth exceeds that of the underlying tissue to which it is attached. The team aimed to investigate whether this process could account for the embryonic crocodile's skin folds and, consequently, scales.
As a result, they created a method for injecting crocodile eggs with Epidermal Growth Factor (EGF), a hormone that promotes epidermal growth and stiffening. The scientists found that the organization of skin folds underwent a dramatic change as the skin's surface became more rigid and stimulated growth.
‘‘We saw that the embryo’s skin folds abnormally and forms a labyrinthine network resembling the folds of the human brain. Amazingly, when these EGF-treated crocodiles hatch, this brain-like folding has relaxed into a pattern of much smaller scales, comparable to those of another crocodilian species - the caiman,” said Gabriel Santos-Durán and Rory Cooper, Post-Doctoral Fellows in Michel Milinkovitch’s Laboratory and Study Co-authors.
Consequently, differences in the skin's growth and stiffening rates offer a straightforward evolutionary mechanism that can produce various scale forms across various crocodilian species.
A 3D Model of Jaw Development
The researchers then employed a sophisticated imaging method called “light sheet microscopy” to measure the collagen fiber organization in the dermis as well as the growth rate and geometries of the different tissues (bone, dermis, and epidermis) that make up the embryo's head.
Using these data, the team created a three-dimensional (3D) computer model to mimic the skin's limited growth. Thanks to this model, the researchers were also able to investigate the consequences of altering the tissue layers' precise growth rates and stiffnesses.
By exploring these different parameters, we can generate the different head scale shapes corresponding to Nile crocodiles both with or without EGF treatment, as well as the spectacled caiman or the American alligator. These computer simulations demonstrate that tissue mechanics can easily explain the diversity of shapes of certain anatomical structures in different species, without having to involve intricate molecular genetic factors.’’
Ebrahim Jahanbakhsh, Computer Engineer and Study Co-Author, University of Geneva
The mechanics of crocodile head scales patterning
Video Credit: University of Geneva
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Journal reference:
Santos-Durán, G. N., et al. (2024) Self-organized patterning of crocodile head scales by compressive folding. Nature. doi.org/10.1038/s41586-024-08268-1.
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