The morphological diversity of living organisms is a captivating puzzle, and while genetics often takes center stage, it is not the sole explanation.
A new study by a team at the University of Geneva (UNIGE) reveals that the mechanics of growing tissues play a crucial role in the development of crocodile head scales. By integrating observations from embryonic development, advanced microscopy techniques, and innovative computer modeling, the researchers demonstrated that mechanical properties, such as skin growth rate and stiffness, significantly influence the variation in head scales among different crocodilian species.
These findings provide new insights into the physical forces that drive the development and evolution of various life forms.
The quest to understand the root of morphological diversity continues to challenge scientists, and Michel Milinkovitch’s lab at UNIGE focuses on vertebrate skin appendages like feathers, hair, and scales to further unravel the fundamental mechanisms behind this phenomenon.
It has been generally believed that the embryonic development of such appendages is primarily controlled by chemical processes involving intricate molecular interactions stemming from gene expression.
Previously, the research team demonstrated that the development of crocodile head scales, in contrast to body scales, arises from a phenomenon similar to the spread of cracks in a material subjected to mechanical stress.
However, the exact nature of this physical phenomenon remained unclear. The scientists have now unraveled this mystery through their innovative and highly interdisciplinary research. Initially, they monitored the emergence of head scales during the Nile crocodile embryo’s development, which spans approximately 90 days.
While the skin on the jaws stays smooth until around day 48, skin folds begin to appear at approximately day 51. These folds then extend and connect, resulting in the formation of irregular polygonal scales featuring larger and more elongated scales on the snout’s top and smaller scales along the sides of the jaws.
The study focused on the relationship between skin growth and the underlying tissue in embryonic crocodiles, particularly how discrepancies in growth rates can lead to skin folds and the formation of scales. Researchers injected crocodile eggs with Epidermal Growth Factor (EGF) to stimulate epidermal growth and stiffening of the skin. They observed that this activation resulted in significant alterations in the arrangement of skin folds.
“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,” explain Gabriel Santos-Durán and Rory Cooper, post-doctoral fellows in Michel Milinkovitch’s laboratory and co-authors of the study.
This phenomenon suggests that variations in the growth and stiffness of skin can serve as an evolutionary mechanism, contributing to the vast array of scale types found in different crocodilian species.
To further investigate this, the scientists employed light sheet microscopy, an advanced imaging method, to analyze the growth rates and geometries of the tissues that make up the embryo’s head, including the epidermis, dermis, and bone, along with the collagen fiber arrangement in the dermis.
Using this data, they created a three-dimensional (3D) computer model simulating the constrained growth of skin, enabling them to examine how changes in tissue growth rates and stiffness impact skin development.
“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,” concludes Ebrahim Jahanbakhsh, a computer engineer in Michel Milinkovitch’s laboratory and co-author of the study.
Journal reference:
- Gabriel N. Santos-Durán, Rory L. Cooper, Ebrahim Jahanbakhsh, Grigorii Timin & Michel C. Milinkovitch. Self-organized patterning of crocodile head scales by compressive folding. Nature, 2024; DOI: 10.1038/s41586-024-08268-1