CAlthough coming face-to-face with a powerful crocodile might scare the daylights out of most people, michelle milinkovic I find them beautiful. Milinković, a biophysicist at the University of Geneva, studies the development and evolution of vertebrate skin in crocodilians, focusing on the biomechanics of scale growth and evolution. In 2012, he and his team discovered that: scales on a crocodile’s head They are produced through physical processes rather than controlled by genetic factors, but the exact mechanism of their development was unknown.1
Michel Milinkovic, a biophysicist at the University of Geneva, studies the development of vertebrate skin in unconventional models like crocodiles.
Gregory Roychaud, University of Geneva
Now, new researchMilinković and his team reported that polygonal head scales form due to skin growth and compression, forming a pattern of self-organized folds.2 These findings show that nature, A simple evolutionary mechanism for changes in scale shape and size in different species of crocodiles has been uncovered.
The skin appendages (hair, feathers, scales) of most vertebrates develop in a pattern determined by the interaction of functionally opposite molecules. This creates points of high protein concentration interspersed with areas of low protein concentration, controlling the growth of precursors from which skin appendages grow. The scales on the crocodile’s body develop by this mechanism. about ten years agoMilinkovic noticed that the scales on the reptile’s jaw and face were different from the scales on its body. He speculated that mechanical processes might underlie its formation.
It was a long and arduous journey to test this theory, as it is not easy to obtain crocodiles for experiments. It took the team more than a decade to gather enough animals. In the new study, the researchers first looked at how head scales develop in the embryo. They imaged Nile crocodile embryos over several days using light-sheet fluorescence microscopy. What they observed was that long rectangular scales appeared on the upper jaw, and smaller polygonal scales appeared on the sides and bottom of the jaw. The research team also noticed that as the embryos grew and more scales formed, the firmness of the skin and the number of folds increased.
As in other vertebrates, the skin is connected to additional layers beneath it, such as connective tissue, muscle, and bone. Milinković hypothesized that if the skin grew faster and thus became harder than the underlying tissue, it would buckle and fold inward. To investigate this theory, he and his colleagues injected epidermal growth factor (EGF), a protein that stimulates skin growth, into crocodile embryos and observed what happened to the scales on their heads. These embryos showed numerous thinner scales on the upper jaw, while the scales on the underside of the jaw showed a labyrinth-like pattern, indicating that increased skin stiffness causes more medial folds. . The researchers hatched some EGF-treated embryos to see if the temporary increase in skin growth had a lasting effect. The scales on the heads of these crocodiles were intermediate, partially polygonal and labyrinthine, similar in shape and size to the scales of another species of crocodile, the caiman. The researchers also hatched some embryos injected with EGF for long periods of time and observed that these crocodiles only had a maze of scales.
Using light-sheet fluorescence microscopy, the researchers found that unlike scales on the crocodile’s body, the growth and physical properties of the skin promote the formation of scales on the crocodile’s head.
Grigory Timin & Michel Milinkovic, University of Geneva
To investigate whether skin growth and firmness play a role in the development of head scales in other crocodile species as well, Milinković investigated parameters of crocodile head growth, physical characteristics, and location of tissue layers. I created a computer model that allows you to input . We test whether scale output resembles patterns observed in various wild species. This model faithfully mimicked the development of Nile crocodile cranial scale patterns and the labyrinthine scales of EGF-treated embryos and juveniles. The researchers also observed cranial scale patterns in marsh crocodiles, spectacled caimans, and American crocodiles by adjusting the mechanical properties of the model’s skin.
“These computer simulations show that tissue mechanics can easily explain the diversity in shape of specific anatomical structures in different species, without the need for complex molecular genetic factors.” . Ebrahim Jahanbakhsha computer engineer at the University of Geneva and co-author of the study, press release.
