It is well-established that the spatial patterning of skin appendage primordia — called placodes — is self-organized through interactions between activatory and inhibitory morphogens, forming a so-called Turing reaction-diffusion system. In other words, the resulting self-organised “polka-dot” arrangement of gene expression provides a template for where skin features such as feathers, scales or hairs will grow.
Crocodiles are a spectacular exception to this paradigm. Unlike the scales of other reptiles and birds, and unlike their own body scales, the irregularly shaped head scales of crocodiles are not individual developmental units patterned by reaction-diffusion among gene products. Instead, these scales seem to form through a mechanical process that generate a pattern reminiscent of those emerging from material cracking [1] —think of crack patterns in drying mud or in cooling pottery glaze.
However, using advanced techniques such as in-ovo drug treatment of crocodile embryos, cutting-edge 3D microscopy, and state-of-the-art computer simulations, we later revealed that the underlying mechanical self-organising process consists of compressive-folding of the skin [2].
The 3D model above has been obtained from the light-sheet microscopy imaging of a Nile crocodile embryo at day 63 of egg incubation.
The segmented tissue layers are the epidermis (brown), dermis (pink), multi-sensory micro-organs [3] (orange), bone (white), and collagen fibres [4] (blue).
Transparency reveals hidden structures.
[1] Crocodile Head Scales Are Not Developmental Units But Emerge from Physical Cracking
Milinkovitch, Manukyan, Debry, Di-Poï, Martin, Singh, Lambert & Zwicker.
Science 339, 6115 : 78-81 (2013)
[2] Self-organized patterning of crocodile head scales by compressive folding
Santos-Durán, Cooper, Jahanbakhsh, Timin & Milinkovitch
Nature 637 : 375-383 (2025)
[3] Crocodylians Evolved Scattered Multi-Sensory Micro-Organs
Di-Poï & Milinkovitch
EvoDevo 4, 19 (2013)
[4] High-Resolution Confocal and Light-Sheet Imaging of Collagen 3D Network Architecture in Very Large Samples
Timin & Milinkovitch
iSciences 26, 106452 (2023)