Secondary palate development is characterized by the formation of two palatal shelves on the maxillary prominences, which fuse in the midline in mammalian embryos. However, in reptilian species, such as turtles, crocodilians, and lizards, the palatal shelves of the secondary palate develop to a variable extent and morphology. While in most Squamates, the palate is widely open, crocodilians develop a fully closed secondary palate. Here, we analyzed developmental processes that underlie secondary palate formation in chameleons, where large palatal shelves extend horizontally toward the midline. The growth of the palatal shelves continued during post-hatching stages and closure of the secondary palate can be observed in several adult animals. The massive proliferation of a multilayered oral epithelium and mesenchymal cells in the dorsal part of the palatal shelves underlined the initiation of their horizontal outgrowth, and was decreased later in development. The polarized cellular localization of primary cilia and Sonic hedgehog protein was associated with horizontal growth of the palatal shelves. Moreover, the development of large palatal shelves, supported by the pterygoid and palatine bones, was coupled with the shift in Meox2, Msx1, and Pax9 gene expression along the rostro-caudal axis. In conclusion, our results revealed distinctive developmental processes that contribute to the expansion and closure of the secondary palate in chameleons and highlighted divergences in palate formation across amniote species.

Department of Anatomy Histology and Embryology Faculty of Veterinary Medicine University of Veterinary and Pharmaceutical Sciences Brno Czechia

Department of Developmental Biology Institute of Experimental Medicine Czech Academy of Sciences Prague Czechia

Department of Experimental Biology Faculty of Science Masaryk University Brno Czechia

Department of Histology and Embryology Faculty of Medicine Masaryk University Brno Czechia

Department of Radiation Dosimetry Nuclear Physics Institute Czech Academy of Sciences Prague Czechia

Institute of Vertebrate Biology Czech Academy of Sciences Brno Czechia

Laboratory of Computed Tomography Central European Institute of Technology Brno University of Technology Brno Czechia

Laboratory of Mammalian Evolutionary Genetics Institute of Animal Physiology and Genetics Czech Academy of Sciences Brno Czechia

Laboratory of Molecular Morphogenesis Institute of Animal Physiology and Genetics Czech Academy of Sciences Brno Czechia

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