Mineralized dermal layer of the Brazilian tree-frog Corythomantis greeningi

Some species of anuran amphibians possess a calcified dermal layer (the Eberth-Kastschenko layer) located between the "stratum spongiosum" and the "stratum compactum." This layer consists of calcium phosphate deposits, proteoglycans, and glycosaminoglycans. Although regarded as a protective layer against desiccation, a calcium reservoir, or possibly a remnant of a dermal skeleton present in anuran ancestors, very little is known about its origin, structure, and function. Thus, we studied the structure and composition of the mineralized dermal layer of Corythomantis greeningi, a peculiar hylid from the Brazilian semiarid region (caatinga), using conventional and cryosubstitution methods combined with transmission, scanning, and analytical electron microscopy. Results show that the dermal layer consists of dense, closely juxtaposed, globular structures. Although the electron opacity of the globules was variable, depending on the type of preparation, crystal-like inclusions were present in all of them, as confirmed by dark field microscopy. Electron probe X-ray microanalysis showed calcium, phosphorus, and oxygen, and electron diffraction revealed a crystalline structure comparable to that of a hydroxyapatite.