The development of palatal rugae in the European pine vole, Microtus subterraneus (Arvicolidae, Rodentia)

I n the European pine vole (Microtus subterraneus, Arvicolidae, Rodentia) eight palatal rugae are formed, three of them are located in the antemolar, five in the intermolar area. The development of these rugae was studied in staged and aged embryos and fetuses using light and scanning electron microscopy. In 12.5 to 13.5 DO (days of ontogeny) embryos, epithelial thickening of ruga 1 was observed, whilst rugae 2 to 4 appeared as epithelial buds in rostral parts of the palatal processes; in their caudal parts, epithelial thickening was discernible (future rugae 5 to 8). The cell surface was characterised by rarely dispersed minute cytoplasmatic projections, sometimes arranged into rows along cell borders. In embryos at 14 to 16 DO, the secondary palate was closed and eight rugae were observed as in newborn voles. Rugae 1 to 3 distinctly protruded into the oral cavity. In fetuses at 17 to 18 DO, formation of the mesenchymal core in palatal rugae occured in rostro-caudal direction. SEM revealed simple- shaped microvilli and microplicae on the epithelial surfaces. In fetuses at 18 to 18.5 DO, the shape of the epithelial base was changed beneath ruga 2 to 6. The rugal core was distinctly developed in all rugae. The microplicae were thicker, acquired more complex shape and became interlaced. In fetuses at 19 to 19.5 DO, the special formation of the epithelial base beneath ruga 2 to 6 continued. The cell surface was covered by a dense network of microplicae the mass of which considerably increased.