Prenatal development of rugae palatinae in mice: scanning electron microscopic and histologic studies.

The development and spatial arrangement of rugae palatinae was investigated using sagittal histological sections through the heads of 12- to 19-day mouse (ICR) embryos (appearance of vaginal plug = day 1) and scanning electron microscopy (SEM) visualization. Three distinct, consecutively occurring types of developing rugae were described: 1) rugal anlage (formed by thickened epithelium mostly burrowed in mesenchyme), 2) primitive ruga (transversally oriented band of thickened epithelium protruding to the oral cavity), and 3) definitive ruga (transversally oriented mesenchymal ridge protruding to the oral cavity, covered by epithelium). As the characteristic configuration and spatial patterns were found on each of days 13-19, rugae could be utilized as a natural positional marker (eg, in odontogenesis or palatogenesis studies). The foremost rugae start to differentiate between days 12 and 13. Between days 13 and 14 the number of rugae conspicuously increases in the anterior third of palatal shelf, and by palatal shelf horizontalization (day 15, a.m.) new rugae originate in the middle third. We presume that the origin of rugae is dependent upon inductive epithelial-mesenchymal interactions. The presence of developing rugae and the timing of their origin (occurrence of tissue interactions) in the middle versus the anterior third of the palatal shelf appears to be reflected in the mode of cellular arrangement, extracellular matrix (ECM) composition, and, probably, even in the elevation mechanism. Six stages of formation of the ruga were defined, and stage-dependent arrangements of epithelial and mesenchymal cells within the developing ruga were documented. A rearrangement of cells precedes the occurrence of the primitive ruga as well as its transformation to the definitive one. These events are discussed in relation to the hypothetical integrated function of cytoskeletal and ECM components. Regarding the developmental relation of rugae to maxillary dentition in the mouse, a comparison of particular stages of teeth and rugae development and analysis of their similarities and dissimilarities may extend the knowledge of general rules of morphogenesis and differentiation in oral biology.