Morphogenesis of Palatal Ridges in the Golden Hamster (Mesocricetus auratus, Rodentia)

Abstract

Rugae palatinae (RP) -transversal mucosal ridges of the mammalian palate -are assumed to play a role during closure of the palate in embryos. The aim of this study was to assess the morphogenesis of palatal ridges in the golden hamster (Mesocricetus auratus, Rodentia) by light and scanning electron microscopy. Cell proliferation was detected by immunohistochemical staining of proliferating cell nuclear antigen (PCNA). In the hamster, three to four antemolar and three intermolar ridges were formed. In ED 11.0 (ED -embryonic day) embryos, RP1 and RP3 were at the epithelial thickening stage, RP2 was a primitive ruga in the rostral part of the palatal processes. In the caudal part of the palate, an epithelial placode represented the prospective RP4-RP7. At ED 12.5, the closed secondary palate bore six ridge primordia. Only RP2 protruded distinctly into the oral cavity. At ED 13.0-14.5 and ED15.0-15.5, the mesenchymal core of the antemolar and intermolar ridge primordia, respectively, started to develop. Strikingly, a local increase of proliferation activity does not seem to be the main process involved in palatal ridge formation and elevation. Although the palatal ridge formation and elevation is based on tissue volume enlargement, strikingly, the proliferation activity was higher in the interrugal epithelium than in the ridge primordia. Rather than the epithelial proliferation activity increase, the change in orientation of mitotic spindles of dividing cells seems to be a reason of palatal ridge formation. Oral cavity, proliferation, embryo, foetus, developmentPalatal ridges (rugae palatinae) are more or less transversal mucosal ridges of the hard palate, which are covered by a cornified stratified squamous epithelium. Number and arrangement of the palatal ridges is specific for the respective mammalian species.Palatal ridges have an important function during pre-and postnatal development. They help to transport the food within the oral cavity (Zietzschmann et al. 1943) and take part in grinding the food between tongue and hard palate (Eisentraut 1975). Palatal ridges contain tactile and taste receptors, so they have also a sensory function (Scott and Symons 1967;Luke 1980). In embryos, they are assumed to stiffen and strengthen the palatal processes during formation of the secondary palate (Pourtois 1972;Brinkley and Vickerman 1982;Luke 1984;Bulleit and Zimmerman 1985).Palatal ridge development has been studied mainly in the mouse. Primordia of the rugae palatinae are as well as dental and vestibular laminae derivatives of the odontogenic epithelial zone, an epithelial placode on the oral surface of the developing maxilla that can be recognised in ED 12.0 mouse embryos (Peterková 1985). The development of individual palatal ridges of the mouse starts at ED 13.0.Step by step eight rugal primordia occur. The