Development of the secondary palate in the rat: a scanning electron microscopic study

Normal palatogenesis was studied in the Sprague-Dawley rat, using fetuses of a precise, individually determined age (between 15 and 21 days of pregnancy) and scanning electron microscopy. Individual fetal age deviated markedly from gestational age determined by the vaginal smear method. Palatal closure, ie elevation and fusion of the anterior and remodeling of the middle zones as well as fusion of the posterior zone all occurred between 16.0 and 17.5 days of fetal age, ie earlier than in Wistar rat fetuses. Prior to fusion, the medial edges of palatal shelves revealed two distinct zones of initial surface alterations: one at the maximum anterior shelf convexity, the other at the caudal end of the posterior shelf portion. A similar zone was found at the free edge of the nasal septum. Subsequent oral aspects of palatogenesis including formation and fate of the midpalatal fusion line, the development and fusion of palatine rugae, the union between primary palate and the secondary palatal shelves, the development of palatine foraminae as well as the development of the soft palate following shelf closure are described in detail. It is suggested that (1) cobblestone appearance of surface cells reflects undifferentiated epithelial status rather than marks future fusion zones; (2) closure between primary and secondary palate involves two different mechanisms, ie furrow closure by growth lateral to and epithelial fusion between the palatine foraminae; and (3) fusion of the posterior shelf portions (soft palate) may occur independently from fusion in the anterior region.