Abstract
Little is known about the pathogenesis of cleft lip, compared with that of cleft palate, because normal developmental processes of the primary palate have not been thoroughly clarified. In this study, scanning electron microscopy was carried out on epithelial cells of the primary palates and mesenchymal cells just underlying the epithelia both in normal and abnormal (cleft lip) embryos in the 9.5-13.5 th gestation days. The latter was induced with i. p. injection of hydantoin to A/J mice. The following results were obtained.
1) The epithelial changes in the normal primary palate formation, were the same as those reported previosly during the secondary palate morphogenesis, but not in cleft lip malformation. These changes in normal animals appear to be associated with the fusion of the medial and lateral nasal processes.
2) Three types of epithelial cells (periderms) were recognized during the primary palate formation. Because these three types of cells might specifically appear according to differentiation of each part of the embryos, this classification may represent some information on inner developmental changes.
3) In hydantoin-treated embryos, both posterior ends of medial and lateral nasal processes did not fuse, and thus mutually separated to cause partial or complete cleft lips according to degree of the breakdown of the isthmus. The main cause of these failures seems to be related to abnormalities in mesenchymal cells adjacent to the surface epithelium.
