Development of the lip and palate in staged human embryos and early fetuses.

Cleft lip and palate are the most common congenital malformations in humans. Using 43 staged human embryos and early fetuses ranging from the 4th to 12th week of development, we investigated the development of the lip and palate in order to provide the basic developmental concepts required for managing these anomalies. The lower lip appeared as bilateral mandibular arches at Carnegie stage 11, and these were completely merged at stage 15. The components of the upper lip, medial nasal prominence and maxillary process, appeared at stage 16, and completely merged at stage 20. The median palatine process appeared at stage 16, and the lateral palatine process, at stage 17. The palatine processes and the nasal septum started to fuse abruptly at stage 23, and from external observation seemed to be fused at the 9th week. However, complete fusion did not take place until the 12th week of development. The tongue was prominent at stage 16, showed differentiation of the muscular tissue at stage 21, and was located superior to the lateral palatine process before stage 23. These results may be used in understanding the different mechanisms present in the formation of various congenital anomalies in this region.