Figure 1. Development of the lip and palate in the mouse. (a) Facial development of an embryonic day 10.5 mouse (equivalent to approximately 4 weeks of development in a human embryo) as revealed by scanning electron microscopy. The development of the nasal pit has divided the lower aspect of the frontonasal prominence into paired medial and lateral nasal process. The maxillary and mandibular processes surround the developing mouth. (b) By embryonic day 12.5 in the mouse (equivalent to approximately 6 weeks of development in an human embryo), the upper lip has been formed by fusion of the medial nasal processes and the maxillary processes. (c) A frontal histologic section through an embryonic day 13.5 mouse head indicates that the paired palatal shelves initially grow vertically down the sides of the tongue. (d) By mouse embryonic day 14.5 (equivalent to approximately 8/9 weeks of development in a human embryo), the palatal shelves have elevated to a horizontal position above the tongue and have begun to fuse together. fnp: frontonasal prominence; mnp: medial nasal process; lnp: lateral nasal process; mx: maxillary process; mp: mandibular process; mand: mandible; p: developing secondary palate; t: tongue; mc: Meckel's cartilage.

Figure 2. Clinical pictures of various types of orofacial cleft: (a) unilateral cleft lip in which only one side of the lip is affected; (b) bilateral cleft lip and palate in which both sides of the lip are affected; (c) isolated cleft of the secondary palate in which the lip is not involved.