ABSTRACT

Early in the fourth week intrauterine, five primordial swellings, consisting of neural crest derived mesenchyme, appear around the stomodeum and are mainly responsible for face development (Figure 9.1). The process starts with the appearance of the nasal placodes on the inferior aspect of the frontonasal process. Proliferation of the ectomesenchyme on both sides of each placode results in the formation of the lateral and medial nasal processes, and the formation of the depression representing the primitive nostril, or “nasal pit.” As the two maxillary processes enlarge, they fuse medially with the medial nasal processes to form the upper lip. The failure of one or both of the maxillary processes to fuse with the medial nasal processes leads to a unilateral or bilateral cleft of the lip, respectively [1–4].