Craniofacial development initiates at approximately gestation day 25, following closure of the anterior neuropore. Neural crest cells migrate and induce formation of the branchial arches. The first branchial arch and neural crest ectomesenchyme give rise to 5 prominences that surround the stomodeum (the future mouth). The facial prominences (or processes) consist of a single midline frontal prominence (median nasal process), paired maxillary prominences, and paired mandibular prominences. The facial prominences progressively fuse by disintegration of the contacting epithelial surfaces, and the intervening grooves fill in by migration of cells and proliferation of mesenchyme. Formation of the palate involves fusion of the primary palate, which is derived from the median nasal process, and the palatine shelves, which are derived from the maxillary processes. Failure of appropriate fusion of the 7 embryonic facial prominences results in persistent facial or palatal clefts.
During fusion of the facial processes, complete lysis of the overlying epithelium occurs. Failure of complete epithelial lysis can result in trapping of epithelial elements between embryonic processes of bone. This epithelial tissue may undergo proliferation and cystic degeneration. This can result in a nasopalatine duct cyst, globulomaxillary cyst, or nasoalveolar cyst. The nasopalatine duct cyst is the most common developmental cyst of the maxilla. This results from entrapment of epithelial cells within the incisive canal during fusion of the horizontal palatal processes and the premaxilla. This cyst is usually asymptomatic. CT shows a well-circumscribed midline low attenuation lesion with sclerotic margins located along the course of the incisive canal between the nasal fossa and the alveolar process of the maxilla. The globulomaxillary cyst is due to entrapment of epithelium at the site of fusion of the median nasal process and the maxillary process. Radiographs and CT show an inverted pear shaped or oval radiolucency located between the maxillary lateral incisor and the cuspid. Displacement of the roots of these teeth is common.
Disorders of facial clefting can be isolated or syndromic. Hundreds of syndromes associated with facial clefting have been described. Facial clefting is usually classified as central versus lateral. The Pierre Robin sequence is an example of a central clefting syndrome. Restricted mandibular development causes failure of the tongue to descend away from the fusion line of the secondary palate. Lateral facial clefting syndromes typically result from deficiencies in the first and second pharyngeal arches. Examples include Treacher Collins syndrome and hemifacial microsomia. Children with velo-cardio-facial syndrome have deficiencies of the frontal nasal process, including palatal deficiency, a broad nasal bridge, a hypoplastic philtrum, and hypertelorism.
The most common type of facial clefting is a simple cleft involving the upper lip or palate. Lip clefting usually extends from the lateral border of the philtrum into the nostril, and is due to failure of the medial nasal prominence to merge with the maxillary prominence. Palatal clefts are caused by failure of fusion between the primary palatal process and the secondary process of ...