Development of the Face
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Direct answer
The face is assembled during weeks four to eight from five primordia around the stomodeum: the frontonasal process above and paired maxillary and mandibular processes at the sides. Nasal placodes on the frontonasal process elevate into medial and lateral nasal processes; the medial pair forms the nasal tip, septum, philtrum and premaxilla, the maxillary processes the lateral upper lip and cheeks, and the mandibular processes the lower lip and chin. Cleft lip follows failure of medial nasal and maxillary fusion; cleft palate follows failure of the palatine shelves to fuse.
What you must remember
- Primordia: frontonasal process (neural crest around the forebrain), paired maxillary and mandibular processes of the first pharyngeal arch, bounding the central stomodeum.
- Nasal development: nasal placodes appear in week five and deepen into pits ringed by medial and lateral nasal processes; the lateral processes form the alae of the nose.
- Fusions: the medial nasal processes merge as the intermaxillary segment — philtrum, central lip and premaxilla with four incisors; maxillary plus medial nasal form the rest of the upper lip; maxillary plus lateral nasal close the nasolacrimal groove into the duct.
- Palate: palatine shelves of the maxillary processes grow down beside the tongue in week six, elevate in weeks seven to eight as the tongue descends, and fuse with each other and the nasal septum by weeks nine to ten, completing about week 12.
- Cleft lip: failure of maxillary-medial nasal fusion, usually unilateral and left-sided, occurring around week six to seven — earlier than cleft palate.
- Other clefts: posterior cleft palate alone from shelf failure; median cleft lip from failure of the two medial nasal processes to merge, often with holoprosencephaly; oblique facial cleft from a persistent nasolacrimal groove.
- Associations: folate deficiency, maternal smoking, anticonvulsants and alcohol; Pierre Robin sequence with micrognathia and glossoptosis; cleft lip commoner in males and cleft palate in females.
Common confusion
Cleft lip and cleft palate differ in timing and mechanism though they coexist: the lip fuses from surface processes in week six to seven, the palate from internal shelves in weeks nine to ten, so either may occur alone. Students also misassign the philtrum and premaxilla — both are medial nasal (frontonasal) derivatives, which is why bilateral cleft lip leaves the prolabium projecting forwards. The lateral nasal process forms only the ala.
Exam-focused takeaway
Theory questions ask for the facial primordia and their derivatives and the embryological basis of unilateral and median cleft lip, oblique facial cleft and the cleft palate types. In the viva, draw the five processes around the stomodeum and label their derivatives. Practicals use models of the developing face.
Frequently asked questions
Which processes form the upper lip?
The two medial nasal processes centrally (philtrum and central lip) and the maxillary processes laterally; failure of their fusion produces the common unilateral cleft lip.
What forms the secondary palate?
The palatine shelves of the maxillary processes, which elevate above the tongue in weeks seven to eight and fuse with each other and the nasal septum by weeks nine to twelve.
How does a median cleft lip arise?
Failure of the two medial nasal processes to merge with each other, often part of holoprosencephaly — a different entity from the common lateral cleft lip.
What is the intermaxillary segment?
The midline mass formed by the fused medial nasal processes, giving the philtrum, central upper lip, premaxilla with the four incisors and the primary palate.
Why does cleft palate occur later than cleft lip?
Because the palatine shelves fuse only after the tongue descends from between them, during weeks nine to ten, whereas the lip processes fuse in week six to seven.