Remember from the first year embryology Trilaminar disc has 3 layers: ectoderm, mesoderm, and endoderm

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Marshall Jordan
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1 Development of face Remember from the first year embryology Trilaminar disc has 3 layers: ectoderm, mesoderm, and endoderm The ectoderm forms the neural groove, then tube The neural tube lies in the mesoderm layer toward the back The neural tube will form the brain and spinal cord The cranial part of the neural tube forms 3 vesicles: forebrain, midbrain, and hindbrain The forebrain enlarges to form the forebrain bulge (head bulge) (surrounded by mesoderm and covered externally by ectoderm) In the area of the neck, the pericardial bulge will form, note that the heart forms first in the neck then descends in the thorax. If you look from front, the face area is a depressed area located between the two bulges, the forebrain bulge and pericardial bulge this area is called stomodeum Remember the pharyngeal arches will form at the sides The first arch has two processes, maxillary and mandibular. Both processes grow forward and the forebrain bulge grows downward to form the face. The part of the forebrain bulge that grows downward forms the frontonasal process So we have 5 processes that will form the face 1- Two mandibular processes 2- Two maxillary processes 3- One frontonasal process

2 Frontonasal process Two thickenings appear on the frontonasal process, called nasal placodes (Proliferation of ectodermal cells).. Remember the outer layer is always ectoderm A depression will form in each nasal placode to form the nasal pit Nasal pit is in the middle of the nasal placode When you have a pit, then you have an elevated area around the pit This elevated area forms two folds around the pit, one medial and one lateral So nasal placode, then nasal pit, then medial and lateral nasal folds or prominences In between the two nasal placodes in the midline is a cleft called nasal cleft (This cleft will disappear later) Now,,, the maxillary process grows forward, as it grows forward it 1- Fuses with mandibular process (partial fusion) 2- Pushes the nasal placodes centrally (note as the placodes move centrally, the cleft becomes smaller) Now the medial nasal folds from both sides fuse together and the nasal cleft disappears completely The lateral nasal folds will form the ala of the nose The fused medial nasal folds will form the nasal septum The medial nasal folds after fusion grows downward This downward growth will prevent the fusion between the two maxillary processes, so this part (the downward extension of the fused medial nasal folds) is called intermaxillary segment

3 The intermaxillary segment will form 1- Philtrum of upper lip (philtrum is the middle part of upper lip) 2- The ridge carrying the Anterior incisor teeth 3- Primary palate (triangular area of palate located behind the incisor teeth) Note that the each maxillary process fuses with the frontonasal process except at the area of orbit (the lower part of orbit) That s why the upper eyelid is supplied by ophthalmic but the lower eyelid is supplied by maxillary nerve (infraorbital branch) At the junction medially between the frontonasal process and maxillary process, ectodermal cord of cells will form a duct----nasolacrimal duct The upper end of the duct widens to for a blind sac (lacrimal sac) The two maxillary processes will fuse medially with the intermaxilllary segment The two maxillary processes send also two horizontal processes to form the secondary palate.. Note that the horizontal plates will fuse with each other and with the intermaxillary segment in front The maxillary process fuses partially with the mandibular process If the fusion is excessive: microstomia If the fusion is minimal: macrostomia Note that at first, the mouth has a broad opening, but later this diminishes in extent because of fusion of the lips at the lateral angles. The mandibular process meets the other mandibular process at symphysis menti forming the lower lip, mandible and the floor of mouth Symphysis means joint between two identical bones\menti: chin

4 The area of skin overlying the frontonasal process and its derivatives receives its sensory nerve supply from the ophthalmic division of the trigeminal nerve, whereas the maxillary division of the trigeminal nerve supplies the area of skin overlying the maxillary process. The area of skin overlying the mandibular process is supplied by the mandibular division of the trigeminal nerve Development of palate Formed by three processes 1- Inward growth of two palatine/ horizontal processes of maxillary process (shelves) (2dry palate) 2- Downward growth of the intermaxillary segment of frontonasal process (1ry palate) 3- The anterior part will ossify to form the hard palate 4- The posterior part will not ossify and form the soft palate Congenital anomalies Dermoid cyst: collection of embryonic cells (mesenchyme), it grows into strange tissue (ex. Hair, teeth, bone or cartilage). It is common at sites of fusion esp. at the lateral edge of the eye between upper and lower eyelids) Oblique facial cleft: failure of fusion between maxillary and frontonasal processes Macrostomia or Microstomia : defective or marked fusion between maxillary and mandibular processes Cleft (hare) lip: cleft lip due to failure of fusion between maxillary process and intermaxillary segment. Can be unilateral or bilateral

5 Median cleft lip: Results from failure of the medial nasal prominences to merge and form the intermaxillary segment Cleft palate: failure of fusion between different parts that form palate The incisive foramen is considered the dividing landmark between the anterior and posterior cleft deformities Cleft of the primary palate Results from failure of the palatine shelves (maxillary process) to fuse with the primary palate (intermaxillary segment) Takes place anterior to the incisive foramen therefore this type is anterior cleft palate Note that Cleft of the primary palate is always anterior Can be unilateral and bilateral Cleft of the secondary palate Results from failure of the palatine shelves (maxillary processes) to fuse with each other Takes place posterior to the incisive foramen therefore this type is Posterior cleft palate Note that Cleft of the secondary palate is always posterior Cleft of the primary and secondary palate Results from failure of the palatine shelves (maxillary processes) to fuse with each other and with the primary palate (intermaxillary segment) Takes place anterior and posterior to the incisive foramen therefore this type is mixed anterior and posterior cleft palates

Essentials in Head and Neck Embryology. Part 3 Development of the head, face, and oral cavity

Essentials in Head and Neck Embryology Part 3 Development of the head, face, and oral cavity Outline General overview of prenatal development Embryonic period phase 1 Formation of bilaminar disk Formation