EMBRYOLOGY OF THE FACE
Intro:
§Facial development occurs between weeks 4—8
§Face develops from 5 prominences

1 and 2:  Pair of Maxillary prominences (right and left) à forms cheek (under eyes and side of nose)  = area innervated by Maxillary division of Trigeminal (CN V)

3 and 4:  Pair of Mandibular prominences (R +L) à lower lip, lower jaw, ear = areas innervated by Mandibular division of Trigeminal (CN V)

5: Frontonasal prominence
 The stomodeum sits in the middle.

 

•In 4th week, a thickening of the mesenchyme occurs on the side of the embryo in the frontonasal prominence = lens placode =area where eyes develop that begins on the sides of the head(eyes will rotate to front eventually)

•A pair of thickenings on the front of the frontonasal prominence = nasal placodes = where nose will form

•maxillary prominence is paired but unlike the mandibular prominence, is not fused.  (Eventually they fuse with medial nasal prominence.  If this doesn’t happenàcleft lip)

How is the gap between the maixillary prominences formed?
 
§No gap exists before this stage—only a groove exists between the maxillary prominences and frontonasal prominences = stomodeum

§This groove deepens and approaches the front of the digestive tract, since at this point there is no connection between the front of the digestive tract and outside world §The deepening of the stomodeum continues until only a thin film remains between the outside world and the digestive tract = oropharyngeal membrane §Eventually the oropharyngeal membrane will rupture and create an opening between the maxillary prominences = primitive mouth (allows the pharynx to communicate with amniotic cavity= “outside world”)

At 5 weeks…
•Eyes move to front of head
•There is a proliferation of mesenchyme that forms the nasal placodes
•Nasal placodes differentiate into a lateral nasal prominence and a medial nasal prominence
•The lateral nasal prominence will become the alae of the nose
•A depression develops in the center of the nasal placodes = nasal pit
Middle of 6 wk…

 

 

 

 

 

 
→becomes primary palate/premaxillary bone (contains incisors)
•Eyes continue to develop and move to front
•Medial nasal prominences grow and merge at midline (at~7wks)-->fused medial nasal prominence =intermaxillary segment
•No nasal cavity exists at this point…
How does the nasal cavity form?
§Nasal pit invaginates and expands, increasing in depth until it approaches the pharynx (this is all happening on both the right and left sides)
§Once the right and left sides erupt into each other, a single nasal cavity is formed
§The oronasal membrane separates the nasal cavity from the oral cavity

§The oronasal membrane eventually ruptures so the nasal cavity is almost completely confluent with the oral cavity.  A small piece of intermaxillary segment at the very front of the developing mouth remains as the only separation between them.

§
How do the oral and nasal cavities separate?  a.k.a   How does the palate form?
Remember…at ~6 wks there is no separation…you get separation during the 7th week:

§The nasal septum forms from a fold/sheet that begins at the top of the nasal cavity (in the midline) and grows down to divide the nasal cavity into left and right halves §Lateral palatine processes are horizontal folds (also running front to back) that develop from the sides and grow medially.  They meet and fuse at the midline along with the nasal septum.

§--> so now we have two separate right and left nasal cavities which are themselves separate from the oral cavity
§the tongue is beginning to develop at the bottom of the oral cavity
§The lateral palatine processes (LPP) can be seen growing in from the sides (toward eachother) as well as forward to meet the IMP
§The palate is formed by this convergence of IMP and LPP’s and has 2 parts:
•Primary palate = part formed by the IMP
•Secondary palate = part formed by the LPP’s
§Primary palate will meet secondary palate → secondary palate will begin to fuse from front to back
§Incisive foramen =  where primary and secondary palates meet
§Closure occurs by the 12th week  (but it is all soft tissue)…
Which bones develop from the secondary palate?
•Maxilla –bears the molars, premolars and canines; not the incisors.
§The intermaxillary process (IMP) is located at the front of future mouth
•Palatine bone forms behind the maxilla
 The back of the secondary palate does not ossify, giving rise to a dip at the end called the uvula.
bones from the primary palate?
1.Premaxilla (premaxillary part of the maxilla)—bears the incisors.
So why don’t all the ossified tissues of the palate make up a single bone?
****Because the embryonic origins of the maxilla and premaxilla are different!****
The maxilla comes from the maxillary prominences (from the 1st branchial)
arch
The premaxilla comes from the intermaxillary segment (from the frontonasal
prominence)
As a result, the premaxilla is considered a separate bone that fuses with the maxilla.
So…

 

Summary/Timetable for Development of the Face
 
Week 4 (early) → 5 facial prominences appear around stomodeum
Week 4 →   Mandibular prominences merge to form lower jaw
          4 (late)  →   nasal placodes present, develop into medial and lateral prominences with
                                pit  (= nostril) in between
Week 5 (end) → External ears begin to develop behind the mandible
Week 6-7 →medial nasal prominences merge with each other and with maxillary
                    prominences, to form intermaxillary segment = premaxilla
Week 7-12 → lateral palatine processes grow medially to fuse I midline with one another,
                        with nasal septum, and with the anterior palate (premaxilla).
                        Fusion proceeds form anterior to posterior ending at uvula at 12 weeks

 

What happens when any of this goes wrong? 

 

Developmental problems of the Face:
Lateral Cleft Lip:
§Most common developmental problem of face (1/1000 births)
§More common in males
§Failure of maxillary prominence to fuse with the medial nasal prominence on affected side
§Unilateral or bilateral
§Range from small clefts to large fissures

 

Cleft Palate:
•Less common (1/2500 births)
•More common in females
•Greatly varied in position and severity
•Clefts of Secondary Palate = Failure of lateral palatine processes to fuse with each other and with the nasal septum at midline
•Clefts of Primary Palate = failure of the lateral palatine processes to fuse with the primary palate

 

Median Cleft Lip:
§Very rare
§Failure of medial nasal processes to fuse in midline
§Result: no intermaxillary segment formation (and therefore do not get anterior palate or premaxilla)

 

Oblique Facial Clefts
§Extremely rare
§Often bilateral
§Failure of the maxillary prominence to fuse with the lateral (and medial)nasal prominence on affected side. 
§Characterized by an open nasolacrimal duct exposed on surface
Various combinations of facial defects can exist (ex. anterior cleft lip with cleft jaw), though cleft palate and cleft lip

These defects can lead to changes in appearance—are repaired surgically, and can also lead to changes in speech, and affect breathing and eating.

Medial clefts are most severe since they are usually associated with other abnormalities.
Cleft palate can cause defective hearing.

These defects can also lead to serious problems with development of teeth (missing or extra teeth) since they affect tooth bearing parts of the upper jaw.