Table 4:

Summary of factors related to facial abnormalities discussed in the text

Factor	Some Observed Abnormalities
RA	Excess leads to fusion of 1st and 2nd branchial arches and acoustic-facial ganglia, small jaws, cleft palate, deformed pinna (Treacher-Collins syndrome); RA controls Shh and Fgf8 levels
Fgf	Controls outgrowth of facial primordia and migration of neural (Fgf) crest cells to facial processes; a decrease in FBGFR1 leads to midline clefting and Kallmann syndrome, small face and skull, achondroplasia, Crouzon syndrome, Apert syndrome
TGF	TGFβ required for fusion of lateral palatal processes; a decrease leads to defects in maxillary and mandibular development
BMP	A decrease leads to short frontal and nasal bones and small pterygoid processes, short stature, ear defects, odontogenic patterning defects, slower neural tube closure, small branchial arches, loss of incisor teeth
Shh protein	A decrease leads to holoprosencephaly, hypotelorism; an increase leads to a wide forehead, frontonasal dysplasia, Gorlin syndrome, Grieg cephalopolysyndactyly, Smith-Lemli-Opitz syndrome
Wnts	A decrease leads to loss of teeth, truncation of jaw, mesencephalic nucleus, and trigeminal nerve
ET-1	A decrease leads to aplasia of 1st and 2nd arches, defects in maxilla and cleft palate, malformations of middle and external ear; 22q11.2 deletion syndrome (CATCH22 syndrome)
Jagged 1 and 2	A decrease leads to Alagille syndrome, failure of palatal shelves to elevate, and fusion of shelves with tongue
Platelet-derived growth factors	A decrease leads to loss of some facial bones
Homeobox-containing genes	A decrease leads to primitive facial morphology, cleft palate, short maxilla and mandible, loss of maxillary molar teeth, ankyloglossia

Note:—RA indicates retinoic acid; FGFR, fibroblast growth factor receptor; PDGFR, platelet-derived growth factor receptor.