The developmental precursors of the nose are the neural crest cells, which commence their caudad migration toward the midface around the fourth week of gestation. Two nasal placodes develop inferiorly in a symmetrical fashion. Nasal pits divide the placodes into medial and lateral nasal processes. The medial processes become the septum, philtrum, and premaxilla of the nose, whereas the lateral processes form the sides of the nose. Inferior to the nasal complex, the stomodeum, or future mouth, forms. (See the image below.)
A nasobuccal membrane separates the oral cavity inferiorly from the nasal cavity superiorly. As the olfactory pits deepen, the choanae are formed. Primitive choanae form initially, but with continued posterior development, the secondary, or permanent, choanae develop. By 10 weeks, differentiation into muscle, cartilage, and bony elements occurs. Failure of these carefully orchestrated events in early facial embryogenesis may result in multiple potential anomalies, including choanal atresia, medial or lateral nasal clefts, nasal aplasia, and polyrrhinia.
Neonates are obligate nasal breathers for the first 6 weeks. When bilateral choanal atresia is present in a neonate, emergency action is needed.