Embryology of the Nose

At the end of the fourth week of fetal development, mesenchymal cells of neural crest origin start to aggregate to form the facial prominences in the midface. On either side of the frontonasal prominences, nasal placodes, bilateral thickening of surface ectoderm, are formed. During the fifth week, the nasal placodes invaginate to form the nasal pits, and the tissue ridges surrounding the pits form the lateral and medial nasal prominences. The maxillary prominences continue to expand medially, shifting the medial nasal prominences toward the midline in the following 2 weeks. The two medial nasal prominences eventually fuse, giving rise to the medial portion of the upper lip and anterior palate ( Fig. 1.1a ). Cleft lip is associated with inadequate contact between the maxillary prominences and the intermaxillary segment. Cleft palate occurs secondary to failure of the lateral palatine processes to properly fuse. The furrow between the lateral nasal prominence and the maxillary prominence involutes to become the nasolacrimal duct. Ultimately, the external nose is derived from five different facial prominences; the frontal prominence forms the nasal bridge, the fused medial nasal prominences give rise to the tip, and the lateral nasal prominences become the alae.2,3

During the sixth week of development, the nasal pits deepen to form a primitive nasal cavity. The oronasal membrane initially separates the nasal cavity from the oral cavity but subsequently breaks down to give rise to the primitive choana. With the development of the secondary palate, the definitive choana is formed at the junction of the nasal cavity and the pharynx. The nasal septum is developed from the frontonasal prominence that extends caudally to fuse with the palate.2

Embryology of the Paranasal Sinuses

The precise embryology of the lateral nasal wall and paranasal sinuses is somewhat disputed in the literature.3–7 However, the traditional teaching is that there are a series of folds on the lateral nasal wall called the ethmoturbinals that appear during the eighth week of gestation ( Fig. 1.1b ). Five to seven folds initially appear, but after a process of fusion and regression, three or four folds remain by week 15. These folds are considered ethmoid in origin, and they ultimately become upper turbinates in the lateral nasal wall.8 The first ethmoturbinal regresses during the development period and remains at its rudimentary form; the ascending portion becomes the agger nasi, and the descending portion forms the uncinate process. The second ethmoturbinal eventually develops into the middle turbinate, and the third ethmoturbinal forms the superior turbinate. The fourth and fifth ethmoturbinals are thought to fuse and become the supreme turbinate. A separate ridge of maxillary origin known as the maxilloturbinal is formed inferior to the ethmoturbinals, giving rise to the eventual inferior turbinate. Interestingly, some researchers hold that the inferior, middle, and superior turbinates are identifiable at week 8 and that they develop directly from the cartilaginous nasal capsule; therefore, they propose that the embryologic terms used above are unnecessary5 ( Fig. 1.1c ).

The primary furrows that form between the ethmoturbinals ultimately give rise to the various meatuses and recesses. The first primary furrow is formed between the first and second ethmoturbinals. The descending portion of the first primary furrow forms the ethmoid infundibulum, hiatus semilunaris, and middle meatus. The ascending portion participates in the formation of the frontal recess. The second and third primary furrows become the superior and supreme meatus, respectively.

In addition to the development from the ridges and furrows, the paranasal sinuses receive contribution from a cartilaginous capsule that surrounds the nasal cavity. Some investigators proposed that this cartilaginous nasal capsule plays the main role in the development of the paranasal sinuses and lateral nasal wall structures, and that the development of the ridges and furrows is a secondary phenomenon.5 The detailed mechanism of the development of the paranasal sinuses is still debated, but it is clear that all the paranasal sinuses originate from the ethmoid region.9

The maxillary sinus develops as an outpouching between the middle and inferior turbinates. It is the first sinus to develop, beginning its invagination process during the third gestational month. It continues to undergo growth after birth, with periods of rapid growth typically at the times of dental development.10 The ethmoid sinus is thought to start out as multiple invaginations from the lateral wall of the nasal capsule around the fifth month of development.10 The sphenoid sinus originates from an outpouching from the posterior aspect of the nasal capsule during the third month of gestation. Though minimal in size at birth, the sphenoid bone undergoes pneumatization during childhood, and the sinus reaches its adult size between the ages of 9 and 12.8 The development of the frontal sinus starts with the anterior pneumatization of the frontal recess into the frontal bone around week 16 of gestation. Several folds and furrows develop within the frontal recess that eventually give rise to the agger nasi cell (first frontal furrow), frontal sinus proper (second frontal furrow), and anterior ethmoid cells (third and fourth frontal furrows).11 Pneumatization into the frontal bone does not start until 6 months to 2 years after birth, and radiologic evidence of the sinus is not usually seen until the age of 6 or 7. The two frontal sinuses are typically asymmetric, with 10 to 12% of the adult population displaying only one pneumatized frontal sinus.12 Up to 4% of the population lacks both frontal sinuses.13

Beyond the scope of this chapter, but worth noting, is that there are a variety of congenital malformations that can occur as a result of abnormal nasal and paranasal sinus development. Notable abnormalities include congenital midline masses such as encephaloceles, nasal gliomas, and dermoid cysts. Also observable at the midline of the posterior nasal airway in the nasopharynx are Thornwaldt cysts.

Surface Anatomy

Familiarizing oneself with the surface anatomy of the nose and its relationship to the facial contours is essential not only for aesthetic nasal surgery, but also for effective communication with other physicians. Traditionally, the ideal face is thought to be divided into aesthetic thirds of approximately equal length: upper, middle, and lower thirds14 ( Fig. 1.2 ). The upper third spans from the trichion to the glabella, where the trichion is the junction between the hairline and the forehead, and the glabella is the anteriormost point of the forehead at the midline. The middle third covers the structures from the glabella to the subnasale, which is the midline point of where columella transitions to the upper lip. The lower third of the face refers to the segment from the subnasale to the menton. The menton is the most inferior point of the chin. These surface landmarks are shown in Fig. 1.2, along with other anatomical landmarks of the face, and their definitions are found in Table 1.1.

Nasal Framework

The nose is a pyramidal structure that consists of bony, cartilaginous, and membranous elements. It sits on an almond-shaped bony opening into the skull called the pyriform aperture, which is bounded by the alveolar processes of the maxillae ( Fig. 1.3 ). The alveolar processes come together in the midline and project upward to form the anterior nasal spine. This fusion of the alveolar processes is where the nasal septum attaches to the floor of the nasal airway.15

The nasal pyramid consists of two nasal bones that articulate with both the nasal process of the frontal bone superiorly at the nasion and with the ascending processes of the maxilla laterally. The deepest point along the nasal profile ascending toward the glabella is called the radix. The nasion is the midline point deep to the radix that represents the suture line between the nasal and frontal bones ( Figs. 1.2 and 1.4 ). The terms radix and nasion are often used interchangeably, but they technically represent two distinct anatomical landmarks. The ascending processes of the maxilla are beveled laterally in an interlocking fashion with the nasal process of the frontal bone, anchoring them firmly to the pyriform aperture16 ( Fig. 1.3 ). Internally, this is also the approximate area known as the agger nasi, or nasal mound (see below).

The lower half of the nasal pyramid consists mostly of paired cartilages: upper lateral cartilages and lower lateral cartilages, along with several smaller sesamoid cartilages (also known as accessory cartilages) ( Fig. 1.4 ). The triangular upper lateral cartilages articulate with the nasal and maxillary bones superiorly and overlap with the lower lateral cartilages inferiorly. They are contiguous with the septal cartilage superiorly, adding to the integrity of the cartilaginous nasal dorsum.16

The lower lateral cartilages are thin, curved structures that form the shape of the nasal tip and define the integrity of the nostrils. Each lower lateral cartilage is divided into the medial crus and the lateral crus. The broader lateral crus extends posterolaterally into the ala of the nose, maintaining the patency of the nostril, whereas the narrower medial crus extends caudally along the free edge of the nasal septum, delineating the projection of the nasal tip. There is dense connective tissue binding the upper lateral cartilages to the lower lateral cartilages, and the multiple small accessory (sesamoid) cartilages embedded within fibroareolar tissue add to the integrity of the nasal alar structure.16

Nasal Musculature

The muscles of the nose can be categorized into elevators, depressors, dilators, and compressors.17 The elevators are the procerus, the levator muscle of the upper lip and ala (levator labii superioris and levator labii superioris alaeque nasi), and the anomalous nasi. The depressor muscles of the nose are the alar portion of the nasalis muscle and the depressor nasi septi labii. The anterior dilator naris works to dilate the nostrils, whereas the transverse portion of the nasalis and the compressor narium minor are the compressors15 ( Fig. 1.5 ). All the nasal muscles are innervated by the zygomatic and buccal branches of the facial nerve (cranial nerve [CN] VII), although the procerus receives contribution from the frontal branch of the facial nerve as well.17

Blood Supply to the External Nose

The blood supply to the external nose varies, but it receives contributions from the external carotid via the facial artery and the infraorbital artery, and the internal carotid via the ophthalmic artery.15,16 The lateral nasal artery arises from the angular artery (from the facial artery) that anastomoses with the dorsal nasal artery (from the ophthalmic artery). This arcade receives additional contributions from the infraorbital branch of the internal maxillary artery and the external nasal artery, which is the terminal branch of the anterior ethmoid artery ( Fig. 1.6 ). The venous drainage of the external nose is performed by the angular vein and the ophthalmic vein, which in turn can communicate with the cavernous sinus.

Innervation of the External Nose

The skin of the external nose is innervated by the trigeminal nerve system. The supratrochlear and infratrochlear branches of the ophthalmic nerve (CN V₁) supply the skin of the root, bridge, and upper half of the side of the nose. The infraorbital branch of the maxillary nerve (CN V₂) supplies the skin of the lower half of the side of the nose. The external nasal branch of the anterior ethmoid nerve exits between the nasal bone and the upper lateral cartilages to supply the skin over the dorsum of the nose ( Fig. 1.6 ).16

Nasal Vestibule and Nasal Valve

The nasal vestibule is a dilation inside the nostril that corresponds to the ala of the external nose. It is lined with skin that contains hair (vibrissae), sweat glands, and sebaceous glands. Separating the nasal vestibule from the rest of the nasal cavity is a ridge along the lateral nasal wall called the limen nasi (limen vestibuli). It corresponds to the caudal end of the upper lateral cartilage and marks the transition from the keratinizing squamous epithelium to the pseudostratified columnar ciliated epithelium of the mucous membrane.16 The mucous membrane contains numerous mucous and serous glands.

The nasal valve itself is a slitlike structure associated with the entrance to the nasal passages. The nasal valve has both external and internal components. It has been described anatomically as the cross-sectional area of the nasal cavity with the greatest overall resistance to airflow, thus acting as the dominant determinant for nasal inspiration. Even the smallest lesion in the area can substantially affect the overall airflow through the nasal passage ( Fig. 1.7a ). The external nasal valve is defined as the area in the nasal vestibule under the nasal ala, formed by the caudal septum, the medial crura of the alar cartilages, the alar rim, and the nasal sill. The internal nasal valve is located ~1.3 cm from the nares (nostril opening) and corresponds to the region under the upper lateral cartilages, bound medially by the dorsal septum, inferiorly by the head of the inferior turbinate, and laterally by the upper lateral cartilage18 ( Fig. 1.7b ).

Although the exact teleological reason for the nasal valve is still debated, several theories exist. Inhalation against resistance in the upper airway yields higher intrathoracic pressure, which in turn promotes the alveolar gas exchange by prolonging the expiratory phase of breathing. Also, the nasal valve disrupts laminar airflow within the nasal cavity, and the resulting turbulent flow increases the interface time between odorants and the olfactory neuroepithelium.

Nasal Septum

The nasal septum serves as both a functional and an aesthetic unit, dividing the nasal cavity into right and left sides and providing major support for the external nose and an extended surface area for the mucosa. The septum extends from the columella to the rostrum of the sphenoid sinus, where the posterior choanae open into the nasopharynx. The septum has three components: the membranous septum, the cartilaginous septum, and the bony septum. The majority of the septum is formed by the perpendicular plate of the ethmoid bone posteriorly and the quadrangular (also known as quadrilateral) cartilage anteriorly. The vomer (Latin for “plowshare”) is a wedge-shaped bone situated in the posteroinferior portion of the septum. In the inferior portion of the septum, the nasal crests of the maxillary and palatine bones attach to the cartilaginous and bony septum at the floor of the nasal cavity ( Fig. 1.8 ).

The membranous portion of the septum, the caudal-most portion, is composed of skin and connective tissue. It is supported anteriorly by the medial crura of the lower lateral cartilages. The cartilaginous septum, which sits just posterior to the membranous septum, is formed predominantly by the quadrangular cartilage. The quadrangular cartilage flares superiorly to fuse with the upper lateral cartilages at the nasal dorsum. Posteriorly, it gives rise to a thin, tail-like process that inserts between the vomer and the ethmoid bone. The cartilage widens inferiorly at the base as it articulates with the maxillary crest and the anterior septal body17 ( Fig. 1.9 ). The anterior septal body is an area of thickened mucosa with underlying pseudoerectile tissue that is located just anterior to the leading edge of the middle turbinate. The pseudoerectile tissue of the nasal airway appears to have an important role in the “nasal cycle” that helps maintain normal nasal physiology. The bony septum lies posterior to the cartilaginous septum and consists of the perpendicular plate of the ethmoid bone and the vomer. It is a common site of septal deviation and septal spurs. Septal deviation is very common and has a variety of shapes. In one anatomical study of adult skulls, only 21% of the nasal septa were straight; 37% were deviated and 42% kinked.19 The perpendicular plate of the ethmoid complex articulates with the frontal and nasal bones superiorly and with the sphenoid bone posteriorly. As seen in Fig. 1.8, it articulates with the vomer and the quadrangular cartilage as well. The alae of the vomer rest on the sphenoid rostrum. Along the inferior border of the quadrangular cartilage lies a small bar of cartilage called the vomeronasal cartilage, which is the site of the rudimentary vomeronasal organ (of Jacobson).17 The superior aspect of the septum is partially covered by olfactory neuroepithelium that is responsible for the sense of smell. The olfactory neuroepithelium provides direct communication between the external environment and the brain by sending fibers centrally through the porous cribriform plate to synapse with the olfactory bulb. The region situated between the uppermost septum and the lateral nasal wall is referred to as the olfactory cleft (see below). Additionally, sensory fibers of CN V descend through the cribriform plate to supply the nasal cavity and even more important through the sphenopalatine fossa.

Lateral Nasal Wall

Whereas the medial wall of the nasal cavity is relatively simple in its anatomy, the lateral nasal wall displays complicated anatomy with multiple raised structures, clefts, and openings, working as the interface between the paranasal sinus cavities and the nasal cavity. The osteology of the lateral nasal wall is depicted schematically in Fig. 1.10a. Seen on the lateral nasal wall are three or four turbinates (or conchae) that are thin, medially projecting scrolls of bone ( Fig. 1.10b ) covered by mucous membrane.2 They are the inferior turbinate, middle turbinate, superior turbinate, and occasionally the supreme turbinate, going from inferior to superior along the wall. The space lateral and inferior to each turbinate is named according to the structure with which it is associated. For example, the inferior meatus lies underneath the inferior turbinate ( Figs. 1.10b and 1.11 ). As stated previously, only the inferior turbinate is embryologically a separate bone. The other turbinates are part of the ethmoid complex.

Like the anterior septal body, the inferior turbinate is lined with pseudoerectile tissue and is covered by a thick mucous membrane. The inferior meatus houses the opening to the nasolacrimal duct (valve of Hasner), which is usually located superolaterally in the anterior portion of the inferior meatus ( Fig. 1.11 ). Thus, any effort to enter the maxillary sinus from the inferior meatus is typically advised through the thinner bone more posteriorly, where the risk of lacrimal duct injury is less.

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