Understanding the anatomy of the paranasal sinuses and surrounding structures is integral to performing safe and appropriate sinus surgery through endoscopic and open techniques. In addition, knowledge of the embryologic development of the paranasal sinuses allows for better comprehension of the spatial involvement of disease processes affecting the sinuses. Because the nasal and paranasal sinus structures develop from multiple bones, rather than a single bone, understanding the developmental relationship between these bones allows the surgeon to better evaluate and treat certain disease processes that affect the sinonasal cavities.
The primary bones from which the paranasal sinuses develop are the maxillary, ethmoid, sphenoid, and frontal bones. There are also lesser contributions to paranasal sinus development from the lacrimal and zygomatic bones. Development of the four sets of paired paranasal sinuses is discussed in detail here. The nasal septum develops from four sources: the perpendicular plate of the ethmoid bone, the maxillary bone (crest), the vomer, and the quadrangular cartilage.
Nasal Cavity Development
Early development of the sinonasal cavity begins during the 8th week of fetal life. At this time, the nasal septum can be seen dividing the right and left sides of the future nasal cavity. At 8 weeks’ gestation, the nasal septum is a mesenchymal structure that is partially differentiated into cartilage.1 Also beginning at 8 weeks, several ridges begin to develop along the lateral nasal wall.2 These lateral nasal wall ridges are the earliest signs of the developing turbinates. Surrounding the embryologic nasal cavity a cartilaginous capsule forms, and at 9 to 10 weeks the cartilaginous capsule contributes finger-like projections to the developing turbinates.2 Also between 9 and 12 weeks’ gestation, a separate cartilaginous and soft tissue bud forms between the developing middle and inferior turbinates.1,2 This bud will become the uncinate process ( Fig. 1.1 ).
The ridges along the lateral nasal wall, which will ultimately develop into the turbinates, have been reported to be of different origin by various authors throughout history. In 1895, Killian described the inferior turbinate as originating from the maxillary process and termed this developing structure the maxilloturbinal.3 Killian further described the more superiorly located ethmoturbinals as forming the middle and superior turbinates, with a small nasoturbinal forming the agger nasi region. Stammberger supports the maxilloturbinal origin of the inferior turbinate, but notes some subtle differences in the ultimate development of the five described ethmoturbinals, with approximately four of the ethmoturbinals remaining throughout development and eventually forming the agger nasi region (superior portion of first ethmoturbinal or nasoturbinal), middle turbinate (second ethmoturbinal), superior turbinate (third ethmoturbinal), and supreme turbinate (fourth and fifth ethmoturbinals).4 In contrast to Killian and Stammberger, Bingham et al. have described the inferior turbinate as arising from the cartilaginous nasal capsule along with the middle and superior turbinates and they do not support separate maxilloturbinal terminology.2
At 15 to 16 weeks’ gestation, the inferior, middle, and superior turbinates are clearly formed and easily visible in embryologic sections.2 As seen from developmental histologic sections, the middle and superior turbinates arise from precursors of the ethmoid bone, whereas the inferior turbinate bone is independent, receiving contributions of its final adult structure from the cartilaginous nasal capsule and the bone of the maxilla.2
Between the ethmoturbinal ridges are primary furrows, which will form the recesses and meatuses that separate the adult turbinates.4–6 The first and second ethmoturbinals are separated by the first primary furrow, which becomes the middle meatus, ethmoid infundibulum, hiatus semilunaris, and part of the frontal recess in the adult. The superior and supreme meatuses are derived from the second and third primary furrows, respectively. The extent of adult paranasal sinus pneumatization and development differs greatly from person to person. This is thought to result from the extent of invagination and evagination between the developing turbinates and their intervening furrows.5
Sinonasal Mucosa and Olfactory Development
In a histologic study of human fetal heads, Wake et al.1 have elegantly described the development of the sinonasal mucosa. In summary, as the nasal cavity begins to develop at 8 weeks’ gestation, a hypercellular mesenchymal capsule forms around the developing nasal structures. Although the majority of the nasal cavity contains undifferentiated cells or stratified cuboidal cells, the nasal septum is partially differentiated into cartilage at this time, and olfactory epithelium can be seen in the superior aspect of the nasal cavity. By 9 to 10 weeks, the cartilaginous nasal capsule has fully differentiated, ciliated pseudostratified columnar or cuboidal epithelium is seen on the septum and inferior turbinate, and primitive blood vessels are present. At 11 to 12 weeks, the septal epithelium has differentiated into characteristic ciliated respiratory epithelium and secretory goblet cells are present, but the lateral nasal wall mucosa continues to be less differentiated. The cribriform plate is present in cartilaginous form at 14 to 16 weeks, with neurovascular bundles penetrating it, and olfactory epithelium is present throughout the superior portion of the nasal cavity. Also at 14 to 16 weeks, stratified squamous epithelium with hair follicles can be seen in the nasal vestibule. The mucosal lining of the developing paranasal sinuses remains spherical or cuboidal with few cilia and glands. By 17 to 18 weeks, the lateral nasal wall and ethmoid sinus mucosa has matured to respiratory epithelium, with higher concentrations of goblet cells anteriorly and ciliated cells posteriorly. At 20 to 24 weeks, secretory cells are more evenly distributed and vascular structures are present throughout the lamina propria, resembling post-nasal development. Postnatally, there is partial regression of the olfactory epithelium such that it occupies only the area of the cribriform plate and superior turbinate.
The ethmoid sinus is the first to develop into detectable pneumatized cells in the fetus. Early anterior ethmoid cells, including the cartilaginous beginnings of the ethmoid bulla, form as a result of budding from the middle meatus around 11 to 12 weeks of fetal life.1,7 At 14 to 16 weeks some anterior ethmoid cells are well formed.1 Wake et al.1 report that, by 17 to 18 weeks, the posterior ethmoid buds begin to develop from the superior meatus. Ossification of the ethmoid sinuses and lamina papyracea has occurred by 20 to 24 weeks’ gestation.1,7
At birth the ethmoid sinuses are the most mature of the paranasal sinuses, being completely developed in the number of cells but not in size.8 In the newborn, the ethmoid complex is 8 to 12 mm long, 1 to 3 mm wide, and 1 to 5 mm in height. The ethmoid sinuses undergo significant growth during the first decade of life. Wolf et al.8 note the most rapid expansion of the ethmoid complex between 1 and 4 years of age, whereas Shah et al.9 have demonstrated the most rapid anteroposterior expansion of the ethmoid sinuses occurs between 3 and 8 years of age. By age 12, the ethmoid sinuses have essentially reached their adult dimensions.8,9 The ethmoid cells can, however, expand beyond the boundaries of the ethmoid bone to extend into the frontal recess (frontal cells, suprabullar cells, and frontal bullar cells), sphenoid bone (sphenoethmoid [Onodi] cell), and maxillary bone (infraorbital ethmoid [Haller] cell).
The ethmoid bone contains more than the ethmoid sinuses. Other structures that are derived from the ethmoid bone include the middle turbinate, superior turbinate, supreme turbinate, cribriform plate, and the posterosuperior portion of the nasal septum (perpendicular plate of the ethmoid).