Embryology of Nose and Sinuses
Developmentally nose and paranasal sinuses are interlinked. They are always considered together developmentally. Developmentally the various sinuses may follow different calenders, their orgin is the same.
Development of head and neck along with face, nose and paranasal sinuses takes place simultaneously in a short window span. At the end of 4th week of development branchial arches, branchial pouches and primitive gut makes their appearance. This is when the embryo gets its first identifiable head and face with an orifice in its middle known as the stomodeum.
The stomodeum (primitive mouth) is surrounded by mandibular and maxillary prominences bilaterally. These prominences are derivatives of first arch. This arch will give rise to all vascular and neural supply of this area. The stomodeum is limited superiorly by the presence of frontonasal eminence and inferiorly by the mandibular arch.
The frontonasal process inferiorly differentiates into two projections known as “Nasal Placodes”. These nasal placodes will be ultimately invaded by growing ectoderm and mesenchyme. These structures later fuse to become the nasal cavity and primitive choana, separated from the stomodeum by the oronasal membrane. The primitive choana forms the point of development of posterior pharyngeal wall and the various paranasal sinuses.
The oronasal membrane is fully formed by the end of 5th week of development. It gives rise to the floor of the nose (palate develops from this membrane).
Figure showing development of nose and pns
The continuing growth of embryo brings both the nasal placodes and the maxillary processes together in midline to form the maxilla and the beginning of the external nose. The frontonasal prominence gives rise to inferior mesodermic projection which goes on to form the nasal septum dividing the nose into two cavities. Simultaneously the skull and facial bones also develop. The skeletal system develops from mesoderm. Each cranial bone is formed by a series of bone spicules that grow from the centre towards the periphery. At birth all the cranial bones are separated by layers of connective tissue which later fuses and ossifies in the postnatal period.
At about 25 – 28 weeks of gestation, three medially directed projections arise from the lateral wall of the nose. This serves as the beginning of the development of paranasal sinuses. Between these projections small lateral diverticula invaginate into the primitive choana to eventually form the meati of the nose.
The medial projections arising from the lateral wall of the nose forms the following structures:
The anterior projection forms the agger nasi
The inferior (maxilloturbinate) projection forms the inferior turbinate and maxillary sinus
The superior projection (ethmoidoturbinate) forms the superior turbinate, middle turbinate, ethmoidal air cells and their corresponding drainage channels. The middle meatus develops between the inferior and middle meatus.
Figure showing infundibulum, and meati of nose developing
The middle meatus invaginates laterally to form the embryonic infundibulum and uncinate process. During the 13th week of development the embryonic infundibulum grows superiorly to form the frontal recess area.
Development of frontal sinus: The frontal sinus may develop as a direct continuation of embryonic infundibulum and frontal recess superiorly during the 16th week. It can also develop by upward migration of anterior ethmoidal air cells to penetrate the inferior aspect of the frontal bone between its outer and inner tables. Pneumatization of frontal bone is a very slow process. The frontal sinus infact remains as a small blind sac within the frontal bone till the child is about 2 years of age, then secondary pneumatization begins. From the age of 2 till the child becomes 9 years old secondary pneumatization of frontal bone proceeds. When the child reaches the age of 9, the development of the frontal sinus has reached completion. Sometimes frontal sinus may be asymmetrical / aplastic as well.
Figure showing development and enlargement of frontal sinus. The numbers indicate the size of frontal sinus at that corresponding age.
The embryonic infundibulum may also invade the mesenchyme in the maxillary process forming the primitive maxillary sinus. Pneumatization of maxillary sinus is faster than that of frontal sinus. Pneumatization occurs at the expense of erupting upper dentition. Abnormalities of maxillary pneumatization is associated with anomalies of upper dentition.