Morphogenic mechanisms in the development of ethmoidal sinuses

The current teaching in regard to the development of paranasal sinuses is that they are evaginations of the nasal cavity; this seems to totally ignore earlier observations.

Serial coronal sections of the heads of 23 human fetuses from 18-mm CR length to 282-mm CR length were stained by various methods and studied.

The ethmoidal sinuses develop by the formation of 'turbinal cushions' at the 33-mm CR stage. By the 45-mm CR stage, the cushions have grown toward each other and made contact. The epithelium covering the cushions also proliferates to fill up the enclosed space, and it proliferates dorsoventrally as well. The cushions fuse at the 50-mm CR stage. The fusion part of the process should be described as a 'constriction' of the nasal cavity. Next, the epithelium disintegrates and results in the formation of the sinus. This part of the process only can be described as an 'evagination' of the nasal cavity. As the advancing epithelium of the sinus encounters cartilage, it causes disintegration of the developing nasal cartilage. This area of disintegration appears as a spot stained pink with H & E, black with mucicarmine, and yellow with van Gieson's stains, respectively. These spots are seen between the 89-mm and the 225-mm CR stages (both inclusive).

Two mechanisms operate in the development of ethmoidal sinuses: constriction of the nasal cavity by a pair of turbinal cushions, and evagination from the nasal cavity by proliferation and subsequent disintegration of the nasal epithelium.