The formation of the primitive choanae and the junction of the primary and secondary palates in the mouse.

This study has followed the development of the primary choanae in the mouse and has shown that they originate at the developmentally strategic position of junction between the primordia of the primary and secondary palates and that this basic anatomic relationship is maintained throughout further development. Involution of the oronasal membrane begins late in the 11th day (stage 19) with the formation of interstitial gaps. The gaps enlarge and coalesce so that a completely patent opening between nasal passage and stomodeum is established by 13 days (stage 21). The membrane consists of two layers of simple squamous epithelium which become separated as involution progresses. The form of the choanal antrum changes from a simple funnel-shaped ellipse early in the 13th day to a complex slitlike opening within the following 24 hours. This coincides with the completion of a definitive primary palate and the enlargement and elevation of the shelves of the secondary palate. The maturation of the incisive papilla and interchoanal columella is related to the final stages of choanal morphogenesis. Thus, by stage 22 (14 days) the shape of the primary choanae and their anatomic relationship to the primary and secondary palates are established, and they remain essentially unchanged in later stages.