Switching of the Laryngeal Cavity From the Respiratory Diverticulum to the Vestibular Recess: A Study Using Serial Sagittal Sections of Human Embryos and Fetuses.

A cecum-like protrusion of the pharynx (the laryngeal cecum or vestibular recess [VR]) develops immediately anterior to the laryngeal part of the respiratory diverticulum. An expansion of the VR has been well described, whereas the fate of the diverticulum is still obscure, although its pharyngeal opening corresponds to the glottis. We observed sagittal sections of 10 embryos (five specimens at 5-6 weeks and another five at 7-8 weeks) and eight fetuses at 25-30 weeks. At 5-6 weeks, a lumen of the laryngeal part of the respiratory diverticulum appeared, and subsequently, the VR opened into the epithelial lamina. Because of this discrete separation, it seemed unlikely that the pharyngeal pouches contributed to the laryngeal epithelium. At 6-7 weeks, the VR exhibited a high boot-shaped lumen with canalization to the diverticular lumen at the level of the cricoid cartilage. Thus, in a midline area between the bilateral arytenoid cartilages, double laryngeal lumina were evident, separated by the thick midline epithelial lamina. At 25-30 weeks, the inferior part of the VR lumen had become enlarged because of the destruction of the epithelial lamina along the arytenoid and corniculate cartilages. In contrast, candidates for the initial diverticular lumen remained as epithelial slits in the anterosuperior side of the transverse arytenoid muscle. Therefore, the final anterior and lateral laryngeal walls seemed to originate from the VR with canalization, in contrast to the part of the posterior wall derived from the initial diverticular wall.