The laryngeal primordium and epithelial lamina. A new interpretation

The laryngeal primordium is present in both the laryngotracheal sulcus (LTS) and the primitive pulmonary sac (PPS). Its early period of development may be subdivided into two phases. The first phase (Stage 11) is represented by what is traditionally referred to as the LTS, located directly beneath the PP4 on the ventral wall of the foregut (primary segment), and by the PPS which is situated at its caudal end. The LTS will represent the primordium of the upper or membranous infraglottic cavity region; whereas the PPS, will give rise not only to the bronchial tree, but also to the primordium of the trachea and the lower or cartilaginous region of the infraglottic cavity. The second phase (Stages 13 and 14) is distinguished by the cranial growth of the LTS above the PP4 and therefore by its absorption into the floor of the primitive pharynx in the mesobranchial area (secondary segment), which will develop into the primordium of the vestibule of the larynx. Similarly, we observed that in the development of the laryngeal cavity there are two temporally and spatially separate epithelial structures: the epithelial septum and the epithelial lamina. In this respect we differ from other authors who are of the opinion that there is a single structure (the epithelial lamina). The epithelial septum is a primary structure responsible for the final configuration of the LTS, as it contributes to the development of the lower end of the primary segment of the LTS and also to the creation of the secondary segment. The epithelial lamina is a secondary structure which appears inside the LTS as a result of pressure exerted by the mesenchyme on its lateral walls, without having any effect on the morphogenesis of the LTS.