The early development of the digestive and respiratory systems is summarized in relation to embryonic staging. It has frequently been emphasized that the digestive and respiratory tubes do not arise from a common chamber, that they pursue separate courses as soon as the lung bud appears, that a mesenchymal septum comes to intervene between them, and that the two tubes rapidly acquire independent outer coats. Some commonly held views such as the supposed caudorostral separation of the trachea from the esophagus, have been shown to be incorrect. These ideas often arose from the use of unstable landmarks during development. It is pointed out that, for 3 weeks after its appearance, the tracheoesophageal separation point remains at a constant level, whereas the tracheal bifurcation descends. The application of valid embryological data to the interpretation of several congenital anomalies has been shown to be of value. Although the modes of origin in some instances are still obscure, considerable advance has been made in understanding the timing of the relevant events.

Congenital Abnormalities↗

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The human larynx at the end of the embryonic period proper. 2. The laryngeal cavity and the innervation of its lining.

The laryngeal cavity was studied in eight serially sectioned embryos of stage 23 and in three early fetuses, and graphic reconstructions were prepared. After the isolation of the tracheal from the pharyngeal cavity during stages 16 through 22, a communication (not necessarily the pharyngotracheal duct) appears again during stage 23. At this time (8 postovulatory weeks) the laryngeal cavity comprises 1) the coronal and parts of the sagittal clefts of the vestibule (uniting later at the laryngeal inlet); 2) the ventricles, which are not yet completely formed; and 3) the subglottic cavity, which appeared already in earlier stages. The characteristic events of stage 23 are the dissolution of the epithelial lamina and the development of the ventricles. The disruption of the epithelial lamina is an active process that comprises rearrangement and growth, but not loss of cells. The ventricles, which begin as solid outgrowths in stage 20, do not represent fifth pharyngeal pouches. They now point toward the middle of the still paired thyroid laminae and are not at the level of the future glottis, which lies more caudally. In the absence of the median part of the soft palate, the nasopharynx communicates widely with the oral cavity. The epithelium of the respiratory tube, including the larynx, resembles that of the pharynx and esophagus in being pseudostratified columnar and showing a clear basement membrane. It is ciliated over that part of the epiglottis that surmounts the arytenoid swellings, and also over the tip and back of the latter. The transitional area between the laryngopharynx and the esophagus is already innervated by the recurrent laryngeal nerve. Nerve fibers have not yet reached the epithelium of the coronal cleft and the ventricles, but fibers are present near the sagittal cleft of the vestibule. The sensory innervation of the pharynx and larynx has been followed and plotted for the first time in an embryo, and previously unrecorded silver-impregnated receptors have been observed.

Adult↗

PubMedDate not supplied

The human larynx at the end of the embryonic period proper. I. The laryngeal and infrahyoid muscles and their innervation.

The larynx at stage 23 (about 27-32 mm C-R, 8 postovulatory weeks) is being studied in detail, and findings on the musculature and its innervation are presented here. Serial sections of six embryos all belonging to the one developmental stage were investigated and compared, and graphic reconstructions were made from five of the specimens. All the infrahyoid and most of the major laryngeal muscles are present, and their innervation follows closely the adult pattern. The ary-epiglottic and thyro-epiglottic apparently may or may not be identifiable. The vocalis is beginning to differentiate and is noted here in the embryonic period proper for the first time. The histological degree of development of the intrinsic laryngeal muscles is not as advanced as that of adjacent muscles in the neck, including the infrahyoid group. Variation within this one stage of development are small but definite. They include differences in the position, level, and shape of the skeletal components, and the precise position and extent of development of some of the muscles.

Humans↗

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Anatomy↗