Material

esophagus, lumen, cells, epithelium and nuclei

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Where the wide pharynx passes into the oral end of the esophagus the lumen becomes triangular in cross-section, the base directed dorsally. The esophagus at this level is lined by 5 to 6 layers of cells throughout. The peripheral and central cells are cylindrical in shape, those of the intermediate layers have a polyhedral form, and many of the latter are in mitosis. At a slightly lower level the dorsal portion of the lining epithelium thickens, while in the ventral portion, i. e., along the tracheal evagination, the epithelium becomes somewhat thinned. The shape of the lumen at this level, including here in con tinuity a portion belonging to the esophagus proper and a portion belonging to the laryngo-tracheal groove, is oval in transection, flattened from side to side.

At the level of bifurcation of the trachea the lumen of the esophagus has a narrow, spindle-shaped outline, compressed from side to side. This lumen is lined by a pseudo-stratified columnar epithelium, con taining 4 rows of nuclei dorsally and 2 to 3 ventrally. The res piratory anlage, of rectangular outline, flattened dorso-ventrally, is lined by a similar type of epithelium with about 3 rows of nuclei. Mitosis is active among the cells immediately surrounding the lumen. Terminal bars are conspicuous, especially in the tracheal anlage.

The ventral laryngo-tracheal groove becomes separated from the primitive esophagus, and is consequently converted into a tube, by a process involving the approximation and ultimate fusion of bilateral mesenchymal plates, which process extends in a caudo-oral direction. In the area of the advancing residual connection between the separating tracheal and esophageal tubes, i. e., at the level of constriction pre ceding the separation, the cells are evidently under considerable pres sure. The nuclei are in general much smaller, of very variable size, and greatly crowded. They are spheroidal in outline and many show signs of degeneration. Many of these nuclei are vesicular and prac tically achromatic, except for a relatively large and very chromatic nucleolus. None of these cells are in mitosis, but many of the nuclei are at some phase of amitotic division. In the surrounding mesen chyme, however, especially in the plates which invaginate the eso phageo-tracheal tube in the process of separation, mitosis is very active. Along the line of separation, the nuclei in the ventro-medial epithelium of the esophagus and in the dorso-medial epithelium of the trachea are much smaller, more nearly spherical, and more crowded than elsewhere in these tubes. A few smaller spheroidal yolk-globules are present in

the lining-cells of both the esophagus and trachea at these stages, but they are relatively more numerous in the esophageal epithelium.

In the 12-day embryo the esophagus practically lacks a lumen (fig. 3). At the level just behind the point of connection between the primitive esophagus and the trachea (definitive pharynx and larynx) the narrow circular lumen of the trachea is lined with 3 to 4 layers of cells, some of which, especially centrally, are in mitosis. The esophagus, just cephalad of this point (i. e., in the region of the glottis) has a T-shaped lumen, lined dorsally with about 9 layers of cells, of cylindric form peripherally and centrally and of polyhedral form in the intermediate layers. Viewed as a whole—i. e., the lumen and its cells—the esophagus is still more distinctly T-shaped, the cross-bar in the esophagus proper being about equal to or even greater in length than the stem or the laryngeal anlage.

At the level here illustrated, figure 3, the minute central opening of the esophagus is surrounded by only 3 to 4 layers of cells, as indi cated by the rows of nuclei. By the thirteenth day even this capillary lumen is obliterated and the esophagus is completely closed for about 0.5 mm. This closure is apparently effected by a process of lateral traction (or possibly dorso-ventral compression) upon the originally cylindric tube, causing thus a change in shape of the lumen from an oval or a circular to a horizontal cleft form. This mechanical factor is assisted by the larger number of cell-layers dorsally, the result of greater proliferative activity, and accomplishes finally a close apposi tion and ultimate fusion of the dorsal and ventral walls. One small lateral vacuole was seen in the epithelial lining of the stenosed esoph agus at the 12-day stage; at the 13-day stage three lateral vacuoles had appeared.

The central area of the closed esophagus of this stage consists of irregular stellate cells, resembling a compact mesenchymal reticulum. These cells come chiefly from the dorsal thicker wall, and represent modification products of originally cylindric cells.

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