The reopening process of the esophagus in the normal chick and the crooked neck dwarf mutant.

In an earlier publication (Allenspach & Hamilton, 1962) an account was given of chick esophageal development and histogenesis up to 10 days of incubation. The esophagus becomes completely occluded at 5 days (stage 26, Hamburger & Hamilton, 1951) and reopens rapidly at 7-J-8 days (stages 33-34). Detailed observations were made of the histological changes in the organs as correlated with the localization of ribonucleic acid and alkaline phosphatase. The occlusion of the developing esophagus was apparently due to collapse and adhesion of roof and floor epithelia to form a solid epithelial bar and not to cell proliferation, as stated in Lillie's Development of the Chick (Hamilton, 1952, pp. 384-5). This process was the object of further investigation in which the role of proliferation in occlusion was determined by the use of colchicine. Results indicated that cell proliferation is not involved in the closing process (Allenspach, 1964). The mechanisms which control the reopening process have also been partially elucidated. Reopening is initiated by the appearance of bilateral primary vesicles. Subsequent fusion of primary and secondary vesicles with concomitant desquamation of degenerating epithelial cells restores the lumen (Allenspach & Hamilton, 1962; Allenspach, 1964). During this period the esophageal epithelium changes from non-oriented cells in the occluded region to a stratified epithelium similar to that of the definitive organ. Reorganization is initiated around the most lateral vesicles. Another subtle mechanism apparently operates in the reopening process. Orientation of dorsal, epithelial cells suggested they were the object of vertical tension. By treating the esophagus with versene, a compound which disaggregates tissues, a cleft was induced between roof and floor. This suggests that (1) adhesions play a significant role during occlusion, and (2) forces exerted on the dorsal epithelium assist in separating the roof from the floor (Allenspach, 1964).