A histological and dynamic study of the gastric region of Discoglossus pictus larvae, cultured with or without thyroxine.

The organotypic culture of the gastric region is carried out on premetamorphic Discoglossus pictus larvae. Adding thyroxine to the culture medium provokes various transformations. On the cytological level, the reactions observed, which are variable depending on the cell category concerned, can be divided into two types of phenomena: histolytic and histogenetic. Autophagia linked to lysosome intervention is frequently found among the histolytic processes. Autophagic vacuoles and residual bodies are observed. The gastric lumen is filled with deteriorated cells that probably come from the degeneration of the tadpole epithelium (primary epithelium). The incorporation of tritiated thymidine makes it possible to study the evolution of cell proliferation in the control and in the thyroxinated cultures. After a 1-2 day latency period, possibly due to the adjustment of the tissue to the culture environment, the incorporation of the radioprecursor H3-thymidine into the epithelium and the tunica muscularis of thyroxine-treated gut tissue increased on day 3, reached a maximum on day 5, and then dropped slightly on day 7. In the control cultures H3-thymidine incorporation showed the same pattern but lower levels on the same days. The histolytic phenomena induced by thyroxine in vitro are comparable to those of natural metamorphosis. On the other hand, the histogenetic phenomena are incomplete. Proliferating and transitional phases occur but neoformated (or secondary) epithelium does not replace the degenerated primary epithelium, whatever the culture time.