Development of the connective tissue in the digestive tract of the larval and metamorphosing Xenopus laevis.

Developmental changes, until the completion of metamorphosis, of the connective tissue of the digestive tract in the South African clawed frog Xenopus laevis were examined by light and scanning electron microscopy. During the pre- and prometamorphosis (stages 50 through 59) the layer of connective tissue was prominent in the typhlosole, while that in the rest of the intestine and stomach remained very thin. Most of the connective tissue cells were fibroblasts and were concentrated mainly in the typhlosole, the boundaries of which were distinct. In the anterior part of the typhlosole just behind the stomach, the layers of connective tissue and muscle were the thickest and the boundary between these 2 layers was obscure. Although the mitotic cells of the connective tissue were rare, if any, and no migrating cells were observed on the surface of the connective tissue, the connective tissue of the intestine gradually increased in cell number during the pre- and prometamorphosis. In the stomach, on the other hand, there was no such increase. At the beginning of the metamorphic climax (stages 60 and/or 61), the layer of connective tissue suddenly became thick in both the stomach and the intestine. The boundaries of the typhlosole became less distinct. Mitotic index of the connective tissue became high, and its cells dramatically increased in number and contained many undifferentiated mesenchyme-like cells. In the stomach, these connective tissue cells invaded the glands of the larval epithelium into just below the surface epithelium and mitotic index of the entire stomach epithelium increased. In the intestine, in contrast, mitotic epithelial cells were localized in the islets which invaginated into the connective tissue and later formed the adult epithelium. With the growth of islets, the intestinal folds were formed as straight rows and, after stage 63, were modified into a zigzag pattern and had mitotic cells localized in the troughs of folds. The present results indicate that dramatic changes of the connective tissue of both the stomach and the intestine occur at the beginning of the metamorphic climax, concomitantly with those of the epithelium. Moreover, the results also show that, before the climax, even when there were no indications of metamorphic changes in the epithelium, cells of the connective tissue gradually increased in number in the intestine (but not in the stomach). It is also suggested that the muscle cells of the typhlosole just behind the stomach may participate in this gradual increase of connective tissue cells.