Light and electron microscopic observations of the development of intestinal musculature in Xenopus.

In the developing intestine of Xenopus the inner circular muscle layer is always thicker and is set up earlier than the outer, longitudinal one. First undifferentiated myoblasts appear between the intestinal epithelium and serosa in the stage 41 (NIEUWKOOP and FABER, 1967). Two crossed muscle layers appear not earlier than stage 50. Then the outer, longitudinal layer consists of single undifferentiated cells, while the inner one is already built of several rows of myocytes. Their ultrastructure and distribution were examined with TEM and SEM. The differentiation of myocytes examined at ultrastructural level was found to be more advanced in the inner layer than in the outer one. However, only after metamorphosis the smooth myocytes of each layer display all the features of mature muscle cells and contain large numbers of the characteristic elements: dense bodies and caveolae. The latter were not found in the larvae and tadpoles. The increase of collagen contents in the submucosa but not between the layers was also observed. Its possible role in the arrangement of the muscular coat is discussed.