Cell proliferation in the intestinal epithelium of Xenopus laevis tadpoles

Light microscope, electron microscope and autoradiographic techniques were used to determine whether, in the small intestinal epithelium of feeding tadpoles of Xenopus laevis, proliferation is limited to particular cells or to particular regions. Mitotic figures were classified as apical, medial or basal according to their position relative to the gut lumen. Apically dividing cells represent 75% of all mitoses and were identified on histological and ultrastructural criteria as specialized principal cells or rarely, as specialized gland cells. This interpretation was confirmed by autoradiographic evidence that specialized principal and gland cells incorporate (3H) thymidine into DNA. Basally dividing cells represent 25% of all mitoses and medial mitotic figures are very rare. Histological and ultrastructural evidence indicated that basal and medial mitotic figures are due to the division of intra‐epithelial lymphocytes and it was demonstrated that lymphocytes incorporate (3H) thymidine into DNA. Growth and replacement in the epithelium are therefore due to the mitotic activity of specialized cells. The results also suggest that all specialized cells can divide and that proliferation is not localized in particular regions of the epithelium as it is in other vertebrates. The significance of these conclusions for the interpretation of nuclear transplantation experiments using donor nuclei from intestinal epithelial cells is briefly discussed.