Apoptosis and cell proliferation in the Xenopus small intestine during metamorphosis

In the amphibian small intestine, the epithelial transformation from the larval to adult type is mainly the result of degeneration of the larval epithelium and development of the new (adult) epithelium. In this analysis at the cellular level, we chronologically examined apoptosis and cell proliferation in the Xenopus intestine by using in situ nick end-labeling of genomic DNA (TUNEL) and bromodeoxyuridine (BrdU) immunohistochemistry. During pre- and prometamorphosis, few apoptotic cells were detected by TUNEL, and a small number of proliferating cells randomly distributed in the larval epithelium were labeled by BrdU. At the beginning of the metamorphic climax, when primordia of the adult epithelium were first detected, numbers of apoptotic cells suddenly increased in the larval epithelium, whereas numbers of proliferating cells increased only in the adult epithelium. Subsequently, a dramatic cell loss of the larval epithelium and a rapid growth of the adult epithelium occurred. Following complete epithelial replacement, the adult epithelium became differentiated into a simple columnar epithelium possessing a cell renewal system similar to that of mammalian intestinal epithelium. These results indicate that larval epithelial apoptosis progresses simultaneously with active proliferation of the adult epithelium during the early period of metamorphic climax, which coincides with the modification of the basement membrane lining both types of epithelia.