APOPTOSIS AND CELL-PROLIFERATION IN THE XENOPUS SMALL-INTESTINE DURING METAMORPHOSIS

In the amphibian small intestine, the epithelial transformation from t he larval to adult type is mainly the result of degeneration of the la rval 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) immu nohistochemistry. During pre- and prometamorphosis, few apoptotic cell s were detected by TUNEL, and a small number of proliferating cells ra ndomly distributed in the larval epithelium were labeled by BrdU. At t he beginning of the metamorphic climax, when primordia of the adult ep ithelium were first detected, numbers of apoptotic cells suddenly incr eased 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 epithel ium occurred. Following complete epithelial replacement, the adult epi thelium became differentiated into a simple columnar epithelium posses sing a cell renewal system similar to that of mammalian intestinal epi thelium. These results indicate that larval epithelial apoptosis progr esses simultaneously with active proliferation of the adult epithelium during the early period of metamorphic climax, which coincides with t he modification of the basement membrane lining both types of epitheli a.