Conversion of red blood cells (RBCs) from the larval to the adult type during metamorphosis in Xenopus: specific removal of mature larval-type RBCs by apoptosis.

The conversion of hemoglobins (Hbs) and red blood cells (RBCs) from the larval to the adult type was monitored during normal metamorphosis in Xenopus laevis, and in artificially induced metamorphosis-arrested and precociously metamorphosed animals by means of SDS-PAGE, Hb immunohistochemistry, and double-staining with in situ DNA nick-end labeling (TUNEL) for detection of apoptosis and Hb immunostain. During normal metamorphosis, larval RBCs gradually decreased and, conversely, adult RBCs increased in number. However, in metamorphosis-arrested tadpoles, the larval-adult conversion of RBCs did not occur within 4 weeks, but did rather within 6 months after the controls metamorphosed. In order to identify possible mechanisms for the specific removal of larval RBCs from circulation in metamorphosing and metamorphosed animals, double-staining experiments with TUNEL and Hb immunostain were carried out. During metamorphic climax, many larval RBCs expressed TUNEL-positive reactions in the spleen, suggesting that the larval RBCs were specifically removed from the spleen during metamorphosis. When the larval RBCs were transferred to the circulatory system of histocompatible control adults, they survived for a long time, and no transferred RBCs showed TUNEL-positive reactions. In contrast, larval RBCs transferred to histocompatible adults that had been treated with T3 were reduced in number in the circulatory system of the recipients. Double-staining experiments demonstrated that the transferred larval RBCs underwent apoptosis in the spleen and liver of the adult recipients treated with T3, indicating that the mature larval-type RBCs were specifically removed from metamorphosing animals by apoptotic cell death under the influence of THs.