Quantification and localization of erythropoietin-receptor-expressing cells in the liver of Xenopus laevis

Erythropoiesis occurs in the African clawed frog, Xenopus laevis and is mediated by erythropoietin (xlEPO), a primary regulator of this process. Previously, we have shown that the xlEPO receptor (xlEPOR), which is expressed by erythroid progenitors that respond to xlEPO, is found predominantly in the liver. The aim of the present study was to determine the dynamics of erythropoiesis in the livers of normal and anemic X. laevis by identifying the number and precise location of mature and immature erythrocytes. We quantified mature and immature erythrocyte numbers by o-dianisidine staining or immunohistochemistry and investigated the dynamics of erythropoiesis in normal, acute hemolytic and blood-loss states by in vivo cell proliferation assays with 5-bromo-2′-deoxyuridine (BrdU). We detected 0.12×108 xlEPOR+ BrdU+ cells in the liver of the normal X. laevis at 24 h after BrdU injection. Frogs presenting with acute hemolytic anemia and pancytopenia show a 10-fold increase in the number of xlEPOR+/BrdU+ cells (approximately 1.30×108 cells) in the liver. The xlEPOR+ cells are found predominantly on the inner wall of hepatic sinusoids. Hematopoietic progenitors that undergo slow cell cycling were also observed in the hepatic sinusoids. This study clarifies the rate of production of mature and immature erythrocytes per day in the liver of X. laevis and the way that these cell numbers change in response to anemia.