Erythropoiesis and conversion of RBCs and hemoglobins from larval to adult type during amphibian development [Review]

摘要

In anuran amphibians transitions of hemoglobins (Hbs) and red blood cells (RBCs) from the larval to the adult type have been reported to occur at metamorphosis, depending on certain influence of thyroid hormones (THs). Contrary to this, transition of RBCs/Hbs from the larval to the adult type during the metamorphosis in a urodele, Hynobius retardatus occurs almost independently of thyroid activity, but dependent on certain pituitary factor(s). All findings reported so far support the idea that the Hb switching in H. retardatus occurs in a single RBC population ("Hb switching" model), rather than the concept that larval RBCs are replaced by new, adult RBCs ("RBC replacement" model) as is known to occur in many anurans. Erythropoiesis in vertebrates occurs with two distinct phases, termed primitive and definitive. Primitive erythropoiesis generally provides embryonic/larval erythroids, and definitive hematopoiesis contributes to adult RBCs. Primitive erythropoiesis in Xenopus laevis occurs in the ventral blood island (VBI), and the dorsolateral plate (DLP) cells remain undifferentiated until later for definitive hematopoiesis. H. retardatus embryos also have two distinct hematopoietic sites, the VBI and DLP. The DLP cells of H. retardatus, however, differentiate in situ to RBCs containing larval globin, suggesting that both the VBI and DLP contribute to "primitive" erythropoiesis. Some DLP cells may be set aside in an undifferentiated state during embryogenesis for future "definitive" erythropoiesis coming to express only adult globin during metamorphosis. A tentative model was proposed to explain similarities and dissimilarities in erythropoiesis and conversion of RBCs/Hbs between anurans and urodeles.