Hemoglobin transition from larval to adult types occurs within a single erythroid cell population during metamorphosis of the salamander Hynobius retardatus.

In amphibians transitions of hemoglobins (Hbs) and red blood cells (RBCs) from larval to adult types have been reported to occur at metamorphosis. The transition of Hbs in the salamander Hynobius retardatus also occurs during metamorphosis, but almost independently of thyroid activity. Changes in several properties of RBCs, including their morphology, buoyant density and Hb phenotypes, were analyzed during the normal development of Hynobius retardatus. Typical larval RBCs were distinguished from typical adult ones by their different morphology and different buoyant density, while RBCs from metamorphosing animals had a single buoyant density and thus could not be separated into two populations on a Percoll density gradient. When RBCs from metamorphosing animals were examined immunohistochemically using larval or adult globin-specific antibodies, all the RBCs from any developmental stages from early larvae (36 days after hatching) to metamorphosed juveniles contained varying quantities of both antigens recognized by these antibodies. Immunohistochemical observation also demonstrated that the erythropoietic organs were the liver and spleen at early larval stages, but limited to the spleen in metamorphosing larvae and metamorphosed adults. These findings support the idea that the Hb switching in Hynobius retardatus occurs in a single RBC population, rather than the concept that larval RBCs are replaced by new, adult RBCs, as is known to occur in many amphibians.