Erythrocyte differentiation during the metamorphic hemoglobin switch of Rana catesbeiana.

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Abstract
Anurans (frogs and toads) switch from tadpole to adult Hb synthesis during metamorphosis. A number of workers attempted to determine whether tadpole and adult Hb types are expressed in the same or different erythroid cells during the switch. If the different Hb types were found in different cells during the transition, the switch in globin gene expression occurred at an early stage of cellular differentiation. Previous studies in which immunocytochemical techniques were used to approach this question were in conflict in regard to the metamorphic Hb switch of the North American bullfrog R. catesbeiana. Newly differentiating erythroid cells from the blood of metamorphosing tadpoles were purified by using Percoll gradients. These new cells had an immature morphology, were very active in the synthesis of adult Hb and contained no detectable tadpole Hb. The tadpole cells had no detectable adult Hb, were synthetically inactive, increased in density during the switch and were cleared from the circulation. Only adult Hb expression is detected in newly differentiating erythroid cells during metamorphosis.