Erythrocyte differentiation during the metamorphic hemoglobin switch of Rana catesbeiana.

Anurans (frogs and toads) switch from tadpole to adult hemoglobin synthesis during metamorphosis. A number of workers have 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 are found in different cells during the transition, the switch in globin gene expression occurs at an early stage of cellular differentiation. Previous studies, in which immunocytochemical techniques were used to approach this question, are in conflict in regard to the metamorphic Hb switch of the North American bullfrog Rana catesbeiana. We have purified newly differentiating erythroid cells from the blood of metamorphosing tadpoles by using Percoll gradients. These new cells have an immature morphology, are very active in the synthesis of adult Hb, and contain no detectable tadpole Hb. The tadpole cells have no detectable adult Hb, are synthetically inactive, increase in density during the switch, and are then cleared from the circulation. Thus, only adult Hb expression is detected in newly differentiating erythroid cells during metamorphosis.

Embryonic and larval hemoglobins during the early development of the bullfrog, Rana catesbeiana.

The main hemoglobin (Hb) found in Shumway (embryonic) stage 25 bullfrogs is that which we have designated Td-4. The other major tadpole Hbs (Td-1, 2, and 3) predominate during Taylor and Kollros (larval) stages I-XVIII. We propose that Td-4 is an embryonic Hb, whereas Td-1, 2, and 3 are larval (fetal-like) Hbs. Embryonic Hb Td-4 continues to be synthesized during the larval stages. During the larval period, the average peripheral blood Hb profile changes very little with morphological stage or general growth. However, there is great heterogeneity in the embryonic:larval Hb ratio among individual tadpoles of a given stage or weight, apparently due to differential Hb and red cell production by the two active erythropoietic sites, mesonephric kidneys (Td-4), and liver (Td-1, 2, 3).