Erythroid cell development during natural amphibian metamorphosis

Abstract

The transition from fetal to adult hemoglobin was investigated during natural metamorphosis of Rana pipiens tadpoles fertilized and grown under laboratory conditions and of Rana catesbeiana tadpoles and frogs obtained from a local pond. Tadpole red cells had a higher degree of pleiomorphism and reticulocytosis, an increased resistance to osmotic lysis, and a lower hemoglobin content than frog erythrocytes. Red cells derived from premetamorphic tadpoles and from animals at the metamorphic climax had an increased capacity to incorporate leucine, uridine, and thymidine as compared to red cells of tadpoles and frogs at other stages of development. Juvenile froglets were found on day 14 after emergence of the front legs to contain only or predominantly tadpole hemoglobin. The transition from fetal to adult hemoglobin began in Rana catesbeiana at emergence of the front legs and was completed 4–10 weeks later. Emergence of the front legs was associated with a sharp increase in the synthesis of red cell proteins, which migrated on polyacrylamide gel with both tadpole and frog hemoglobins. This intermediate pattern of protein synthesis was observed throughout the metamorphic climax, and was different from that in blood samples from adult frogs or from tadpoles at earlier developmental stages. The peripheral blood smear of animals during the metamorphic climax contained small erythroid cells, the distinctive feature of which was their selective labeling after incubation with tritiated thymidine. The observed changes in hemoglobin synthesis during metamorphosis are compatible either with the appearance of a new cell line synthesizing temporarily adult hemoglobin in proportions different from those in the adult frog, or with the renewed proliferation of the tadpole erythroid cell line, which eventually produces cells capable of synthesis of frog hemoglobin.