The Effects of Thyroxine on Ribonucleic Acid Content in the Brain of Rana Pipiens Tadpoles

Considerable knowledge concerning RNA formation is known for developing embryos of the frog, Rana pipiens, and the clawed toad, Xenopus laevis. Esper (1962) employed the cytophotometric method of determining the quantity of RNA in pronephric, epidermal, and endodermal cells in haploid and diploid embryos of Rana pipiens (Shumway Stage 24). The cytophotometric measurement revealed that the concentration of RNA per unit area of cytoplasm is the same for haploid and diploid embryos studied. However, cell volume determination indicated that diploid cells of each tissue are twice the volume of haploid cells. The total amount of RNA in the diploid cell is therefore twice the volume of haploid cells. The study of RNA systhesis in synchronized type cells by Klevez and Stubblefield (1967a) revealed an increase in the rate of RNA synthesis after the synthesis of only a small portion of the DNA. They suggested that this phenomenon was the result of replication of functional DNA templates in the cell. Klevez and Stubblefield (1967b) conducted a similar study on synchronized mammalian cell cultures with similar findings; that most of the DNA involved in transcription was replicated in the first third of the DNA synthesis period. Therefore the rates of RNA and protein synthesis increased because of the doubling of the active template population of each cell. Since the literature revealed a marked increase in mitotic activity in the neural axis, which is accompanied by an increase in DNA, during metamorphic climax and following administration of exogenous thyroxine, it seemed feasible to investigate further the role of the thyroxine in stimulating nucleic acid synthesis in the brain of Rana pipiens larvae, with emphasis on the quantity of DNA and RNA produced