Induction of Pronephric Tubules by Activin and Retinoic Acid in Presumptive Ectoderm of Xenopus laevis

Correlation between activin and retinoic acid (RA), both of which affect early amphibian development, was studied using Xenopus laevis embryos. In the first set of experiments, two isomers of RA, all- trans RA and 13- cis RA, were compared in terms of stability of biological activity against light. Xenopus blastulae were dipped in RA solutions which had either been kept away from light, or had been exposed to light for a few hours. At doses ranging from 10^–4to 10^–6M, RA elicited head deformity. All- trans RA, under both dark and light conditions, had similarly potent effects. On the other hand, 13- cis RA under dark conditions had much weaker effects than it did under light conditions.
In the second set of experiments, activin was mixed with all- trans RA, and the inducing effects on the animal cap explants were investigated. Activin at a concentration of 10 ng/ml induced notochord. In combination with 10^–6M RA, muscle was well induced instead of notochord. In combination with 10^–5M RA, pronephric tubules were markedly induced. Pronephric tubules were never induced by activin alone, at any of the various concentrations employed. This is the first report on the very high frequency of induction of pronephric tubules by the combination of activin A and all- trans RA in the Xenopus ectoderm.