The effect of temporary treatment of animal half embryos with lithium and the modification of this effect by simultaneous exposure to actinomycin D.

Sixteen-cell stages of the sea urchin Paracentrotus lividus were separated into animal and vegetal halves. The former were reared in four different media: pure sea water, sea water with 10 μg/ml actinomycin D, sea water with 0.043 mol lithium, sea water containing the same concentration of lithium together with 10 μg/ml actinomycin D. The main result of the study was that the development had a more animal character after combined exposure to actinomycin D and lithium than after exposure to lithium alone. The inference from this result is that transcription processes are involved in the vegetalization of lithium treated sea urchin embryos.The differentiation of the animal halves occurring in normal sea water is widely repressed in the presence of actinomycin D. Particularly obvious is the fact that the transformation of the acron with ensuing substitution of the long stereocilia by short motile cilia fails to occur in the presence of actinomycin D. The transformation of the acron and its stereocilia seem thus to be dependent on transcription processes.In animal halves reared in normal medium droplets, dark in phase contrast, may appear near the tip of the stereocilia. This is followed very soon by the shedding of the stereocilia. In animal halves treated with actinomycin D, the droplets appear in a somewhat later stage than in the halves reared in normal sea water. After their appearance, the stereo cilia acquire a certain motility. The droplet formation may correspond to the elimination of a component which stiffens the stereocilia. The elimination of this component does not seem to be directly dependent of transcription processes. On the other hand, the stability of the stereocilia decreases with the distance from the animal pole.The data obtained in this investigation may be integrated into the double gradient concept, also in its recent elaboration (Runnström, 1967).The present investigation was carried out at Stazione Zoologica, Naples. We are deeply indebted to Professor M. Pantaleo, head of this institution, for his interest and generous support. We express also our gratitude to Professors R. Martin and R. Rocca for their help and friendly interest. One of us expresses his gratitude to Professor M. Pantaleo for the award of a research grant from the Stazione. He recognizes also his indebtedness to Professor M. De Vincentiis, director of the Department of Histology and Embryology at the University of Naples. The second of us expresses his thanks for financial support from the "Swedish Natural Sciences Research Council", from the "Swedish Cancer Society" and from the "Research group of embryology for the study of cellular differentiation " of the Department of Zoology, University of Rome. He expresses his sincere thanks to the Director of this Department, Professor P. Pasquini, for his stimulating interest.