The influence of the nucleus on sequential determination in frog ectoderm following induction by lithium ion.

It appears certain that the process of cellular differentiation is an outcome of interactions between the cell nucleus and cell cytoplasm. Differentiation of embryonic amphibian ectoderm involves two fairly distinct phases: during the first short period an inductor (or some intrinsic factor, if an inductor is absent) determines the course of future differentiation in a multipotential cell; during the second, longer interval of time presumably a complex sequence of reactions leads to physiological and morphological differentiation. Little is known about the nature of reactions which take place during the first phase when the cells become developmentally determined by the inductor. It appears that the first step in translation of the inductive instruction in the competent cell is accomplished during the first 2 or 3 h following the treatment with the inductor (Ansevin, 1966). The step is not actinomycin-sensitive (Ansevin, 1965), as was shown by cells that completely recovered after actinomycin treatment (in conditions when it was unlikely that they could have failed to take up some inhibitor). This suggests that transcription of RNA does not have to occur during cell determination, but it does not exclude the possibility that induction removes biological repressors from specific genes which thus become available to direct cellular differentiation along a specific line. Barth (1965) has drawn attention to a striking similarity between the power of several inorganic ions to bind to DNA and the capacity of the same ions to act as embryonic inductors. He suggested that ions might compete with histones for active sites on DNA molecules, and by binding on these sites protect the molecules from the inactivating action of histones. With the present lack of definite evidence for or against the involvement of genes in cell determination, no hypothesis concerning the nature of induction can be more than a speculation, and two possibilities have to be explored: (a) that cell determination involves nuclear activity; (b) that the phenomenon is restricted to cytoplasmic reactions. The following report presents an attempt to analyse further the role of cell nucleus and cytoplasm in determination of frog 1 Author's address: Department of Biology, Rice University, Houston, Texas 77001, U.S.A.