Inducing activity of subcellular fractions from amphibian embryos

The homogenate from unfertilized eggs, gastrulae, neurulae and hatched embryos ofXenopus laevis was fractionated by differential centrifugation and subsequent repeated centrifugation on discontinuous sucrose gradients. A high archencephalic-neural inducing activity was found in RNP particles, which were released from the high-speed ("microsomal") sediment by treatment with EDTA, and in a fraction of heterogeneous small vesicles. The highest archencephalic inducing activity was observed in RNP particles from unfertilized eggs and from gastrulae. RNP particles isolated from hatched embryos had a lower inducing activity. The neuralizing factor can be extracted from the small vesicles with pyrophosphate buffer at pH 8.6, but it is not solubilized with a non-ionic detergent (Triton X 100). The high-speed supernatant from the gastrula homogenate contains soluble neuralizing factor, whereas the supernatant from egg homogenate has a low inducing activity. The plasma membrane fraction (isolated from gastrulae) also has only a low inducing activity. The possible significance of the subcellular distribution of neuralizing factors for the transmission of neuralizing inducer from the mesoderm to competent gastrula ectoderm and the processing of signals which are generated on the plasma membrane of induced cells is discussed.