Cytoplasmic determinants localized in particular regions of the egg seem to be important in cell determination during early embryogenesis1,2. Although their composition is uncertain, there is evidence that some are partly composed of maternal RNA molecules3-6. Consistent with this possibility, maternal poly(A)+ RNA and certain specific RNA sequences are reported to show an uneven distribution in the cytoplasm of oocytes and developing embryos7-11, suggesting that maternal RNA sequences may contain signals required to distribute themselves in specific cytoplasmic regions of the egg. We have now tested this idea by analysing the distribution of RNA sequences localized in the vegetal pole cytoplasm of Xenopus laevis eggs after microinjection into fertilized zygotes. We report that exogenous vegetal pole poly(A)+ RNA accumulates along a concentration gradient from the vegetal hemisphere to the animal hemisphere of the injected egg and provide evidence that the concentration gradient is set up by active migration of the injected vegetal pole poly(A)+ RNA into the vegetal hemisphere cytoplasm between fertilization and the first cleavage.
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