Temporal Variability of Morphogenetic Nuclear Activity

The spatial-temporal pattern of transcription determines the specificity of onto-genic processes nd is realized in the variability of morphophysiological nuclear activity. This was described in detail by Neyfach (1962). Nuclear activity is characterized by a relatively high sensitivity to irradiation with X-rays and to actinomycin treatment (Neyfach, 1963, 1964). In this case, dosages of X-rays which suppressed morphogenetic nuclear activity also inhibited the synthesis of mRNA. The synthesis of different RNA fractions was inhibited to an equal degree (Kafiani et al., 1966). Nevertheless, the rate of RNA synthesis in the whole embryo is apparently 

Corresponding variability in nuclear morphogenetic activity was demonstrated in later developmental stages and during postnatal ontogenesis, when the processes of cell and tissue specialization were complete. This was demonstrated for the differentiation of the crystalline lens (Reeder and Bell, 1965), muscle differentiation (Fieldman et al., 1964), hemopoiesis (Marx and Kovach, 1966), and early neuroem-bryogenesis (Akimova and Diban, 1967). The cortex of 1-5-day-oId newborn rats is sensitive to the injection of the RNA synthesis inhibitor actinomycin D. Under these conditions cortex development is slowed. Injection of actinomycin D after the fifth day postpartum does not affect development of 5-6 layers of cortex (Korochkin and Olenev, 1966 Korochkin, 1970). 

Differential Activity of Genes as a Basis for Cell Differentiation 

These differences lead us to suggest that, in this case, the peculiarities of nu-cleo-cytoplasmic interactions in the different parts of the embryo played an important role. Apparently, nuclear activities differ in the presumptive cells of any embryo component. Such differences determine the further development of the various presumptive layers in gastrulation (Korolev and Neyfach, 1965). 

Morphogenetic nuclear function both began and became apparent earlier in mosaic eggs. It began at the 12 and 22-24 blastomere stages in Mollusca, and in ascaris at the two blastomere stages (Fig. 9 after Neyfach, 1962). 

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The discovery of temporal variability of morphogenetic nuclear activity makes possible the explanation of classical postulates on critical periods of ontogenesis... 

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