The Nature of Cytoplasmic Factors Controlling Nuclear Function

3 The Nature of Cytoplasmic Factors Controlling Nuclear Function 

Gurdon (1967) specified the stage of egg development when the cytoplasm becomes inducible for DNA synthesis. For this purpose he injected nuclei isolated from Xenopus laevis brain into eggs of these animals at various periods after egg activation. The incorporation of H -thymidine into the transplanted nuclei was 

Direct internuclear transport Fig. 56. Nuclear-cytoplasmic interactions in a heterokaryon. (After Ringertz et al., 1972) 

In chicken erythrocyte - HeLa cell heterokaryons, growth and changes of erythrocyte nuclear morphology depend upon the accumulation of HeLa cell cytoplasmic proteins in the nucleus. These proteins can migrate from the cytoplasm of the HeLa cells or can go directly from the HeLa nucleus into the nearby erythrocyte nucleus (Fig. 56). There is a characteristic negative correlation between the number of erythrocyte nuclei and the amount of cytoplasmic proteins incorporated into each erythrocyte nucleus. The RNA synthesis in these nuclei is also reduced proportionately. Therefore in this case nuclear reactivation is affected by cytoplasmic proteins (Ringertz et al., 1972). 

According to Tiedemann (1970), protein inductors are present in the unfertilized egg in an inactive form. The discovery of the inhibitor pointed to a complex mechanism of regulation of inductor action. The basic experiments which led to the separation of the inductor from the inhibitor are as follows. Frozen and defrosted chicken embryos were homogenized in two volumes of 0.1 M NaCl and centrifuged for 2 h at 105,000 g. The supernatant, after precipitation with ethanol. 

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