Segregation of Ooplasm

The transport into the oocyte of RNA synthesized by nurse cells has been demonstrated (Fig. 50). There is very little endogenous synthesis of RNA s in such oocytes. Radioautography was used to demonstrate that RNA is synthesized in the nurse cells and then transferred into the oocyte (Bier, 1963). 

In many cases, these nurse cells have polyploid nuclei and synthesize large amounts of RNA s. In oocytes with metabolically inactive nuclei, the amplification of rDNA does not occur and rRNA are synthesized by trophic cells (Bier, 1965, 1967). In some beetles, for instance Dytiscus marginalis, active RNA synthesis is going on in trophic cells as well as in oocytes. Oocytes contain extrachromosomal DNA that is represented by Djiardine bodies (Bier et al., 1967). 

In addition, it has been demonstrated that RNA synthesis in the maturing oocytes of mammals is under the control of the surrounding follicule cells. RNA synthesis in various stages of oocyte maturation was changed (Fig. 51) and was terminated in large oocytes. It was found that the anthrum fluid, which is produced by follicular cells, contains compounds that inhibit DNA dependent RNA-polymer-ase activity in vitro (Moore et al., 1974). A proteinous factor was extracted from embryonic vesicles of amphibian oocytes. This factor inhibits transcription of the ribosomal genes (Crippa, 1970 Crippa et al., 1972). 

Apparently, the differences in various parts of the cytoplasm are caused by the maternal genotype (Child, 1948 Neyfach, 1962 Raven, 1964). These differences are due to the action of certain gene groups and to the synthesis of various RNA fractions in the oocytes. These RNA s are stable in Xenopus laevis and remained in the cytoplasm for a long time during embryogenesis. Approximately 90% of this 

In Xenopus, from early embryogenesis until the 500-2000 cell stage (early blastula), the store of mRNA that was synthesized on the lampbrush chromosomes did not change. However, 28% of this store disappeared during the 1.5 h-2.0 h that separate the early and late blastual stages (7 and 9 respectively) (Crippa et al., 1967). 

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