Changes in somatic cell nuclei inserted into growing and maturing amphibian oocytes.

When somatic cell nuclei are injected into enucleated unfertilized eggs they very soon cease RNA synthesis (Gurdon & Brown, 1965; Gurdon. 1967 a), and commence DNA synthesis (Graham, Arms & Gurdon, 1966). By means of such experiments it has been shown (i) that components of living egg cytoplasm can substantially change the nature of nuclear activity, (ii) that nuclear activity is altered in such a way as to conform to that of the host egg cell, and (iii) that these effects are obtained with the nuclei of adult as well as embryonic cells. The experiments to be reported here were designed to find out whether the same generalizations apply to nuclei inserted into cells other than eggs, and whether they are true of changes in nuclear activity other than the repression of RNA synthesis and the induction of DNA synthesis. For this purpose growing and maturing amphibian oocytes have been used. Growing oocytes are characterized by the very active synthesis of RNA (Brown & Littna, 1964), and by the absence of nuclear DNA synthesis whether [H]thymidine is supplied intraperitoneally (Izawa, Allfrey & Mirsky, 1963 a), or is injected directly into living oocytes (Gurdon, 19676). Maturing oocytes are in the process of completing meiosis and contain condensed chromosomes inactive in both RNA and DNA synthesis. If cytoplasmic components are of general importance in controlling nuclear activity, it would be predicted that nuclei inactive in RNA synthesis, or unaccustomed to division, should display these activities following insertion, respectively, into growing or maturing oocytes. The results reported here show that this expectation is realized, and permit some general conclusions to be drawn on the widespread importance of the cytoplasmic control of nuclear activity during cell differentiation.