Cytoplasmic states of different cytoplast recipients

1359-6004/96 $8.50 Embryo reconstruction by the transfer of a donor nucleus to an enucleated one-cell egg was first proposed by Spemann (1938) to answer the question of nuclear equivalence or ‘Do nuclei change during development?’ By transferring nuclei from increasingly advanced embryonic stages, these experiments were designed to determine at which point the developmental potential of nuclei became restricted. Owing to technical limitations and the unfortunate death of Spemann, these studies were not completed until Briggs and King (1952) demonstrated that certain nuclei could direct development to a sexually mature adult. Their findings led to the current concept that equivalent, totipotent nuclei from a single individual could, when transferred to an enucleated egg, give rise to genetically identical individuals. In the true sense of the meaning, these individuals would not be clones, as unknown cytoplasmic contributions in each may vary and the absence of any chromosomal rearrangements would have to be demonstrated. Since the demonstration of embryo cloning in amphibians, similar techniques have been applied to mammals (see Box 1). The ability to produce genetically identical clones has obvious advantages for research (i.e. as biological controls) and in commercial applications (i.e. uniformity of meat products, animal management). After reconstruction, embryo development depends on many factors, including the ability of the nucleus to direct development, i.e. totipotency, nuclear reprogramming, developmental competence of the recipient cytoplast (i.e. oocyte maturation), oocyte activation and embryo culture (reviewed by Wilmut and Campbell, 1992; Campbell and Wilmut, 1994). Another group of factors can be described as cell cycle effects. Many reports on both amphibians and mammals have shown that the cell cycle stage of both the donor nucleus and the recipient cytoplasm, at the time of transfer, can have substantial effects upon the development of the reconstituted embryo. Cytoplasmic states of different cytoplast recipients