Abstract
The concept of transferring nuclei from multicell stage embryos to enucleated oocytes was first proposed by Spemann in 1938 (1). Spemann was a developmental biologist whose concern at the time was that of nuclear equivalence. The idea of the basic composition of the nuclei changing during early embryo development gave rise to the theory that nuclei from different tissues were not equal. To answer this question, Spemann proposed an experiment to transfer nuclei from progressively more advanced embryos to enucleated, activated oocytes. The conclusion of the experiment would be determined when inequivalence was achieved and the transferred nucleus would not support development to a mature adult. Results from Spemann's experiment were first reported in 1952 by Briggs and King (2) in the amphibian Rana pipiens. They (2) showed that nuclei from preblastula stage embryos could promote development to the blastula stage. In addition, they alluded to the observation that nuclei from beyond the blastula stage had a lesser probability of promoting development after nuclear transfer.
The spin-off of such a project is the possibility of cloning. Since all of the nuclei of the early embryo are presumed to be identical (all containing the same complement of genetic material), subsequent transfer to enucleated, activated oocytes with development to term should result in genetically identical individuals (the reality of identical individuals will be discussed later). This procedure, in combination with serial nuclear transfer (i.e., growing the first nuclear transfer embryos to the donor cell stage and then repeating the procedures) could, theoretically, result in an unlimited number of identical individuals. The possibility of this in mammals became much more likely after the successes of such procedures were reported in 1986 by Willadsen (3) and Prather et al. (4) in sheep and cattle, respectively.
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