Differentiation (2002) 69:154–157
C Blackwell Wissenschafts-Verlag 2002
James A. Byrne ¡ John B. Gurdon
Commentary on human cloning
Human cloning refers to the production of genetically identical humans. This cloning is possible via either ‘‘embryo splitting’’ or ‘‘nuclear transfer’’. Embryo splitting involves the separation of an early human embryo into two or more parts. Each of these parts has the potential to develop into a blastocyst (late embryo), which, if implanted, can develop into a child. This is how genetically identical monozygotic twins are created. Artiﬁcial embryo splitting has been successfully implemented in various mammals including ...view middle of the document...
The reconstituted zygote has the potential to divide into a blastocyst, and if implanted, develop into a child genetically identical to the nuclear donor. There are two fundamentally distinct types of human cloning by somatic cell nuclear transfer: reproductive cloning and therapeutic cloning. The objective of reproductive cloning is to produce a child genetically identical to an individual. This has been suggested as a last resort when an infertile couple are unable to conceive a biologically related child via any other method (Zavos, 2001). The objective of therapeutic cloning is to produce embryonic stem cells that are genetically identical to a patient. These stem cells could then be differentiated into precursor replacement cells to treat one of a variety of degenerative diseases from which the patient might suffer (Gurdon and Colman, 1999). Several scientiﬁc and related ethical issues surrounding both types of human cloning are addressed in this commentary. However, many of the religious (Turner, 1997) and moral arguments (Pence, 1998) that have been associated with human cloning, are beyond our present scope.
Reproductive human cloning
The ﬁrst Vertebrate reproductive cloning (nuclear transfer) experiments were on Amphibia. Initial success involved using embryonic donor cell nuclei (Briggs and King, 1952), but it was soon discovered that differentiated cell nuclei could also result in cloned offspring, proving that there was no loss of genetic material as differentiation occurred (Gurdon, 1962). Nuclear transfer work progressed into the use of mammals in the 1970s (Bromhall, 1975) and 80s (Willadsen, 1986), and resulted in the conception of the ﬁrst mammal cloned from an adult cell nu-
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cleus in 1996. The birth of the cloned sheep ‘‘Dolly’’ was announced in Nature in 1997 (Wilmut et al., 1997) and sparked worldwide discussion about the possibility of cloning humans. To date, sheep (Wilmut et al., 1997), cattle (Cibelli et al., 1998), mice (Wakayama et al., 1998), goats (Baguisi et al., 1999), and pigs (Betthauser et al., 2000; Onishi et al., 2000; Polejaeva et al., 2000) have all been cloned from differentiated cells. Suggesting that somatic cell nuclear transfer may eventually be successful in all mammals, including humans. However, the majority of scientiﬁc opinion is opposed to the reproductive cloning of humans in view of the developmental, morphological, and physiological problems observed in mammals that had been cloned (Jaenisch and Wilmut, 2001). The ﬁrst problem observed in mammalian cloning is a consistently low efﬁciency of reconstituted eggs developing to parturition (birth). Typically, to get one cloned animal to parturition, approximately one hundred eggs must be enucleated and reconstituted with donor somatic cell nuclei, either by electrofusion (Wilmut et al., 1997) or microinjection (Wakayama et al., 1998). Thus nuclear transfer from adult or specialized cells is...