Theriogenology 69 (2008) 48–54
Lessons learned from nuclear transfer (cloning)
Department of Animal & Avian Sciences, University of Maryland, College Park, MD 20742, USA
Somatic cell nuclear transfer (SCNT) has been accomplished in an ever-growing list of species. In each case, an enucleated
oocyte has successfully reset the nucleus of a somatic cell such that the embryonic program could progress to the production of a live
offspring. The overall efﬁciency of the process remains low due to a combination of biological and technical challenges, some of
which are known and others remain to be elucidated. Comparative studies between livestock and laboratory species may help
improve not only nuclear transfer efﬁciencies but also uncover basic underlying developmental principles.
# 2007 Elsevier Inc. All rights reserved.
Keywords: Oocyte; Embryo; Reprogramming; Somatic cell nuclear transfer
Nuclear transfer (cloning) in mammals was not
achieved until over three decades after the initial reports
from Briggs and King of the production of adult frog
clones using embryonic nuclei [1,2]. There is no doubt
that researchers working with mammalian eggs were
intrigued and envious of these reports of amphibian
cloning, however, various aspects of the in vitro systems
and equipment required improvement before cloning
could be attempted successfully using mammalian eggs.
The much smaller mammalian oocytes required ﬁner
tools and better pressure control. More critically, culture
systems which allowed continued development of
embryos needed to be developed. And, of course, there
is the numbers game—the number of eggs that can be
obtained from a single frog (and which can be grown
under relatively simple conditions) is mind boggling to
someone happy to obtain 10 embryos from a superovulated sheep, cow or goat. So considering the
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