Methods

1
Animal Cloning (bad layout) Stuart Johnson and
Jon Scott (stuart.johnson_at_le.ac.uk,
jon.scott_at_le.ac.uk)
Introduction Animal cloning is a contentious
topic. The history of cloning dates back to the
19th century (Driesch, 1892) however, animal
cloning came to particular public prominence in
1997 when Wilmutt et al. demonstrated that the
cloning of mammals from adult cells, rather than
an embryo, was possible. The authors of the study
derived nuclei of cultured mammary epithelial
cells from an adult ewe, resulting in the birth
of Dolly the first cloned sheep.
Methods Cloning from adult cells involves taking
the nucleus of a cell from the animal intended to
be cloned and transferring it into an
unfertilised ovum which has had its metaphase
chromosomes removed. If this step is successful,
the transferred nucleus is re-programmed to
initiate the development of a new embryo. This
embryo is genetically identical to the animal
from which the cell was obtained. The embryo is
then transferred into a surrogate mother for
gestation to term and birth (see Figure
1). However, there are many factors affecting
success of cloning, including
Results Table 1 shows the success rates of animal
cloning from mammalian adult cells. The figures
are taken from a review article by Solter (2000).

• the type of nuclear donor cell used
• the treatment of donor cells prior to nuclear
  transfer
• the source of recipient ova
• the techniques employed
• and the competency of the laboratory group
  working on it.

Cloning in Cattle There are more laboratories
worldwide cloning cattle than all other species
combined. This is partly due to the knowledge and
expertise of assisted Reproductive techniques in
cattle developed over the last 20 years. Whilst
the success and repeatability for cloning from
embryonic cells is now well established in
cattle, cloning from adult cells still remains
more problematic. Cloning in Pigs The cloning of
pigs has illustrated well the fact that assisted
reproductive technologies and techniques for
nuclear transfer do not directly apply from one
species to another. In a study by Onishi et al.
(2000) the transfer of 110 cloned embryos to four
surrogate mothers produced only one piglet.
It is evident from the above figures that, even
given the significant progress made in the field
of animal cloning from adult cells in recent
years, success rates are still very low.
Conclusion Techniques used to clone animals are
still developing and cloning to reproduce certain
genotypes can represent a significant challenge.
With a variety of cloned animals being produced,
the demand for cloning specific animals will
increase significantly in the coming years.
Differences in the ability to clone a particular
species are a result of the time and resources
which have been invested into research.
References Driesch H (1892), Entwicklungsmechanisc
he Studien. I. Der Wert der beiden ersten
Furchungszellen in der Echinodermentwicklung. Z.
Wiss. Zool. 53, pp. 160-178 Onishi A, Iwamoto M,
Akita T, Mikawa S, Takeda K, Awata T, Hanada H,
Perry A (2000). Pig Cloning by Microinjection of
Fetal Fibroblast Nuclei. Science, 289(5482), pp.
1188-1190 Solter D, (2000). Mammalian cloning
advances and limitations. Nature Reviews Genetics
1, pp. 199-207 Wilmut I, Schnieke AE, McWhir J,
Kind AJ, Campbell KH (1997). Viable offspring
derived from fetal and adult mammalian cells.
Nature, 385(6619), pp. 810-813.