Animal Biotechnology and Modeling
227
cells themselves) can be transferred by
electrofusion, polyethylene glycol-induced
fusion, or by direct injection into enu-
cleated metaphase II oocytes. Following
culture,
the
reconstituted
embryos are
transferred to the reproductive tracts of
host animals. Since
in vitro
culture of
reconstituted sheep embryos is relatively
inef±cient, they can instead be transferred
to an intermediate (temporary) host. After
in vivo
incubation, they can be recovered
and transferred to the ±nal host to develop
to term.
If
cloning can
be
done
with
differ-
entiated cells from adult animals with
proven
phenotype/genotype
superiority,
then many genetically characterized clones
can be produced as an important experi-
mental/commercial resource (particularly
for the requisite ‘‘between batch’’ con-
sistency needed for biomedical reagent
production). However, there are still a
number of procedural hurdles to reduce
the costs associated with nuclear trans-
fer – extending the overall utility of the
methodology. With both sheep and cattle,
there has been a very low success rate with
a high percentage of fetal loss. With sheep,
especially using adult donor cells, there is
a signi±cant loss of embryos, fetuses, and
newborns and the process is very inef±-
cient. Only 10–50% of nuclear-transferred
embryos develop to the blastocyst stage, in-
dependent of
in vitro
or
in vivo
developmen-
tal concerns and again, there is less than a
5% yield in total live offspring (on the ba-
sis of embryos needed to generate founder
animals). Lastly, the developmental conse-
quences of nuclear transfer procedures are
still controversial, with issues regarding
birth weight of cloned animals, premature
aging phenomena, and relative health and
immunocompetence coming under signif-
icant scrutiny as various animal models
entertain commercialization prospects.
3.3
Traits Affecting Domestic Animal
Productivity
Interest in modifying traits that deter-
mine productivity of domestic animals
was greatly stimulated by early exper-
iments
conducted
by
Ralph
Brinster,
Richard Palmiter, and their colleagues,
in
which
body
size
and
growth
rates
were dramatically affected in transgenic
mice expressing growth hormone trans-
genes driven by a metallothionein (MT)
enhancer/promoter. From that starting
point, similar attempts followed in swine
and sheep studies to enhance growth by
introduction of various growth hormone
(GH) gene constructs under control of a
number of different regulatory promot-
ers. Use of these constructs was intended
to allow for tight regulation of individual
transgene expression by dietary supple-
mentation. However, although resulting
phenotypes included altered fat compo-
sition, feed ef±ciency, rate of gain, and
lean/fat body composition, they were ac-
companied by undesirable side effects (e.g.
joint
pathology,
skeletal
abnormalities,
increased metabolic rate, gastric ulcers,
infertility). Such problems were attributed
to chronic overexpression or aberrant ex-
pression of the growth-related transgenes
and could be mimicked, in several cases,
in normal animals by long-term treatment
with elevated doses of GH.
Subsequent efforts to genetically alter
growth rates and patterns have included
production of transgenic livestock harbor-
ing a vast array of genes – from growth-
related
transgenes
to
immune
system
modulators. Other productivity traits that
are major targets for genetic engineering
include altering the properties or propor-
tions of caseins, lactose, or butterfat in
milk of transgenic cattle and goats, more
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