Animal Biotechnology and Modeling
very much in line with results reported by
other groups.
Currently, estimates for the cost of pro-
duction for each transgenic pig run from
$25 000 to $125 000 (U.S.$), with the broad
range reflecting the low overall experimen-
tal efFciency. The costs are compounded
(doubled or even tripled) if functional
transgenes and pig survival are factored
into the cost estimates. To address the ef-
Fciency of the gene-transfer experiments,
a modiFed scheduling can be employed
to decrease some of the Fxed and variable
costs, particularly in the use and schedul-
ing of research animals. By overlapping
days of experimentation rather than syn-
chronizing one group of donor animals per
week, actual experimental costs can be re-
duced by more than 25%. Superovulation
can induce an average of 25–30 embryos
per sow. While cost-effectiveness dictates
the speciFc regimen, all pigs do not re-
spond similarly, and only 60% respond
within a speciFc window of opportunity for
embryo collection. This low response rate
may appear unacceptable, but it should
be noted that the actual yield of suitable
embryos is still higher than might be avail-
able using naturally synchronized females
or other superovulatory schemes.
Surgical transfer of microinjected em-
bryos usually follows the microinjection
efforts. The mechanical insult to the em-
bryos is severe, however, and only 15–25%
of them will still be viable 5 days after
transfer. Therefore, 30–50 microinjected
embryos are routinely transferred per re-
cipient sow, with the expectation that 50%
of the recipient females will maintain
pregnancy. While the number of embryos
transferred may seem excessive, the basis
is derived from classical studies that estab-
lish a requirement of four viable embryos
at the time of implantation for a sow to ini-
tiate and maintain successful pregnancy.
±or transgene expression studies tar-
geting pigs, the use of outbred domestic
pigs is the most practical way to produce
and evaluate potential models. However,
miniature or laboratory swine are now
used with increasing frequency in biomed-
ical research in which desirable, well-
characterized background genetics make
these animals more suitable for human
modeling studies (e.g. xenotransplantation
research). While miniature swine are com-
mercially available, research in developing
methodologies to explore or enhance re-
productive performance has been limited.
Reproductive efFciency in miniature swine
is low compared to commercial swine and
is characterized by a low ovulation rate, low
birth weight, and small litter size. In ad-
dition, extensive management is required
to maintain neonates. Average litter size is
between 4 and 7 pigs at birth, with each
breeding sow producing 12 to 18 pigs per
year. Estrous cycles and gestation length
are similar to standard commercial swine;
however, sexual maturity in males and
females occurs between 4 and 6 months
of age in some breeds, which is sooner
than the 6- to 9-month norm of commer-
cial swine.
It is important to evaluate the potential
breed differences and ultimate production
efFciencies using a superovulatory proto-
col on groups of donor animals, to be able
to assess and possibly enhance the repro-
ductive performance prior to embarking
on a gene-transfer program. Trials were
conducted using Yucatan miniature pigs
to ascertain the feasibility of a superovula-
tion regimen, artiFcial insemination, and
embryo transfer to standard commercial
crossbred females. Yucatan semen was
collected and used to successfully fertilize
ova obtained by hormonal stimulation of
donor females. As it happened, only 25%
of the potential eggs were recoverable and
previous page 230 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 232 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off