Animal Cloning: A Draft Risk Assessment - Biotechnologie.de

Chapter II: Technology Overview 26
550,000 in vivo derived bovine embryos were transferred worldwide in 2004 (Thibier
2005). Most of those transfers occurred in North America (39.5 percent), with the rest
taking place in Asia (~21.6 percent), South America (~21.1 percent), and Europe (~15.9
percent). The numbers of embryo transfers for other species (sheep and goats) were
considerably lower, with approximately 68,000 sheep embryos transferred, mostly in
Australia, New Zealand and South Africa, and fewer than 1,000 goat embryos
transferred, mostly in South Africa and Asia. According to IETS statistics, approximately
16,016 swine embryos, most of which were either transgenic or embryo clones, were
transferred in 2004, with almost all occurring in Korea and Canada.
4. In vitro Fertilization
The first in vitro fertilized (IVF) offspring was a rabbit born in 1959 (Chang 1959). Since
that time, IVF offspring have been born to mice, rats, hamsters, cats, guinea pigs,
squirrels, pigs, cows, monkeys, and humans (Bearden and Fuquay 2000). IVF allows for
the production of offspring from animals where other ART methods fail due to
difficulties with either the female (blocked oviducts, non-responsive ovaries) or male
(marginal semen quality and/or quantity), or where disease is present. In cattle, it is also
used for the production of embryos from sexed semen because of the low sperm counts
resulting from current sexing protocols, and for the further extension of the semen of
superior sires due to the relatively low level of sperm required for in vitro fertilization.
(IVF procedures are also used to assist human couples with limited fertility.)
The overall technique for IVF is similar among species, and involves significant
manipulations in vitro, or outside the body of animals. In livestock species, oöcytes are
collected from the ovaries of either living or deceased animals whose genetic potential is
desirable (Goodhand et al. 1999). Ovaries can be obtained by transvaginal aspiration
from live animals, or from a deceased animal at time of slaughter. Slaughterhouse ovaries
are cross-sectioned and the contents of all of the follicles are collected; mature oöcytes
are collected, evaluated for quality, and used for fertilization. Immature oöcytes must be
allowed to continue to develop in a maturation medium.
Either fresh or frozen-thawed semen can be used for fertilization. Sperm need to be
capacitated in vitro in order to penetrate the zona pellucida and fuse with the ovum or to
undergo the same maturation process that they would normally undergo in the female
reproductive tract. Capacitation involves a series of cellular changes to the sperm
including increased motility, calcium uptake and protein binding (binding to proteins
produced by the female reproductive tract). In vitro capacitation is accomplished by
creating a medium designed to simulate the female reproductive tract and allowing the