Challenges and progress in the production of transgenic cattle.

The production of transgenic cattle presents a number of unique challenges not encountered in other species. First, the survival of microinjected zygotes is low; only 15% in vivo-derived develop into morulae and blastocysts and, of these, only about 18% yield live calves. Second, transgene integration frequency is relatively low, around 3%. Thus, more than 1000 zygotes must be injected to produce a single transgenic calf. Obtaining sufficient zygotes from donor cattle to sustain a transgenic cattle programme is logistically and financially prohibitive, since the average superovulated donor yields only about four microinjectable zygotes per collection attempt. In vitro oocyte maturation and fertilization techniques may be used to alleviate this problem, although initially the developmental potential of in vitro-derived microinjected zygotes is lower than their in vivo-produced counterparts (8% v. 15%, respectively, yield morulae and blastocysts). Since only 3-5% of calves born from microinjected zygotes produced in either fashion yield transgenics, at least 20-30 pregnancies must be carried to term for every transgenic calf born. These conditions require that large herds of donor and recipient cattle be maintained. Recipient requirements could be reduced if transgene integration frequency could be increased, but improvements in the near future are unlikely since the mechanism of integration after pronuclear microinjection is poorly understood. Alternatively, embryos could be screened for integrated transgenes before transfer; however, efforts in this area have been complicated by high frequencies of false positive results. Although yet to be developed, bovine embryonic stem cells would alleviate many of these problems and permit a wider range of genetic manipulations.