The team of researchers, headed by Dr. Takashi Shinohara, injected the retrovirus into the seminiferous tubules in the testes of immature male mice. The retrovirus was taken up by the sperm stem cells, which went on to produce sperm with the transgene.
According to Dr. Shinohara and his colleagues Mito Kanatsu-Shinohara and Shinya Toyokuni, 86 percent of the males injected with the retrovirus became fertile. Eight of 31 of these males (26 percent) later mated normally with wild-type female mice and sired offspring. An average of about 3 percent of these offspring had the transgene.
The transgene remained stable and was detected in some offspring in the next generation. When transgenic offspring from injected ("founder') males were mated with wild-type mice, six of eight second-generation offspring from a transgenic male mouse showed the presence of the transgene, as did one of three offspring of a transgenic female mouse.
Because stem cells treated with retroviral transgenes can continuously generate large numbers of transgenic sperm, many transgenic offspring can be produced from a single "founder" male.
In a paper accepted for publication in the journal Biology of Reproduction, the Japanese team notes that other methods have been described for introducing transgenes into animals, including techniques based on eggs or embryos from female animals. The rate of success for these methods, however, is generally less than 1 percent.
The new method of using male sperm stem cells is simple and has a relatively high rate of success. It should be directly applicable for producing a wide range of animals, including pigs and cattle, with modified germlines.
The same technique does not work with mature male mice, the scientists point out. Apparently the retrovirus injected into seminiferous tubules of immature mice is able to penetrate potential barriers and reach the sperm stem cells.
Biology of Reproduction, published by the Society for the Study of Reproduction, is the top-rated peer-reviewed journal in the field of reproductive biology.