Spermatogonial stem cells are the only stem cells in the body that can transmit genetic information to the offspring. Current methods to create transgenic animals are based on oocytes or eggs. Although germline can be efficiently modified in mice, it is inefficient or impossible in other animal species. Our long-term goal is to use spermatogonial stem cells for genetic modification.
　In 1994, a technique to transfer spermatogonial stem cells was developed. Testis cells transferred into the seminiferous tubules of infertile animals form colonies of spermatogenesis and produce donor-derived offspring by mating with females. Using this technique, we have developed several new methods to manipulate spermatogonial stem cells. It is now possible to 1) purify stem cells, 2) introduce retrovirus into stem cells for transgenic animal production, and 3) treat male infertility by gene therapy. However, due to the absence of culture technique, genetic modification of stem cells was not efficient.
　We have recently succeeded in long-term culture of mouse spermatogonial stem cells. Due to their unique morphology, we named them germline stem (GS) cells. GS cells have different morphology from embryonic stem (ES) cells, and can grow exponentially in vitro for more than 6 months. Upon transplantation into infertile animals, GS cells can produce normal fertile offspring, indicating that they are real stem cells.
　GS cells have several advantages over ES cells. First, although ES cells are only available during the embryonic period, GS cells can be derived from postnatal animals. Second, they are not tumorigenic and committed to the germline lineage. Given that ES cells with germline potential have been obtained only from mice, our GS cell technology may resolve current challenges with ES cells and greatly contribute to the development of new transgenic technologies. We are now trying 1) to modify the genome of GS cells and 2) to derive GS cells from other animal species.

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2. Shinohara, T., Orwig, K. E., Avarbock, M. R. and Brinster, R. L. 2000. Spermatogonial stem cell enrichment by multiparameter selection of mouse testis cells. Proc. Natl. Acad. Sci. USA 97, 8346-8351.

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5. Kanatsu-Shinohara, M., Ogonuki, N., Inoue, K., Miki, H., Ogura, A., Toyokuni, S. and Shinohara, T. 2003. Long-term proliferation in culture and germline transmission of mouse male germline stem cells. Biol. Reprod. 69, 612-616.