By David N. Leff

For each heartbeat in a sexually mature male, that man pours about 1,000 sperm cells into his testis. They are produced by his unique precursor stem cells ¿ the spermatogonia. What makes them unique is SSC¿s sole responsibility for perpetuating the human race.

¿The only cells, including all other stem cells in the adult body that divide and can contribute genes to the next generation, are the spermatogonial stem cells [SSCs],¿ observed reproductive biologist Ralph Brinster, at the University of Pennsylvania in Philadelphia. ¿They divide throughout adult life, whereas female germ cells stop dividing before birth. Stem cells in the adult body are essential for survival, and have received considerable attention lately. They can (1) self-renew and (2) produce daughter cells.¿ (See BioWorld Today, July 18, 2000, p. 1.)

Brinster, a professor of animal biology at the university¿s School of Veterinary Medicine, is senior author of a paper in today¿s Proceedings of the National Academy of Sciences (PNAS), dated Oct. 23, 2001. Its title: ¿Transgenic mice produced by retroviral transduction of male germ-line stem cells.¿

Pursuing the male/female dichotomy, Brinster told BioWorld Today, ¿All germ-line studies so far have been done with what you might call female-derived cells ¿ oocytes, eggs, blastocysts ¿ and primarily in the mouse. So this PNAS paper reports for the first time that a genetic modification has been introduced into an animal¿s genome using a male germ-line stem cell. The efficiency of generating transgenics by spermatogonial stem cell modification proved similar to the efficiency with female-derived cells, 5 percent, and time necessary, five months.

¿One aspect,¿ Brinster continued, ¿is that these SSCs are available throughout the life of the male, from when he¿s born to when he dies. And they continue to divide nonstop. All female germ cells are made in the fetus, and they cease dividing by the time of birth, so the supply is limited, and you cannot get them to divide once the animal is born. SSCs are a very readily available source of stem cells that researchers can use. Also, transplantation assays and genetic modification can help us understand more about spermatogenesis and fertility.¿

This does not mean that nature is politically incorrect, implying male superiority, but rather signifies the division of reproductive labor between the sexes.

Title Holder: The Body¿s Premier Cell

¿Biological science considers spermatogonia to be one of the most important cells in the body,¿ Brinster went on. ¿That¿s what biology is built around,¿ he explained. ¿The shape of the animal is almost irrelevant to biology, as long as the DNA in its germ cells is passed from one generation to the next.

¿In the hierarchy of stem cells,¿ he related, ¿the embryonic stem cell [ESC], now so popular, is at a very early developmental stage. All other stem cells derive from that ESC. Some people think that the spermatogonial stem cell may be the first, or most closely associated, partly because it arises at about 7.5 days after the ESC is isolated in the mouse. And also because it¿s been shown that SSCs from the fetus are cells that develop into ESCs. So in biology it has to be considered the premier stem cell because it¿s the one that passes the genetic information, by way of the sperm it makes, to the next generation, and thus is responsible for species continuity.

¿There are only two stem cell systems in the body ¿ hematopoietic and spermatogonial,¿ Brinster explained. ¿The definitive proof of having isolated a stem cell,¿ he pointed out, ¿is that you can transplant it and show regeneration of its function in a new body. And that could only be done for hematopoiesis ¿ now joined by SSC.¿

He made the point, ¿When people talk about transgenic animals and their value, most of them are mice. In other species, it¿s very difficult to make transgenic animals, especially primates, which are probably the best models for human disease. But in the case of spermatogenesis, that¿s a highly conserved process, and the stem cells continue to divide throughout life. So I think in the future transgenesis will be more easily done with male germ-line stem cells than through eggs.

¿You can develop models of human disease in pigs and monkeys, for example,¿ Brinster observed. ¿They are larger, closer to some of the organ systems in tissues, and resemble humans more than mice do. And there¿s a big effort now, particularly in pigs, to develop organs for transplantation.

¿In order to do that,¿ he went on, ¿you have to make genetic changes in the animal, not just to add genes but to remove some. We hope to culture SSCs where we not only insert genes but also delete genes. This would greatly facilitate making animal models of human diseases, and production of replacement organs.

¿Questions have been raised,¿ he observed, ¿regarding whether male germ-line stem cells can be transduced with a retroviral vector [RV] ¿ as used in somatic cell gene therapy ¿ and whether any gene introduced would be silenced. Our article [PNAS¿s cover story] shows that it¿s relatively easy to infect them with an RV, and probably other viruses likewise. The stem cell was transduced at relatively high efficiency ¿ 20 percent ¿ and expression was not silenced. About 10 percent of the stem cells carried active genes that were transmitted for at least three generations.¿

Caveat: Gene Therapy And The Germ Line?

¿Many think that if germ-line gene therapy is done in human patients,¿ Brinster said, ¿it will be through the spermatogonial stem cell. But I don¿t know how many years in the future that might be. So in that sense it¿s a step along a very valuable resource ¿ the SSC.¿

¿If you treat the patient for a liver or a blood disease,¿ he suggested, ¿that¿s fine, and the patient is cured. But you may not want that curative retroviral gene to remain in the germ line, and able to be transmitted to progeny, to children. I think people are very conservative about manipulating the germ line,¿ he added. ¿The possibility of eugenics has been known for thousands of years, and it hasn¿t gained much popularity. Animals have been bred and selected for thousands of years, but people don¿t do that to themselves.¿