"This advance opens up an exciting range of possibilities for future research, from developing new treatments for male infertility to enhancing the survival of endangered species," said Duane Alexander, M.D., Director of the NICHD.
Their research, funded in part by the National Institute of Child Health and Human Development, will be published online this week in an upcoming issue of Proceedings of the National Academy of Sciences.
Led by Hiroshi Kubota, D.V.M., Ph.D., the team of researchers from the University of Pennsylvania School of Veterinary Medicine in Philadelphia, also included Mary Avarbock and Ralph L. Brinster V.M.D., Ph.D. The researchers succeeded in developing the culture medium containing the precise combination of cellular growth factors needed for the cells to reproduce themselves outside the body. Known as spermatogonial stem cells, the cells are incapable of fertilizing egg cells but give rise to cells that develop into sperm.
In 1994, this same research team developed the means to transplant spermatogonial stem cells from one mouse into another. The recipient mice then produced sperm--fully capable of fertilizing egg cells--with the genetic characteristics of the donor mice.
Because they can now grow spermatogonial stem cells in culture, researchers have a ready source of cells that they could manipulate genetically, explained the study's senior author, Ralph Brinster.
For example, researchers could implant a new gene into a spermatogonial cell, reproduce a large number of spermatogonial cells in the culture medium, and then implant the cells into recipient animals. These animals could then pass the new trait on to their offspring. The ability to introduce a new trait into animals would greatly assist breeders of both livestock and laboratory animals.
Moreover, by culturing and freezing spermatogonial stem cells from a valuable livestock animal or an endangered species, researchers could extend the reproductive life of that animal indefinitely. (The researchers developed a technique for successfully freezing and thawing spermatogonial cells in 1996.)
By manipulating the culture media that contains the spermatogonial stem cells, researchers might also be able to induce the spermatogonial cells to develop into sperm cells that could be used to fertilize eggs, providing a method to treat some types of infertility.
"This finding is likely to be applicable to humans," Dr. Brinster said. He added that the same growth factors needed to culture the mouse stem cells would likely foster the growth of human spermatogonial cells as well as the cells of other mammals.
Currently, males who undergo chemotherapy that renders them infertile can store their semen so that it can be used at a later date, should they wish to father children. However, this approach results in a less than 50 percent success rate. Boys who are too young to provide a semen sample but who also need such chemotherapy treatments could also be helped by the new technique. Their spermatogonial stem cells could be cultured to increase their numbers, frozen, and reimplanted at a later date, restoring their fertility.
Moreover, the new culture technique would allow researchers to further investigate the potential of spermatogonial stem cells as a source for more versatile adult stem cells to replace diseased or injured tissue. The replacement tissue might be used to help patients with spinal cord injury, or disorders like Parkinson's disease or heart disease.
To conduct their study, Dr. Kubota and his colleagues began with mice that had been genetically altered to express green fluorescent protein, or GFP, which gives off a green light in the presence of a certain wavelength of light. During key stages of the experiment, tissue from the donor mice gave off a green light.
At the first step, the researchers could distinguish spermatogonial stem cells from the cells used to nurture them in lab cultures by the green light the spermatogonial stem cells gave off.
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