A male achievement that is perhaps insufficiently celebrated is that with every heartbeat a man generates 1,000 sperm, each of which has taken two months to produce.

In a step that brings closer the possibility of making inheritable genetic changes in humans, scientists have succeeded in growing outside the body the special stem cells that direct the remarkably prolific process of sperm production.

Although the method now works just for mice, it may well apply to human cells, since they use the same genetic signals as mouse cells.

Cultivation of the sperm production cells has been a 10-year goal of Dr. Ralph L. Brinster, a reproductive biologist at the University of Pennsylvania School of Veterinary Medicine. The ability to culture the cells is a first step that leads in a number of possible directions. One is correcting the sperm of infertile men. Another, if ethically acceptable, would be genetic engineering in humans. A third is generating embryonic stem cells without the controversial step of making an embryo.

The new method was developed by Dr. Brinster and his colleagues Hiroshi Kubota and Mary R. Avarbock and is reported in the current issue of Proceedings of the National Academy of Sciences.

The ability to cultivate the sperm production cells in large numbers would make it possible to try swapping mutated genes in the cells for normal or improved versions. With infertile men, the sperm production cells could be removed, genetically treated, and put back in the testis, where they should produce normal sperm.

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