Male infertility--the inability of sperm to fertilize an egg--has several causes. If during fetal development the testes do not descend into the scrotum, the higher temperature of the abdominal cavity or inguinal canal causes the developing sperm cells in the seminiferous tubules to degenerate. Certain diseases, such a mumps, may inflame the testes (orchitis), producing infertility by destroying cells in the seminiferous tubules.

Both the quality and quantity of sperm cells are essential factors in the ability of a man to father a child. If a sperm head is misshapen, if a sperm cannot swim, or if there are simply too few sperm cells, completing the arduous journey to the well-protected female cell may be impossible. Sometimes even a sperm that enters an egg is unsuccessful because it lacks the microtubules necessary to attract and merge the nuclei of the two cells.

Until recently, sperm analysis was rather subjective, based on a person's viewing the cells under a microscope. Now, computer-aided sperm analysis (CASA) is standardizing and expanding criteria for normalcy in human male seminal fluid and the sperm cells it contains.

To analyze sperm, a man abstains from intercourse 2 to 3 days, then provides a sperm sample, which must be examined within the hour. The man must also provide information about his reproductive history and possible exposure to toxins. The sperm sample is placed on a slide under a microscope, and then technology intervenes. A video camera sends an image to a videocassette recorder, which projects a live or digitized image. The camera also sends the image to a computer, which traces sperm trajectories and displays them on a monitor or prints a hard copy.

Hundreds of CASA systems are now in use, mostly at fertility clinics. The devices are also helpful in using sperm as biomarkers of exposure to toxins. For example, in one study, the sperm of men who work in the dry cleaning industry and are exposed to the solvent perchloroethylene (believed to damage sperm) were compared with sperm from men who work in the laundry industry and are exposed to many of the same chemicals except this one. CASA showed a difference in sperm motility that was directly related to level of exposure, as measured by exhalation of the chemical. This result supported the reproductive evidence: Although the men in both groups had the same numbers of children, the dry cleaners' wives took far longer to conceive than did the launderers' wives.