David Katz pops a favorite cassette into one of his lab’s VCRs. The image is of a pool of fresh semen. At first glance the sperm look like swirling black minnows, zipping crazily in all directions. Their motion appears almost random. Some loop, some zigzag, some march across the screen like so many Energizer bunnies. Each wears a faint halo, an artifact of the background lighting and optics that make individual sperm cells easier to track.
After the novelty wears off, what’s most striking about the video is that only half the sperm are swimming. The others hang motionless, apparently paralyzed or dead, some of them hideously misshapen by deformed heads, kinked tails, or even extra tails and heads. And all these sperm, Katz says, came from a fertile man.
The scene makes it easier to understand why pregnancy is such a chancy thing. Men produce a lot of bad-looking sperm, says Katz--and apparently a lot of bad swimmers too. Up to 300 million sperm get deposited at the opening of the cervix after ejaculation, but fewer than 20 arrive anywhere near the unfertilized egg just five inches away. If you didn’t know that this process actually worked, you’d probably never guess it just by looking.
Yet looking--with all its room for individual judgment and variation--is just how physicians traditionally take a measure of sperm vitality. One measure they use is the well-known sperm count, but this by itself doesn’t predict male fertility. Many clinicians put at least as much emphasis on sperm shape and mobility, using subjective scales with such categories as slow or sluggish versus rapid progressive.
Katz--a chemical engineer turned reproductive biologist--is out to change all that. He and his colleague James Overstreet have established a high-tech sperm lab on the outskirts of the campus of the University of California at Davis. Here, in a mixture of well-weathered farm buildings and brand-new custom-built labs, researchers are bringing consistency to sperm analysis by attaching precise numbers to video images.
Katz’s pioneering use of CASA--computer-aided sperm analysis-- hasn’t yet found its way into routine sperm testing, but it’s now being used in hundreds of research labs worldwide. We can see things with CASA, says technician Jane Andrew, that you simply can’t see with the human eye. Andrew is seated in front of a personal computer, watching a small monitor showing another scene of swirling sperm. The image, magnified 100 times, was made with a video camera attached to a standard microscope. (Well, not entirely standard: the stage is warmed to keep the sperm happy.)
With a few keystrokes, Andrew attaches numbers to the sperm. First a digital image processor replaces each cell with a curly silhouette of dots. The silhouettes make tracks across the screen for less than a second. Then another program transfers an image of one sperm cell’s trajectory to an adjoining computer and monitor. Alongside the erratic trajectory, this computer spells out the cell’s stats: the linearity or curvature of its course, its speed, the number of side-to-side head swings, the degree of head wobble. Group statistics include the percent of sperm in motion and their concentration.
The images aren’t as clear as live sperm under a microscope, but that’s okay with Katz. It’s a slightly myopic view, but we can repeat the experiment a zillion times, he says--meaning they can rerun the video endlessly to refine their measurements. With live sperm, you’d be hard pressed to find the same cell again.
Katz is one of a number of researchers who over the past two decades have done much to further our understanding of how sperm cells go about their herculean task. Since the 1970s, using microscopes and more or less standard cameras, film, and stroboscopic lighting, Katz and other researchers have been finding that sperm behavior is far more subtle than once believed. After ejaculation, it’s not full speed ahead until the deed is done. Instead, sperm cells swim with different strokes suited to different environments.
Moving is most of what a sperm does. Once switched on during ejaculation by a bath of sugars from the prostate gland, the sperm’s tail drives it on an often erratic path toward way stations in the cervix and the lower part of the fallopian tubes, where it can wait four or five days, if necessary, for a maturing egg. But the sperm’s swimming style is not self-directed. Chemicals it encounters inside the female reproductive tract influence the tail’s motion. Katz’s group found that potassium suppressed the tail’s motion, while another group demonstrated that calcium speeded it up.
Researchers also found that sperm have a repertoire of different swimming strokes--three, in fact. When first deposited near the cervix, those sperm that seem able and inclined to swim (about half of them) lash rapidly with their tails--an exuberant motion that sends them darting forward. Many, though, never seem to get anywhere, and for most the journey ends right there at the cervix.
But a few struggle through the mucus of the cervix, using a second swimming stroke. Here the tail works like a propellor, with just its tip spiraling around. Cruising slowly, the sperm seem guided by long, elastic protein molecules in the mucus. The drapes of mucus lead upward toward reservoirs, called crypts, on the walls of the cervix; some researchers believe that the female reproductive tract adjusts its chemistry to make sure the sperm are collected there, with tails temporarily halted, while the female body prepares to release an egg.
It’s not certain that the sperm are responding to chemical cues here, though it’s plausible, Katz says, and potassium could play a key role. Recent studies do suggest that the waiting sperm are capable of picking up a large number of chemical signals. Early this year researchers in Belgium announced that the precursors of sperm cells in the testes have all the molecular instructions needed to grow 20 different odor receptors. Whether the odor receptors in fact develop and function remains to be seen, but these are the same kind of receptors that allow the nose to smell. That the nose and sperm could have receptors in common is not unusual. Molecules that work well in one place are often pressed into service for similar tasks elsewhere in the body.
A third, final swimming stroke is used by those pitifully few sperm that get near the egg. There they become hyperactivated by calcium ions in the oviduct. In this environment they thrash and flop violently like fish pulled from water--an odd sort of swimming, to be sure, but a movement that probably raises their odds of bumping into an egg.
With CASA, Katz and his team plan to build on previous findings by detecting far more subtle differences in sperm shape and motion. For instance, after studying CASA videos of the sperm of hundreds of veterans, Katz’s group found that an abnormal sperm shape--an elongated head--showed up more often in veterans with fertility problems. Their sperm also tended to swim both more slowly and less straight than sperm from fertile men.
Those videos were originally made by the Centers for Disease Control as part of a search for evidence that exposure to Agent Orange might have affected the health of Vietnam veterans. Katz believes that sperm are in fact sensitive biomarkers--that is, they give early warning of subtle damage from toxins--and CASA measurements showed slight statistical differences between the sperm of Vietnam veterans and that of former servicemen who had not been to Vietnam. But no links to Agent Orange were established, because for one thing, there wasn’t enough information about who was or was not exposed to it in Vietnam.
Katz’s team recently used the same technique to study the sperm of men who work in dry-cleaning establishments and breathe the solvent perchloroethylene (PERC). They found subtle changes in sperm mobility among those men too, and the differences increased with greater PERC exposure. The researchers aren’t sure that breathing PERC fumes lowers fertility, but they do know that the wives of men who had higher exposures took longer to become pregnant.
Now Katz is working with the California health department on a CASA study looking for connections between fertility and caffeine, nicotine, and trace compounds in tap water. After that, it’s on to nuclear power plants; Katz’s group will examine workers’ sperm for effects of routine, low-level exposure to radiation that other tests may have missed.
To further these studies, Katz’s colleague Russell Davis is designing the next generation of CASA. His aim is to standardize measures for sperm appearance as well as for sperm motion. The shape and size of the head are crucial. No matter how well a sperm cell can move, if its head deviates much from the standard oval, its trip will be short: it won’t be able to burrow through the jellylike mucus in the cervix. As Katz notes, It’s primarily just the good-looking sperm that are able to get in.
For a traditional evaluation of sperm shape, technicians focus a microscope on a slide of dead sperm. They examine the heads of 100 cells chosen at random and, like modern-day phrenologists, assign the heads to the somewhat vague categories of large, small, pointed, or normal. But with Davis’s newest computer program in charge, the viewing is all done electronically. Microscope controls whir softly as the optics zoom in and out, the viewing stage glides from side to side, and video images play on the computer screen. In 15 seconds the program finds and photographs about two dozen sperm, then tosses their fuzzy heads up on the screen, row after row, like criminal mug shots. Then it groups them under the traditional four categories, using not subjective judgment but objective measurement of such parameters as length, width, area, perimeter, length-to-width ratio, and so on.
The aim now is to assemble such portrait galleries for enough men, fertile and infertile, so that researchers can find connections between head shape and fertility. Also in the works is a subtler examination to determine whether a sperm head has prematurely popped its top--a cap called the acrosome. Without this cap, which contains an enzyme needed to loosen the tissue surrounding the egg and thus allow the sperm access, a sperm cell has little chance of achieving fertilization.
Some researchers suspect that while CASA can greatly improve sperm testing, gross appearance and mobility measurements alone will never pinpoint what makes good sperm. If odor receptors, for example, turn out to be important, they would be much too small to be seen with CASA. There will never be a single test that guarantees fertility, Katz predicts. Will the clinician be able to use this to improve diagnosis and therapy? The answer is we’re not sure yet.