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Randi Hutter Epstein, MD, MPH Headshot

How to Tame Wild Sperm

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I whacked sperm the other day. In a scientific sort of way.

I was in an embryology lab using a microscope to spot them and a skinny pipette to bop them on their heads. My job, or really my pretend job: to stop a few sperm in their tracks as if I were a real embryologist capturing one and cramming it into an egg.

It was shockingly enervating, more than any other lab work I'd ever tried. I was like the queen bee taking charge of an army of little worker bees. I'd eye one, usually a straggler, trail it with my pipette, and whack! I'd stun the thing.

"Don't hit, just brush them," the embryologist told me.

In real life, embryologists slow sperm as part of a process called Intracytoplasmic Sperm Injection, or ICSI for short. That's when a single sperm is injected into an egg. After practicing with foul, bunnies, and other farm animals, a Belgian team reported the birth of the first human baby by ICSI in The Lancet in 1992. Since then, hundreds of thousands of babies have been born using the technique. In the early days, fertility experts resorted to ICSI when they believed that none of the sperm in the batch would be able to crack into the egg without a lot of extra guidance. Nowadays, they use it in more cases than not: ICSI accounts for upwards of two-thirds of all the test-tube baby cycles in America, according to the most recent statistics of the Society for Assisted Reproductive Technologies.

In my longtime utter fascination with all things sperm and egg, I had written about the original Belgian feat, and after that I wrote about concerns that ICSI babies would come out damaged (they don't), and I also wrote about the nitty-gritty details of the process, such as how the egg is cracked to allow the sperm to penetrate. Funny thing is that after all of these years and interviews galore, it never dawned on me that you would have to catch the sperm in the first place. But it makes so much sense.

You can't put a wiggly sperm into an egg. First of all, how would you? It's only natural to slow the thing down. (Or actually, quite unnatural.) And secondly, if you shove an entire moving sperm into an egg, the entire egg explodes. Scientists discovered that when they were tinkering with sea urchins, long before the Belgian human success story.

If "you send this projectile thing into the egg, it's just like putting a bomb into the egg, and you kill if from the inside with a sperm," explained Dr. Orhan Bukulmez, reproductive endocrinologist at University of Texas Southwestern Medical Center and the author of a sperm review article in the February 2012 issue of Current Opinion in Obstetrics and Gynecology.

The tail, moving this way and that for, possibly, hours, damaged all egg's innards. Ironically, an intact sperm cannot fertilize an egg. You have to dismantle the sperm's outer membrane, its sheath, something that typically happens when sperm and egg are left to their own devices. This disrobing process is one of the mysteries of childbirth, a crucial step necessary to excite the egg. They have to do it so that the two of them can start the next process: getting their genes ready for fertilization. In other words, just plunking a fully formed, intact sperm is not going to fertilize an egg. So ultimately in these early experiments, as you can imagine, the egg didn't fertilize.

Scientist first attempted to put just a sperm head into an egg using a human freeze-dried sperm head and a hamster egg. The point, of course, was not to create tiny humans addicted to exercise wheels but just to see if the sperm DNA got into the egg and began to unravel, the first step toward fertilization. That's called "spotting the pronucleus." (Ultimately, sperm DNA would combine with egg DNA, creating, once again, a full set of 23 chromosome pairs in humans, but scientists weren't waiting for that to happen, this time. To combine DNA -- the goal of mating, on a microscopic scale -- requires that the partners be from the same species.) They saw what they needed to see, so they moved on to other mammal trials, eventually putting human sperm into human eggs.

As a first-time pretend sperm catcher, decked out in full lab regalia (sky-blue paper robe, booties, and hair net), I was shocked by how hard it was to catch the quivering little bean sprouts:

Then, with a bit of beginner's luck, I was paralyzing one sperm after another, bopping them on the head like Little Bunny Foo Foo hopping through my sperm forest.

Until I learned I was doing it all wrong. (Not that it really mattered; the embryologists gave me a dud batch of sperm to play with, the leftovers or perhaps the losers destined for the waste bin.)

When you whack sperm, the objective isn't to hit them on the head. You don't want to kill them or damage the head, but you want to tame them so that you have complete control and are able to manipulate them. (The marriage metaphor is too obvious to elaborate.)

You are supposed to aim for a point between the head and the tip of the tail, said Dr. Bukulmez, which seemed to me an impossible task. (I was not in his lab, but he went over the play-by-play in a telephone call afterwards). What's more, in real life (or, really, unnatural life-making procedures) you can't waste a moment.

"You can't just fool around once you get it," said Dr. Bukulmez. "You have to put it in the egg, because it can degenerate. You must have the dexterity and get the egg ready, and boom: You inject without losing time. You are not selecting sperm and breaking the tails of 20 and keeping them for 30 minutes while preparing eggs. Everything is time-dependent."

The remarkable thing about all these experiments (besides providing infertile couples with babies) is the amazing mysteries we have unraveled about how sperm and egg do it when left to their own devices. Besides figuring out that only the sperm head gets into the egg, scientists also discovered that that the egg, but not the sperm, has repair mechanisms. Scientists think that means that if a slightly faulty sperm is injected into an egg, the egg may be able to fix it up and use it to make a healthy baby, explained Dr. Bukulmez. Or, as he put it (and this should come as no surprise to anyone), "the female often takes care of things."