Sperm Cells Gain Respect
Millions of tadpole-like sperm cells rush to fertilize one egg. They deserve more respect than they often get.
They look kind of adventurous in a way– sperm cells, like marathoners in a contest to the finish line, with only one winner. We humans are ambivalent about them. Our attraction to sex is mixed with squeamishness about its details sometimes, particularly the male’s ejaculation of sticky seminal fluid with its seeming wasteful profligacy of millions of competing sperm cells. It’s a fact of life, but not often the subject of polite conversation. We can learn to appreciate sperm more by applying the more detached attitude of doctors and scientists who see the incredible design involved.
Researchers at MIT, for instance, found that sperm cells are “extremely efficient at swimming against a current,” comparable to salmon leaping the falls heading upstream to their spawning grounds (where the males will ejaculate sperm, incidentally—a wonder within a wonder). Like a salmon, each sperm cell is equipped to travel a long distance, often through difficult terrain.
Of the hundreds of millions of sperm cells that begin the journey up the oviducts, only a few hardy travelers will ever reach their destination. Not only do the cells have to swim in the right direction over distances that are around 1,000 times their own length, but they are exposed to different chemicals and currents along the way.
While we know that sperm cells can “smell” chemicals given off by the egg once they get very close to it, this does not explain how they navigate for the majority of their journey, says Jörn Dunkel, an assistant professor of mathematics at MIT, and a member of the research team.
So how do they do it? “We really know effectively nothing about how sperm cells navigate, so this gives us more information about a potential mechanism that may be important,” the MIT scientists said. They investigated the question by observing live sperm cells traveling through artificial microchannels they made, a kind of sperm obstacle course. They were able to control the flow of fluid and watch the cells react.
They discovered that at certain flow speeds, the sperm cells were able to swim very efficiently upstream. “We found that if you create the right flow velocities, you can observe them swimming upstream for several minutes,” Dunkel says. “The mechanism is very robust.”
One difference they noted was the way the cells swam in a spiral pattern rather than a straight line, and swam in the slowest flows near the walls of the channel. This is different from the motions of other things that propel themselves with a tail. There’s also an element of cooperation in the competition:
“It is a commonly held belief that there is competition between sperm cells, with the fittest reaching the egg first,” Dunkel says. “But recent studies by our team and others show that sperm practically always accumulate at the surface of a tube, and you can end up with a high local concentration of sperm cells, so there could actually be cooperation among these cells that allows them to swim faster collectively.”
The MIT work is not the only research that might pay off in fertilization enhancement technologies and engineering. The BBC News reported a successful “artificial sperm” robot that can be manipulated with magnets, demonstrating that engineers find the design of sperm cells worth imitating. Scientists in the Netherlands believe their work “will be useful in medical and manufacturing applications.” Impressive as this feat is, though, it’s a far cry from the living counterpart:
For its size, the robot can hardly compete with its biological inspiration for pace: it wiggles along at up to 0.5 body lengths per second, whereas a human sperm can cover several times its body length in that time.
Human sperm are also 6 times smaller than the engineered robots, named MagnetoSperm. The imitations are also far simpler, consisting of just a polymer and magnet, lacking the genetic code and proteins that sperm deliver to the egg. Furthermore, the robots have to be manipulated by external controls, whereas live sperm are self-contained.
One other source of respect for sperm cells comes from considering their information content. Sperm are more than tails with DNA packages to deliver. In a new open-access paper in the intelligent design journal Bio-Complexity, developmental biologist Jonathan Wells of the Biologic Institute demonstrates that sperm and egg cells deliver much more information, independent of DNA. One example:
After fertilization the dorsal-ventral axis is established by a process of “cortical rotation,” in which the cortex rotates approximately 30º relative to the interior of the zygote. The movement is microtubule-dependent, and the cortex normally rotates away from the sperm entry point, which has suggested to some that the aster nucleated by the sperm centrosome initiates cortical rotation. The rotation movement orients microtubules in the vegetal cortex, and these form a parallel array that provides tracks to transport various proteins from the vegetal pole to the future dorsal side of the embryo, thereby establishing a dorsal-ventral axis.
This means that in addition to the vast library of information in DNA, each gamete passes along additional epigenetic information vital to the proper spatial development of the embryo. In this case, the sperm cell gets things rolling along the proper axis.
“Sperm” simply means seed (a plant metaphor), and egg is the animal metaphor. For centuries, they were just terms used as placeholders for ignorance. Semen doesn’t look like seeds, and egg cells were too small to be seen (except for large ones, like bird eggs or fish eggs). Until Leeuwenhoek and the invention of the microscope, followed by its phenomenal development and the dawn of era of molecular biology, people really didn’t have a clue what was going on. Even after Leeuwenhoek made the first drawings of sperm cells with their long tails, some thought they housed a miniature version of the adult: a homunculus. What a bombshell it was in the 1950s to discover that gametes carry a molecular alphabetic code! That was the secret: information. What a phenomenal thought to realize that the information in trillions of cells of one generation can be squeezed down into a single cell that can grow back into trillions of cells again in a new individual. That speaks of intelligent design on a scale unfathomable to human minds.
The discovery that both sexes contribute half of the vital information to create a new life represented women’s liberation at a molecular scale. No longer is a woman just a vessel for the man’s seed; both contribute equally to the next generation. Both sperm and egg cells are equally well-designed, too. Individual sex cells, containing just half the human complement, can be lost without harm, like many other tissues of the body; the number doesn’t matter until fertilization takes place. So what if hundreds of millions of sperm are continuously molded in the male, only to be ejaculated into oblivion? We don’t question the purpose of millions of skin cells that are continuously created, only to be sloughed off in the outer layers, or the blood cells that come and go. Here, though, we see a glimpse of design in the apparently wasteful numbers. If the collective sperm actually cooperate in delivering the information to the egg, they can be considered a kind of fluid “tissue” with a collective function.
Many a sperm and egg cell is born to die without consequence. This design is necessary to allow sexual union under a variety of circumstances. Once the gametes fuse, though, the critical mass of genetic (and epigenetic) information is reached for a new, unique, individual life. A marvelous sequence of events dedicated to its development ensues.