Homer Simpson said: "I won't lie to you. Fatherhood isn't easy like motherhood."
Yet in our insistence that men are more than elaborately engineered gamete vectors, we neglect the marvels of their elaborately engineered gametes. As the scientists who study male germ cells will readily attest, sperm are some of the most extraordinary cells of the body, a triumph of efficient packaging, sleek design and superspecialization. Human sperm are extremely compact, and they've been stripped of a normal cell's protein-making machinery; but when cast into the forbidding environment of the female reproductive tract, they will learn on the job and change their search strategies and swim strokes as needed.
Sperm are also fast and as cute as tadpoles. They have chubby teardrop heads and stylish, tapering tails, and they glide, slither, bumble and do figure-eights. So while a father may not be entitled to take the same pride in his sperm as he does in his kids, it's fair to celebrate the single-minded cellular commas that helped give those children their start.
Sperm are pretty much the tiniest cells in the human body. The head of a mature, semen-ready sperm cell spans about 5 microns, or two-thousandths of an inch, less than half the width of a white blood cell or a skin cell. And a sperm cell is absurdly dwarfed by its female counterpart, the egg, which, fittingly or not, is among the biggest cells in the body. At 30 times the width of a sperm, the egg is massive enough to be seen with the naked eye.
But men have the overwhelming quantitative edge in the gamete games. Whereas current evidence suggests that a human female is born with all the eggs she will have, and that only about 500 of her natal stock of 1 million will ever ripen and have a shot at fertilization, a male from puberty onward is pretty much a nonstop sperm bakery. Each testicle generates more than 4 million new sperm per hour, for a lifetime total of maybe 12 trillion sperm per man (although the numbers vary with the day and generally slope downward with age).
The average ejaculation consists mostly of a teaspoon's worth of nonspermic seminal fluid, a viscous mix of sugars, citric acid and other ingredients designed to pamper and power the sperm cells and prepare them for difficult times ahead; the sperm proper account for only about 1 percent of the semen mass. Yet in that 1 percent may be found 150 million sperm, 150 million human aspirants yearning to meet their mammoth other halves.
To which one can crack, dream on. Not only are there far too few eggs to go around, but also the majority of sperm couldn't fertilize an ovum if it were plunked down in front of them. "Only a perfectly normal sperm can penetrate an egg," said Dr. Harry Fisch, a urologist at Columbia University Medical Center, "and the majority of sperm are abnormally shaped." Some may have pinheads, others have two heads, some lack tails, a third don't move at all. As a rule, Fisch said, a man is lucky if 15 percent of his sperm are serviceable. "One guy I saw had 22 percent," he said, "but that's rare."
Creating sperm is a complex, multistep operation in which immature cells spend one or two months wending through a labyrinth of tubules coiled in the testes, at each stage losing a bit more of the blobby contours and yolky contents of standard cells and assuming the streamlined profile of sperm cells. The operation is a delicate one that must be performed at temperatures some 2 degrees below that of the body, which is why the testicles hang outside the body, where breezes can keep them cool; why a man hoping to become a father is advised to skip the hot baths and saunas; and why a bout of high fever can disrupt fertility for months.
The model sperm that emerges at tubule's end has, like an insect, three basic body segments. Of crowning importance is the head, which is taken up largely by a supercondensed tangle of 23 chromosomes, half the complement of DNA found in a normal body cell and thus the right number to merge with an egg's 23 chromosomes and begin tapping out a whole new body. At the tip of the sperm head is the acrosome, a specialized sack of enzymes that help the sperm penetrate through what Joseph S. Tash, a male fertility expert at the University of Kansas Medical Center, calls the "forest" of ancillary cells and connective tissue that surrounds the ripe, ready egg.
Below the head is the midpiece, which is packed with the tiny engines called mitochondria that lend the sperm its motility, and below the midpiece is the tail, a bundle of 11 entwined filaments that thrashes and propels a sperm forward at the estimable pace of one-twelfth of an inch per minute, the equivalent of a human striding at 4 mph.
Sperm do not really hit their stride until they are deposited in the female reproductive tract, at which point chemical signals from the vaginal and cervical mucus seem to spark them to life. Released from the buffering folds of their seminal delivery blanket, they at first swim straight ahead, torpedo-style, "with very little back and forth of the head," Tash said. They may linger in the cervical mucus for a couple of days, or cross the cervix and enter the uterus.
If an egg has burst from its ovarian follicle and been plucked by a fallopian tube, sperm can sense its signature, a telltale shift in calcium ions. The sperm become "hyperactivated," said Moira O'Bryan, a sperm expert at Monash University in Australia, switching to "a crazed figure-eight motion" ideal for boring through barriers. The ovum eggs them on, signaling some to play the sacrificial kamikaze and explode their enzyme sacks prematurely, loosening the corridor for other, shapelier sperm to pass through intact. A few dozen fine-figured sperm find their way to the final barrier, the egg's plasma membrane, where they waggle with all their crazy-eight might and beg to be chosen -- but only one will be taken, will fuse with the egg and be absorbed into its rich inner sanctum.
In a fraction of a second, an electrical, ionic jolt dramatically changes the egg's outer coat, to forestall the lethal intrusion of additional sperm.
The wheels are in motion.
New York Times News Service