Human eggs: 9 fascinating facts

Know much about eggs? If you're like many women and men, you know more about the eggs that come in a carton than the ones a woman's body makes. That's too bad. As cells go, human eggs are truly amazing. Keep clicking as Dr. Randi Epstein, author of the new book Get Me Out: A History of Childbirth from the Garden of Eden to the Sperm Bank, shares nine fascinating facts about eggs.

Eggs are made early

The female body starts making eggs at nine weeks. Not nine weeks after birth, but nine weeks after conception. By the time it's five months old, a female fetus has made upward of 7 million oocytes. By the time of birth, the vast majority of these immature egg cells have died. That's normal..

Eggs are huge

The human egg is a giant compared to other cells in the body. It has a diameter of about 100 microns (millionths of a meter), or roughly the thickness of a strand of hair. No other cell in the body is anywhere near that big. That egg at left is NOT a human egg.

Eggs are precious

On average, women ovulate a mere 400 to 500 eggs in her lifetime. That makes eggs far rarer than sperm. In fact, more sperm cells are released during a single ejaculation than a woman produces her entire life.

Maybe that's one reason why eggs are worth so much more than sperm. An egg donor might make several hundred dollars for a single egg. A man might earn only a few hundred dollars per ejaculation - which translates into a pittance for each sperm cell.

Eggs have an extended adolescence

Unlike other cells in the body, egg cells take years to "grow up." That is, they spend years inside the ovaries in an immature state, maturing just before they are released during the process of ovulation.

Among the hundreds of eggs in this half-mature state, what causes one egg and not the others to be released? No one knows.

Eggs are fragile

A technique called vitrification, doesn't actually involve freezing eggs but hardening their outer later, like encasing them in a glass container.

Eggs age fast

Young women have plenty of healthy eggs. In fact, about 90 percent of the eggs of a 21-year-old woman are viable. Only about 10 percent of the eggs of a 41-year-old woman might be viable. That's why some young women are having their eggs extracted and frozen - just in case it takes a while to find Mr. Right.

Eggs are monogamous

While a million sperm may be trying to get into the egg, eggs have special powers to ensure that once one sperm gets inside, no others are allowed in. Eggs house a special "organelle" that releases proteins and enzymes to ensure that another sperm won't get in.

What decides which sperm will be the lucky one? No one knows.

Eggs have all the baby-making machinery

In ancient times, people figured that the "life force" was contained inside sperm. Now we know that the eggs run the show. In addition to supplying half the genes of the baby-to-be, eggs have the power to rev up the sperm-egg union. The egg's DNA dangles in its center, held together by a set of spindles (think cobweb).

Egg donation - not so fun

Sperm donation involves not much more than masturbating. Egg donors must undergo a complicated medical procedure. First, the donor gets hormone injections that "hyperstimulate" the ovaries so that they'll create not one egg but dozens. When the timing is right, doctors insert a catheter inside the birth canal to suck the fluid from the follicle, hoping to snatch a few eggs.