It’s crazy to think about, but many of us know more about the eggs in our kitchen than we do about the eggs in our own body. Here are a few egg cell facts to get your started on your educational journey!
Egg cell fact #1: The egg is one of the biggest cells in the body.
Egg are larger than any other cell in the human body, at about 100 microns (or millionths of a meter) in diameter, about the same as a strand of hair. That means you could, in theory, see an egg cell with the naked eye. The fact is that egg cells are about 4 times the size of skin cells—and about 20 times the size of sperm!
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Here’s a really cool interactive tool that lets you compare the size of different biological and chemical units.
Egg cell fact #2: Human eggs are full of instructions.
Unlike birds or reptiles, whose eggs are full of food, mammals don’t develop on their own—they get a cushy uterus and a placenta with plenty of nutrients. So what’s filling up that huge egg cell?
Here’s what we do know: human eggs contain lots of RNA, which transfers genetic code out of the nucleus of a cell, preventing the DNA from having to leave the nucleus. The RNA in an egg cell has a few jobs: it help the egg’s nucleus fuse with a sperm’s during fertilization, it guides the fertilized eggs through its initial cell divisions, and it tells the cells inside a developing embryo—which are all the same, at first—how to specialize, and what kind of cell they need to become.
And the fact is that egg cells need a lot of energy, especially after they’re fertilized and they start dividing and developing. So we know, also, that human eggs contain lots of mitochondria, which anyone who paid attention in 8th grade biology should recognize as the powerhouse of the cell—they convert oxygen and nutrients into chemical energy.
Egg cell fact #3: An egg doesn’t live very long after ovulation.
Once released—a process known as ovulation, which usually occurs around 2 weeks after the first day of your period—an egg cell has a pretty short life span. First, it’s pulled in by the finger-like appendages at the end of the fallopian tube, through which it travels down into the uterus over a period of 12–24 hours.
In the case of unprotected sex around the time of ovulation, the fallopian tube becomes the venue for fertilization, and the fertilized egg will implant in the uterus. But if no fertilization occurs within that 24-hour period, the egg disintegrates. It’ll later be shed—along with endometrial tissue, vaginal secretions, cervical mucus, and blood—during the menstrual period, as the body gets ready for a new cycle of ovulation.
Learn more about ovulation.
Egg cell fact #4: We’re born with all the eggs we’ll ever have.
Most of the cells in our body “regenerate,” or get cleared out and replaced by younger, healthier ones, throughout our lives. The one exception is your eggs. Those—all 1–2 million of them—were born with you, and they’ve been enduring the elements, so to speak, ever since. In fact, egg cells are actually created in utero, at just nine weeks after conception. That means that the egg that created you was inside your mother—when she was inside your grandmother. Whoa.
This can be troublesome for your eggs. As egg cells age, they’re more likely to contain genetic abnormalities—mistakes in their DNA—that happen during the division process. Since DNA is like an instruction manual for our cells, any damage to your DNA can prevent that cell from doing what it’s supposed to do—which, in the case of the egg cell, is make a healthy baby. That’s why instances of infertility, miscarriage, and genetic disorders like Down syndrome increase so dramatically with the mother’s age.
Learn more about the importance of the age of the egg.
There’s good news, though:
Egg cell fact #5: Freezing eggs doesn’t affect how likely they are to result in a pregnancy.
Egg freezing works because it allows a woman to preserve her eggs while they’re still healthy and plentiful, and use them to attempt a pregnancy later. The fact is that egg cells remain just as likely to result in a pregnancy after they’re frozen and thawed as they were at the time they were frozen—allowing a 40-year-old woman to use her 30-year-old eggs, which are much more likely to result in a healthy pregnancy. This is confirmed by several studies, including a large study published this September, that conclude that frozen and “fresh” eggs result in essentially equal embryo quality, pregnancy rates, and live birth rates.
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