Seldom is there a single cell in your body that holds as much interest as your egg cells. Even sperm cells, pretty much the male counterpart, are a dime a dozen. Or even a dime a million, once you think about it. The egg, however, is quite an elusive and even valuable thing. Most of the time, only one of it is available each month. And, once we get to menopause, they just seem to run out.
If you were listening in biology class, you might know quite a bit about these special cells. Even so, there are a few cool and interesting things you still may not know about them.
15 All the Eggs
Scientists used to believe that women already have all the eggs that they will ever need at birth. In fact, some even estimate that all the two million eggs that you will ever have already exist when you’re just 12 weeks in your mother’s womb!
This is unlike men, who will be capable of producing sperm throughout their lifetimes. This is one reason why women go through menopause when the number of healthy eggs are at a decline.
14 Could Be Questioned
Despite the previous fact, however, some exciting new research seems to suggest that women may be able to produce eggs throughout their reproductive years. This was due to the discovery of a new ovarian stem cell that can divide to create brand new, healthy egg cells.
This discovery, however, was made in laboratory mice and research is still ongoing to determine whether they are also present in humans. If they are, however, this could mean that menopause is simply due to aging and not due to running out of egg cells. It could also mean that scientists could one day be able to activate these cells, extending women’s reproductive years.
On the average, a woman will have about 350 to 400 menstrual periods over her lifetime. This means that, multiple ovulation aside, only about 350 to 400 egg cells will be released, hopefully awaiting that a lucky sperm will come her way. This is despite the fact that she has over 2 million egg cells available, not counting those that scientists feel could be produced afterwards!
So what does a woman do with those extra egg cells?
12 The Select Few
We often think that only the sperm cells go through a process of selection, a survival of the fittest, if you will, prior to fertilization. Eggs, however, go through a similar process where the healthiest are pre-selected to actually mature.
Through a single menstrual cycle, more than one of the egg cell precursors, known as the primordial follicles, begin the process of maturing into egg cells. However, only one (or, in the case of multiple ovulation, a bit more) of these primordial follicles, usually the one that the body has determined to be the healthiest, actually reaches maturity and is released as an egg cell.
All the rest remain arrested in development but still contribute to producing fertility hormones until they die off and are reabsorbed by the body at the end of the cycle in a process called ovarian follicle atresia.
11 Big and Proud
Egg cells are among the largest cells of the human body. To give you an idea of just how big they are, take note that most of your body cells cannot be seen with the naked eye. You would have to use a microscope to be able to see a sperm cell or a liver cell, for instance.
An egg cell, however, is visible to the naked eye, at 0.12 mm in length. It’s just at the point where you almost can’t see it as the minimum size that a human eye can see without any help is 0.1mm. It is, however, definitely visible.
10 But Only Half the Genes
Despite being one of the largest cells of the human body, egg cells only have half the genes. It has only 23 of the usual 46 pairs of chromosomes in human cells. And that’s really no surprise as this is what makes human reproduction and genetics work the way it does.
The other half only gets to the egg by the means of sperm. During fertilization, these 46 chromosomes combine to form a brand new human being with exactly half of the genes from mom’s egg cell and half from dad’s sperm cell.
When the egg cell is released from the ovary, it is actually surrounded by a protective outer layer of tiny cells collectively known as the corona radiata, meaning a crown with rays. This layer of cells provides support and nourishment to the egg cell as it makes its way down the reproductive tract. In particular, it supplies the egg cell with proteins.
To get to the egg cell, a sperm literally has to digest the corona radiata and the underlying zona pellucida. To help with this, the sperm’s head or acrosome actually contains digestive enzymes.
8 Picking the Sperm
When it comes to the race to the egg cell, the first to come isn’t always the winner. For one thing, getting through the corona radiata can be tough, and a lucky sperm may be able to swim right through once plenty of others have already broken it down.
Research also seems to suggest that the egg truly favors the healthiest sperm, making conditions just right for the best to fertilize over the ones that may not be as healthy.
7 Three’s a Crowd
Once a sperm has penetrated the egg cell, the outer layers change instantaneously, instantly rejecting all other sperm and preventing multiple fertilization. These changes are so distinct that scientists have been able to observe this through a microscope. This is mainly because it takes 46 chromosomes to make a human. This is just right for a sperm and an egg. A microscopic threesome would result in – wait for it – 69 chromosomes.
Sadly, however, in the rare case that this does happen, this condition, called triploidy, is just incompatible with life. The pregnancy will usually end spontaneously early on or, if it does carry on until term, the baby will die soon after birth.
6 The Egg’s Life Span
Despite the fact that it takes a long, long time to produce a single egg that’s ready for fertilization, the unfertilized egg’s life span is only about 12 to 24 hours. Fortunately, or unfortunately, depending on how you look at it, healthy sperm cells can live for up to three to five days in the female reproductive tract. This extends the fertile period for a few days.
Basically, this means that if you have sex before the egg has died out, it’s still possible to be pregnant. Afterwards, however, fertilization cannot occur.
Some women can actually feel ovulation. In German, this is termed Mittelschmerz, or mid-cycle pain. It is often described as a sharp pain or a mild discomfort in the lower abdomen. Often, it occurs only on one side, likely the side where the offending ovary has just released an egg awaiting fertilization.
For most women, the pain is pretty manageable and lasts only a few hours at most. In some, however, the pain is unbearable and can last for several days, requiring medical pain relief.
4 Freeze for Later
Some women opt to freeze their eggs for a future pregnancy. This process is called oocyte cryopreservation. This is sometimes done for couples who want to get in vitro fertilization. Some women who opt for this procedure may not intend to get pregnant now, but still want to reserve their healthy eggs just in case. Yet others expect to undergo treatment, such as chemotherapy, which could possibly damage their ovaries making them infertile in the long run.
3 Egg Ripening
Unless the healthy eggs are needed immediately, oocyte cryopreservation is done through a process that involves egg ripening. This involves injections of fertility hormones that triggers the maturation of more than one primordial follicle into mature egg cells. Once the eggs are mature and have been released by the ovary, they are collected with the use of a needle and then frozen immediately.
The more eggs that you release during this process, the better your chances of getting pregnant later on when you want it.
2 Your Health and Eggs
If you want to have kids, that should be one motivator to live a healthy lifestyle. Your egg cells can be affected by your general health, which is in turn determined by diet, exercise and habits.
In particular, smoking has a significantly negative effect on your ovaries, among other things. Smoking can contribute to speedy degeneration of your egg cells, and can even make you have a menopause that arrives one to four years earlier than it should. The chemicals in cigarettes can also damage the chromosomes in your egg cells, making you more prone to miscarriage or giving birth to babies with birth defects.
1 It’s a Human Thing
Speaking of menopause, it’s something that is seems to be unique to humans. In fact, only two other species in the animal kingdom experience menopause: killer whales and pilot whales. It’s not exactly clear why this happens, but scientists certainly have their theories.
One theory is that it prevents inbreeding as all three species that go through menopause tend to live with family members. Another is that it’s a way to save on resources, giving the next generation an opportunity to reproduce successfully as well.
In any case, your eggs are still pretty mysterious things and we’re still learning more cool things about them as time goes by!