Egg cells, or ova, are the defining characteristic between the male and female of every species on Earth. Though some males, like the seahorse, will carry fertilized embryos in their specialized pouches, it is the creation of the eggs itself that is used to define the female. Though they may seem simple, ova are remarkably complex and sophisticated, as the following female egg facts illustrate.
Eggs are Large
The human egg cell is the largest single human cell type.
Eggs are Complex
The typical egg that most people are familiar with, the chicken egg, has three layers– the shell, white and yolk. The ova is similar in structure, with one additional feature. The layers include:
- Corona Radiata: This is a thin layer of cells outside of the ova’s “shell”. It is developed by the follicle and is used to supply nutrition to the egg.
- Zona Pellucida: This is the outer structure, or “shell.” As women age, this layer can become harder and more difficult for a sperm to fertilize through. Assisted hatching is a fertility technique used to create small openings in these hardened eggs to allow fertilization opportunity.
- Cytoplasm: The “white” of the egg, it is a gel-like substance that holds the cell’s other organs, like the energy-producing mitochondria. Sperm do not transfer mitochondria to an embryo, so every child has mitochondria genetically identical to that of their mother.
- Nucleus: This is the home of the egg’s chromosomes, half of what is needed to create a person. When the sperm’s 23 chromosomes meet with the egg’s 23 chromosomes, fertilization occurs and an embryo can begin to grow.
Eggs are Choosy
It may not only be the sperm that wins the race to the egg, as was once believed, but the egg which also chooses a suitor from available sperm. A 2011 study shows that the egg will release calcium-high cells into one specific sperm, causing its tail to whip frantically enough to penetrate the egg’s cell.
Eggs are Limited
Dr. Shahryar Kavoussi says that although a woman is born with around 1 million to 2 million immature eggs, only around 400 to 500 of them will ever mature and ovulate, on average, during a woman’s reproductive lifespan. Most remain in an immature follicular state and will die off as a woman ages. In cases of diseases like polycystic ovarian syndrome, a woman may have a large number of partially developed follicles surrounding the ovary’s inner wall, but it may be difficult for her to produce a mature egg. It is not known why the body will choose one particular follicle to mature over another.
Eggs can be Long-Lived
When a female is born, all of her existing follicles have already been created and will sit waiting for puberty when she will begin ovulation. From there, the process of turning an immature follicle into a mature egg will take approximately 85 days. Once an egg has been ovulated, its lifespan is rather short, and the egg is thought to only last for 24 hours or so before it begins to decompose, unless fertilization occurs. Eggs and embryos can both be cryopreserved, or frozen, for long periods of time. The oldest recorded fertilized embryo to result in a healthy baby had been on ice for 25 years.