Egg Development & Ovulation
Reliability of Ovulation
As we’ve already seen, for successful implantation to occur, eggs have to be available. The reliability of ovulation depends on hormone balance within the body successfully recruiting eggs, creating a dominant follicle, and eventually releasing a mature a ready for collection by the fallopian tube. The condition polycystic ovaries affects many women and may affect fertility. An imbalance of hormones prevent the ovary from reliably recruiting follicles in the follicular phase, resulting sometimes in either complete lack of ovulation or certainly irregular ovulation, which is not only unpredictable, but then makes it much harder to conceive.
On average, a woman runs out of eggs in her ovaries at around 51 years. And that’s what we call the menopause. Very occasionally a woman’s egg store will deplete much quicker than this, and this can occur in her reproductive years. Now this is called primary ovarian insufficiency or POI. It’s incredibly rare though for this to happen, and it impacts only on a few people.
Any disruption to a woman’s egg store could result in a reduced ovarian reserve and the delicate balance of hormonal control that regulates ovulation additionally can disrupt successful release of eggs.
A woman’s ovaries contain her entire egg store for her lifetime. During very early fetal development inside the womb, the fetal ovaries form. Most girls are born with two ovaries and these contain all of her eggs. They’re also responsible for manufacturing the reproductive hormones. A woman’s eggs begin life in the developing fetus as germ cells.
As early as six weeks into fetal development, these germ cells divide and migrate to the area where the ovaries will form. And in the 12th week of fetal development, primitive germ cells form primordial follicles, which contain immature eggs.
This, however, is not the whole story of egg development. These primordial follicles are arrested in their development and remain so for up to 45 or 50 years. The final process of egg maturation actually begins each month in a woman’s menstrual cycle as follicles are recruited and a mature egg is released in a process called ovulation.
As a fetus, there is estimated to be around 7 million eggs in the ovaries, but by the time a woman starts menstruating, this has fallen to just under half a million. A woman’s egg store is therefore at its maximum as a fetus.
There is a steady decline in the number of eggs after birth. As menstruation begins, a woman’s egg reserve gradually falls until no follicles remain in the ovaries, culminating in the menopause. When ovulation and periods stop.
This happens on average around 51 years of age.
Egg Development & Quality
Let’s look in a bit more detail at the journey of the egg. The process of egg maturation and development is called oogenesis. Now, this process occurs in several stages with completion of egg maturation occurring during menstruation. The egg cells are suspended in time until they’re recruited during a menstrual cycle.
Initially, the germ cells in the fetal ovary divide continuously to form oogonia and these early egg cells still contain a full female chromosome complement 46 XX. Every cell in our body should contain 46 chromosomes. We should all have 22 pairs of chromosomes and two sex chromosomes. So girls have two X chromosomes and boys have an X and Y chromosome.
Half of our chromosomes come from the egg and the other half come from a sperm.
At the point an egg is released, it has halved its chromosome content to 22 chromosomes and one X chromosome. The egg is then ready for full maturation to occur during fertilisation following fusion with a mature sperm.
As a woman gets older, her eggs age, abnormal eggs increase in quantity with age, miscarriage rates also increase, and the success rates of pregnancy decrease. An embryo has to be chromosomally normal or euploid for it to create a healthy pregnancy. As women age, the chances of creating euploid embryos decreases.
When we think about our chromosomes, it’s important to use an analogy to try to understand how important they are to the health of the eggs, the sperm, and ultimately the embryos that we create. If we use the analogy of a bookshelf, on that bookshelf, are many books like a library. The bookshelf itself may contain an entire encyclopaedia.
In the encyclopaedia, each book represents a chromosome. The pages of the chromosomes in each book represent the DNA and the words on the page represent our genes. A chance abnormality in the genes or the, actually, you can probably split that off the end. I’ll just go.
When egg and sperm combine, the DNA from the egg and the sperm come together, and it’s at this point that a chance chromosomal abnormality is more likely to occur. With increasing age, it’s much more likely that you’re going to have abnormal embryos created, unfortunately.