The ovules are fascinating cells, both for their size (they are the largest human cells) and for their function. These female sex cells or gametes are spherical and immobile, visible to the human eye (which has left us some impressive images) and the most striking is that the fetus starts making eggs from nine weeks of gestation.
It has just passed from the stage of the embryo to the fetus, but the organs are developing and the organism begins to produce eggs. He will not be born yet and the fetus will have millions of oocytes. But before knowing what this impressive process is like, let's look a little more at how the ovules are.
They were first discovered in 1827, by biologist Karl Ernst von Baer, who managed to identify them as female gametes in their research with mammals. As we say, they are the largest cells in the human body, they measure about 0.14 millimeters, while their "partners" sperm are about 10,000 times smaller.
The mature ovule, spherical in shape, is full of fluids, contains a large amount of cytoplasm and a nucleus with 23 chromosomes (it will join the nucleus of 23 sperm chromosomes if fertilization occurs). The oocyte is wrapped in a follicle full of electrically charged cells arranged to release the mature ovum.
After nine weeks of being conceived, female fetuses begin to produce eggs and around the fifth month of gestation they already have an impressive number: around seven million oocytes. We call them oocytes because it is the name that immature ovules receive.
When we are born, however, that number drops considerably in a process called "apoptosis" whereby a programmed cell death occurs and caused by the same cells. It is a kind of "cellular suicide" that probably aims to control its development and growth, although there are varied hypotheses.
Then, at birth, babies have about one million oocytes and new ones will no longer be produced. In addition, it is a number that continues to decline over the years. The vast majority of oocytes inside the ovaries die continuously, until they are depleted in menopause.
In the end, this dance of numbers does not seem too far fetched, since the fertile life of women is quite limited. A woman will produce between 350 and 450 ovules throughout her life. In each rule, about a thousand oocytes are lost that have not matured in ovules. The comparison with male sex cells is abysmal: a single ejaculation contains a greater number than the total "reserves" of women.
From puberty, approximately every 28 days, in the menstrual cycle, an oocyte matures in one of the ovaries and passes to one of the fallopian tubes. If it is fertilized by a sperm at the end of the fusion of the two nuclei (male by the head of the sperm and female located in the cytoplasm of the ovule) the cell becomes zygote.
If the ovum is not fertilized, it will be expelled, along with the membrane that covers the uterus, when the period comes, the menstruation: the cycle begins again. Ovarian stimulation (for example, in a fertility treatment) can release numerous eggs in a single cycle, hence increasing the chances of multiple pregnancy.
Many people may think that with such a number of reserve eggs, fertilization should not be so complicated, but this is not always the case. A large number of ovules does not guarantee pregnancy, we must also take into account other factors (together with male fertility) such as the quality of these ovules.
More properly, the ovarian reserve is the resulting combination of the number of ovules and their quality. And that ovarian reserve descends a lot from a certain age of the woman.
Why is it difficult to conceive from a certain age?
It is known that the fertility of women drops considerably after 35 years and plummeting after 40. However, at these ages there are still many oocytes willing to be ovules and the menstrual cycle continues for years. What's happening? Have these oocytes aged? We could say yes.
Oocytes are very fragile, vulnerable cells against very different stimuli. They are not equipped with "self-repair" mechanisms that other cells do have and when there is some stimulus, some "discharge", the oocytes are damaged. When ovulation occurs in women over 35 years of age, there are many more chances that the released egg is damaged.
Mitochondria, organelles responsible for providing energy to the ovules, over the years weaken and are not able to provide so much energy.
For an egg to become an embryo, it needs to be strong and healthy, with the right amount of chromosomes and the ability to combine them with those of the sperm. It needs energy to divide after being fertilized. If these conditions are not met, fertilization may not occur or the pregnancy may not proceed successfully.
Certain health problems, genetic inheritance, radiotherapy, chemotherapy or some habits such as smoking can cause damage to the eggs (and not only after 40).
But, in general, we can say that women between 15 and 25 years of age have a 40% chance of being able to conceive during each of their cycles. However, over 40 years women have less than 25% chance of becoming pregnant naturally.
In short, although the female fetus begins to produce thousands of eggs at nine weeks of gestation and after five months it has millions, this number begins to decrease considerably after birth and healthy eggs that can fertilize and move on begin to run out as the years go by.
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In Babies and more | Curiosities about ovulation
