Successful conception is possible only with the participation of two full–fledged germ cells – male (sperm) and female (egg). The contribution of each of them to the formation of a new organism is equivalent – 50/50. But there was a time when the question of the role of father and mother in this case was controversial. Aristotle believed that a woman only provides a place for the development of the embryo, and he gets all the material from his father. This point of view dominated for quite a long time. But now the time has come when it became clear: all female mammals produce eggs. There was even a statement: “everything is from the egg!” (Latin Ex ovo omnia). It is attributed to the English physician and scientist William Harvey (1578-1657). And it was only at the beginning of the XIX century that it was established that the sperm and egg are equally necessary for the formation of the future organism.
Today, the process of fertilization and its participants – germ cells, have been studied in some detail. The egg dossier is presented in this publication.
So, her name is: egg, oocyte (ovocyte). The word has Greek roots: Ōón – egg, Kýtos – cell
The term “egg” is used when it comes to the female germ cell in general.
The term “oocyte” is used to refer to the different stages that an egg goes through during its formation.
Function: eggs serve to procreate. Mega important mission!
Place of residence: ovaries. There are two of them in a woman’s body – right and left. Each egg lives in a separate house – follicle. The ovaries work alternately – every month one of them grows one dominant follicle. Although it is noticed that most often the egg matures in the right ovary, where there are more large vessels. It happens that a woman has only one working ovary.
In the ovaries, in “home conditions”, the egg lives for about 50 years. A real centenarian! But at the same time, it ages along with the body. The older a woman is, the fewer eggs she has, and their quality is worse. This explains why it becomes increasingly difficult to conceive a child over the years. Even with IVF, the ovaries sometimes work poorly. Even under the influence of drugs, a sufficient number of eggs are not formed. They do not reach the desired stage of maturity and are unsuitable for fertilization. “Elderly” eggs can also be the cause of genetic disorders of the fetus, which is the main cause of miscarriage or the birth of a child with a genetic disease.
After leaving the follicle (ovulation), the egg has only 24 hours to live. If during this time she does not meet with a sperm, pregnancy will not occur.
The egg is the largest cell in the human body. Its diameter is about 130 microns. For comparison: the size of the sperm is about 55 microns, and some cells of the body are even smaller. It is proved that the larger the egg, the higher its ability to fertilize.
The shape of the egg body is spherical. These are real fatties! They are completely immobile and filled with reserves for the future embryo.
The egg cell consists of cytoplasm, nucleus and membrane.
Its shell is three-layered:
- the outer layer is a radiant crown (corona radiata)
- middle layer – shiny shell (zona pellucida)
- the inner layer is the egg cell membrane
1st – protection against mechanical damage
2nd – protection against the penetration of excess sperm. Only one person should take part in fertilization!
3rd – help with implantation, when the embryo is attached to the inner surface of the uterus – the endometrium.
The egg nucleus contains:
– proteins that can ensure the division and development of new life
– the genetic material that contains all the information about the future organism
Number of chromosomes: 23 (haploid set). Of these, 22 carry hereditary information. One is responsible for the sex of the unborn child – this is the sex chromosome X.
The nucleus is located exactly in the center of the egg, under the protection of the shells and cytoplasm. When the hereditary information of the mother and father merge, the connections between the individual sections of the DNA chain are broken and formed again. So a new, completely unique genetic set is formed.
Near the nucleus of the egg there is a polar body – this is a small cell that is formed together with the egg as a result of meiosis. Its protoplasm contains RNA and nutrients that are needed at the crushing stage after fertilization.
Where do these come from?
The process of egg formation is called oogenesis. Oocytes of the 1st and 2nd order are formed sequentially, and as a result, a mature egg cell appears. The formation of eggs begins in the womb. The female fetus at the time of birth has a certain supply of eggs – oocytes of the 1st order. Their number determines the ovarian reserve or follicular reserve. A girl is born with it and spends it for the rest of her life. A newborn baby has about 2 million eggs, but by the time of her puberty, only about 400 thousand will remain. With the onset of menstruation, every month the eggs will decrease. After 35 years, it is especially intense. This is usually the case, but there are exceptions. Some women at the age of 30 no longer have their own eggs, and some at 45 can conceive naturally. Therefore, if motherhood is postponed for later, it is better to evaluate your ovarian reserve before the age of 35.
Worth its weight in gold
Each egg is priceless, because it is impossible to replenish the ovarian reserve, as well as to improve the quality of aging oocytes. There is only one way to preserve healthy young eggs – freezing or cryopreservation. In the case of eggs, the most advanced technology of cryopreservation of biomaterial is used – vitrification.
What contributes to the loss of the quantity and quality of eggs.
In addition to age, which is the reason # 1, a decrease in ovarian reserve occurs for a number of other reasons – this is:
- genetic factor (hereditary predisposition)
- gynecological diseases (pelvic infections, endometriosis, polycystic ovary syndrome or PCOS, etc.)
- consequences of surgical interventions on the ovaries
- endocrine disorders
- toxic effects, including alcohol abuse, smoking, poor ecology, work in harmful production, chemo or radiation therapy
Her Majesty is an egg! Queen of Fertilization
The female body prepares for conception monthly. Under the influence of hormones, several follicles begin to grow in the ovaries, of which only one will become dominant – an egg will mature in it, ready for fertilization. As the dominant follicle grows, the others disappear – they can be safely written off.
After 2 weeks, the dominant follicle reaches a diameter of 18 to 24 mm. For 15-16 days, it will burst, releasing an egg. This proud queen herself will not move – the villi of the fallopian tube will pick her up and literally carry her to the uterus.
In the fallopian tube, her Majesty may already be waiting for spermatozoa – the strongest and bravest, who passed the “Crimea and the ring” for her sake. They can live in a woman’s genital tract for 3-4 days. Therefore, if sexual intercourse occurred shortly before ovulation, then they still have time and hope. The optimal time for sexual contact in order to conceive a baby is the day of ovulation. With a standard menstrual cycle of 28 days, ovulation occurs in the middle of the cycle.
The egg is simultaneously attacked by several spermatozoa. Only in this way, by joint efforts, they will be able to break through its outer shell – the radiant crown. The next obstacle will be the shiny shell, followed by the egg membrane. As soon as the fastest and most resourceful sperm penetrates through it, the process of cell fusion will begin. The remaining spermatozoa will be blocked from entering the egg. For a while they will still circle around the egg, and then they will stick together, weaken and die. It is believed that they still benefit – they create the necessary chemical environment that helps the fertilized cell along the way in the fallopian tube.
The interaction of germ cells is facilitated by substances – gamons, which are secreted by the egg. They attract spermatozoa to themselves, stimulate their motor activity, cause gluing and immobilization of those who were out of work.
According to the latest scientific findings, the egg itself chooses the most suitable sperm for fertilization. What influences her choice is not yet clear. But scientists hope that new data on the interaction of sperm with egg will help in the future to overcome some forms of infertility.
Beyond the norm
It happens that the follicle matures, breaks, and there is no egg in it. This phenomenon is called “empty follicle syndrome”.
It happens that an egg is formed and even begins to mature, but never leaves the follicle. Many follicles with immature oocytes turn into cysts. In this case, they talk about a disease called “polycystic ovary syndrome” (PCOS, polycystic). Pathology develops against the background of hormonal dysfunction and often leads to infertility.
A fairly common phenomenon is an anovulatory cycle. The cycle is regular, but there is no ovulation and the yellow body is not formed.
It is rare, but there is also such a thing: two or three eggs mature at the same time, all of them fall into the fallopian tube and are fertilized. In such cases, twins or triplets are born.
And what about after fertilization?
The fertilized egg becomes a zygote. At this stage, intensive cell division occurs, the embryo goes through several stages, overcomes the block of development. Regarding the latter, an explanation: it is believed that until the third day the embryo lives at the expense of the resources of the egg, and on the 3rd day its own genome is activated. If there are serious breakdowns in it, then the embryo will stop developing.
On the 4th day, the embryo consists of already 16 cells – this is the stage of morula. By the fifth day, the morula turns into a blastocyst. At this age, the embryo enters the uterus, where it faces a difficult task – to attach, hold on and continue development as a fetus. In this case, pregnancy will occur.
If fertilization does not occur, the upper layer of the endometrium, intended for embryo acceptance, will be rejected and will come out with blood during menstruation. The cycle will start again.
The process of egg maturation, ovulation, fertilization, implantation and pregnancy development is under the control of the endocrine system. Hormonal dysfunction usually leads to cycle disruption and the development of pathological conditions that, without proper correction, sooner or later lead to infertility.
The most effective method of overcoming infertility is in vitro fertilization or IVF. For this procedure, the eggs are surgically extracted. The manipulation is called “transvaginal puncture of follicles”. The evaluation of the obtained biomaterial is carried out in an embryological laboratory. High-quality oocytes are fertilized either by classical IVF in a Petri dish, or by ICSI – the sperm is injected into the cytoplasm of the egg using a special needle.
One of the most severe forms of female infertility is the absence of eggs suitable for fertilization. There are cases when they may not be at all (removal of the ovary, congenital abnormality of organ development, etc.). In such a situation, pregnancy is possible through IVF with a donor egg.