1 What does transparency do?
As shown in the figure, a transparent and slightly thick structure around an egg or embryo is a zona pellucida. The zona pellucida is composed of four glycoproteins, ZP1, ZP2, ZP3, and ZP4, which are synthesized by the egg cells themselves and secreted by the cells, and then coincide outside the eggs.
So what is the role of this vine-like part?
Usually, many sperm gather around the egg during fertilization, but the egg only binds to one sperm, and the pellucida blocks the rest of the sperm out. This behavior is called guarding against the entry of multiple sperm, and the zona pellucida plays an indispensable role in the selection of sperm
If multiple sperm are fertilized, the egg is only self-destructive, so it can be said that the presence of a zona pellucida plays a crucial role.
Another important function of the zona pellucida is to protect the fertilized embryo from attack by maternal immune cells.
Usually after fertilization in vivo, the embryo moves through the egg tubes to the uterus, but in the process the embryo becomes the target of attack by the maternal immune cells. The fact that the mother herself will attack the embryo must be incredible to most people, right? This is because the mother is an ordinary person before her body becomes a mother, and therefore has immunity to the invasion of pathogenic microorganisms.
Because the immune system is designed to prevent the invasion of external pathogens and to remove invading pathogens (such as bacteria, viruses, fungi, mycoplasma, chlamydia, parasites, etc.) and other harmful substances, embryos containing sperm components are identified as non-self objects and attacked.
Hearing this, many people will think that it is not impossible to get pregnant? This is not the case, because the pellucida will hide the factors containing the paternal component in the embryo, and the mother will recognize that the fertilized egg is formed in the body before the fertilized egg immediately reaches the uterus, so as to change the immune system, even if the zona pellucida disappears, the embryo will not be excreted as an attack target.
2 What happens if there is no transparent tape?
Here I would like to explore two aspects of what would happen without a piriform belt. The first is when the pellucida is shed when the egg is taken or tested, and the second is when the pellucida is naturally lacking the genes that make up the pellucida.
First of all, if the pellucida is shed during egg collection or egg testing, a sperm is used for micro-fertilization, and if the egg itself is not abnormal, then it can be fertilized and divided smoothly as a normal egg until it becomes an embryo.
The innate lack of zona pellucida constituent genes is more serious.
A study of the fertile capacity of mice without the pellucida gene found that the result was infertility. The reason is neither the increased chance of multispermia fertilization mentioned in the first episode, which leads to the self-destruction of the egg, nor the attack of the mother's immune cells, but the abnormal development of the follicle itself.
As shown in Figure A, there are many follicles in a normal ovaries, and oocytes and follicular cells are present in the follicles. Figure B is the development of follicles in mice without zona pellucida, which does not develop normally and causes degeneration.
Figure A shows the development of follicles in the ovaries of normal mice. You can see 4 developing follicles (where the blue arrow points), and the center is round and large with an * sign indicating oocytes.
Figure B shows the ovaries of mice with the missing zona zona gene, where there are only 2 normally developing follicles, while oocytes marked with the * symbol do not have zona pellucida. In addition, many degenerated and atrophied follicles can be seen. In other words, follicles do not develop without zina pellucida.
A closer look at Figures A and B reveals that there are more follicles in Figure B, including degenerative eggs. This is so because after the follicle degenerates, the ovaries supply the follicles from the stock of the original follicles. But even so, because there is no zona pellucida, these follicles cannot be fully developed, which will only lead to an increase in degenerative follicles.
3 Eggs without the zona pellucida gene have also been found in humans
In the previous chapter, it was mentioned that follicles do not develop without zona pellucidas, and the reason for this can be considered is that the zona pellucida is not only a membrane sandwiched between the gap between oocytes and follicular cells, but may also have the function of promoting follicular cell development. That is, oocytes and follicular cells are assisted in the development of both through the intervention of the pellucida.
In animal experiments, mice without pellucida are infertile due to follicle infertility due to follicle failure. In a study of humans, it was found that the offspring did not have a pellica due to genetic variation in the zona.
Figure A represents a family tree.
The 32-year-old patient who received infertility treatment is the fourth generation, and the expression of ●3 is the third generation, that is, the mother of the patient, who has given birth to 7 children, of which 6 daughters, the second daughter (〇2) has the ability to become pregnant, but all four daughters, including the patient, are infertile.
After investigating the pellucida gene of the patient himself (●3), it was found that the pellucida gene that existed in pairs was mutated on both sides, and then the pellucida gene of the patient's mother was investigated and found that the zona pellucida gene was unilaterally mutated in pairs, because only one-sided variation was formed, so the zona pellucida was formed to give birth to seven children, and the second daughter was also only a unilateral zona pellucida gene variant, so the next generation was successfully born. Although the youngest daughter is unmarried, it is also a genetic variant of the pellucida of both parties.
Although the patient's father has passed away, the patient's grandfather is still alive, so it is also possible to investigate the zona gene (the male himself has no eggs and zona, but has the zona piriformum gene). The findings are that the pairwise pellucida gene is also a unilateral variant, so it can be assumed that the patient's father inherited the patient's grandfather's genetic variation.
That is to say, the patient's parents are people who hold genetic disease genes, but they do not have the disease themselves. Although it is normal, it causes genetic abnormalities in its descendants.
Figure B shows the patient's egg without a zona pellucida after egg collection, where 4 represents the state of immature oocytes with follicular cells attached to them, and no zona pellucida is seen, and the follicular cells are directly attached to the surface of the egg.
Cases like the one mentioned above are very rare and hardly ever appear in actual clinical data.
4 Why does the transparent tape harden?
Why does the zona pellucida harden after fertilization?
Multiple sperm are gathered around the egg during fertilization, but only one can be fertilized.
The figure below is a diagram of the mechanism of the pellucida to prevent multiple sperm fertilization.
In the upper right of the figure is the sperm that initially enters the zona pellucida, which reaches the gap between the egg and the zona pellucida (within the perietal cavity) and the egg cell membrane fuse. After this, the surface particles will begin to decay, and the decay will release the material into the perietal cavity, causing the zona pellucida to change.
This is the principle of operation to prevent the rest of the sperm from entering, and the two sperm on the left are in a state of being blocked out.
But at this time, not all the surface particles in the egg will decay, and the surface particles will decay further with the division of the fertilized egg, and the zona pellucida will become harder as a result. For eggs, it is necessary to avoid the entry of multiple sperm, so it is natural that the zona pellucida hardens after fertilization. However, too much will make it difficult for embryos to hatch, especially for cryopreserved embryos, where a part of the remaining surface particles on the egg will remain directly on the embryo, and the pellucida will be further hardened due to the stimulation of thawing and re-dissolving.
In infertility treatment facilities, if the hardness of the zona pellucida affects embryo hatching, the embryo is given assisted hatching therapy (AHA).