reception basket

The fertilized egg, which embarks on a four-day journey from the ovaries to the uterus, eventually becomes a tiny ball of cells called a blastocyst. The next stage in her development is fraught with danger: about half of the blastocysts will fail in their attempt to be absorbed into the uterine wall

The fertilized egg, which embarks on a four-day journey from the ovaries to the uterus, eventually becomes a tiny ball of cells called a blastocyst. The next stage in its development is fraught with danger: about half of the blastocysts will fail in their attempt to be absorbed into the uterine wall. In some cases, the rejection is "justified", and occurs because the blastocyte is not normal. But in other cases the uterus itself, for some reason, is not able to provide the essential conditions for the growth of the fetus. From the moment the blastocyst settles comfortably in the wall of the uterus, the cells in the uterine tissue adjacent to it begin to divide and change in their properties, thus creating a spongy tissue rich in blood vessels called decidua. Most of this tissue disappears when a normal placenta is formed, but until the stage of placenta formation, in the first stage of pregnancy, this tissue is essential to nourish the developing fetus.

One of the many changes that occur in the uterine wall at the beginning of pregnancy concerns the composition of the immune system cells residing there. Thus, for example, certain cells called dendritic cells accumulate in the decidua tissue, which envelops the young blastocyst during its rooting. Dendritic cells of various types are found in all body organs, and they are usually part of the first line of defense against invaders. When they recognize foreign cells that pose a threat, the dendritic cells engulf them, and present the foreign cell proteins on their outer surface - like "red flags" that activate various cells of the immune system, which are responsible for eliminating the invaders.

What do the "fighters of the immune system" do near the young blastocyst? The most accepted hypothesis holds that these cells play a double game, and actually protect the rooting process of the embryo in the wall of the uterus. According to this theory, the dendritic cells in the womb protect the blastocyte from attack by other immune cells, which may recognize the fetus as a foreign and dangerous body, because only half of its genes are identical to those of its mother, while the other half is indeed foreign: originating from another person.

Research students Tal Birenberg and Vicky Flex from the research group of Dr. Stefan Jung from the Department of Immunology, Prof. Nava Dekel and Prof. Michal Naman from the Department of Biological Control at the Weizmann Institute of Science, in collaboration with Dr. Gil Moore from the School of Medicine at Yale University, recently tested the theory this. They raised mice and bred them so that the fetuses were genetically identical to the mothers - and in this way they created a situation where the immune system does not recognize the fetuses as foreign, and has no reason to attack them and prevent their absorption in the womb. After that, they used the method developed by Dr. Jung in his post-doctoral research at New York University, by which they removed all the dendritic cells from the uterus. The researchers discovered that despite the genetic identity between the mother and the fetus, the process of absorption in the womb failed. Moreover, even when they tested mice lacking an active immune system, the embryo's rooting in the uterus failed.

The findings clearly refute the immune rejection theory, but the scientists were in for another surprise: they noticed that in the absence of dendritic cells, the normal development of the decidua was prevented. To test the relationship between dendritic cells and decidua, they went back and removed the dendritic cells from the uterus, but this time they did it using the method in which decidua develops in the absence of a blastocyst during a simulated pregnancy. The results of this experiment showed a direct relationship between the absence of the dendritic cells and the lack of development of the decidua.

Magnetic resonance imaging (MRI) and histological staining showed that the cells of the uterine wall that are supposed to become decidua proliferate at a slow rate and do not differentiate normally. In addition, the development process of new blood vessels is impaired - which is an essential step in the development of a normal decidua. This, following the decrease in the production of proteins that control the processes related to the creation of the blood vessel network.

These findings - recently published in the Journal of Clinical Investigations - led the scientists to a surprising conclusion: the dendritic cells in the womb have adopted a completely new role. They no longer serve as warriors of the immune system, nor even as the bodyguards of the new fetus, but rather "renovate the baby's room", that is, they help to reshape the tissue surrounding the implantation site, so that it meets the needs of the new fetus. The scientists intend to continue to study this phenomenon, and to accurately identify the factors involved in the creation of normal decidua. These future studies will clarify an essential, and not completely understood, stage of pregnancy, and will also pave the way for the development of effective treatments for human fertility problems.