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Chimerism, existence of two individuals in one body

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Chimerism, existence of two individuals in one body

Have you ever thought that not only you but someone else can also exist in your body in the same way? No, it is not fiction, it is really possible. After so much research and testing, it has been found that some people have more than one DNA set present in their body. That means the existence of two in one body. 

What is Chimera?

In the ancient Greek mythological stories, there is a fire-breathing creature. The creature is a mix between a lion, goat and serpent, this fearsome beast is called Chimera. The word Chimera resembles the existence of multiples in a single body. 

Apart from mythology, if any human or animal has more than one DNA set in the body, that individual is  known as Chimera.

What is chimerism?

Usually a person carries a specific DNA set in his whole body which is responsible for all his physical characteristics. But In case of Chimeras (humans or any other animals), they carry more than one DNA set in different parts of their body. One of which is their own and the other one comes from another individual, such a phenomenon is known as chimerism. In case of chimerism the same tissues contain more than one type of cell population in it. That means one person can develop different blood groups in the body or one person can develop different coloration in skin cells of the body and many more. 

How common is chimerism?

In humans chimerism is very rare. As it is only diagnosed with genetic testing and usually no one does a genetic test without some medical reasons, so it is very difficult to determine the exact number of cases. About  100 or so cases have been recorded till now. But due to modern childbirth technologies like IVF, the tendency is gradually increasing.

What is meant by the chimerism in Animals?

In the case of animals, chimerism is more common. They are composed of cells with more than one genotype. They can posses blood cells of different blood types, even thay can posses different sex organs in a single body. Many of cases, an individual shows variations in the colour of body, eyes etc. In animals Chimeras are formed by the fusion of multiple fertilized eggs. Body parts transplantation also leads to chimerism, where the individual can achieve some distinct genetic properties from the donor one. 

How is chimerism and mosaicism different from each other?

Initially, chimerism and mosaicism both were considered as genetic combinations. Now after the development of genetics and molecular biology it is said that these two are completely different concepts. However in both cases body cells carry different genomes. 

In case of chimerism, most Chimeras are formed by the fusion of two different zygotes. That means different cell populations originate from two zygotes.

In case of mosaicism the same tissues contain two or more cell populations which originate from a single zygote. Mutation during early development is the reason for mosaicism. 

What Is meant By the chimerism in humans? 

In humans chimerism is a very rare phenomenon. As genetic testing is the only way to find out chimerism and without any medical problem no one goes for the test, it is impossible to say about the exact number. In modern medical literature, only about  100 cases of chimerism are recorded. 

What are the causes of chimerism?

There are so many causes of chimerism in the human body. According to the cause of chimerism, symptoms are varied.

Microchimerism:

In most cases, this type of chimerism is seen in the human body. During pregnancy sometimes the mother absorbs some tissues from the fetus. Also in some cases the fetus absorbs some cells from the mother. These foreign cells travel into their body and migrate to different organs and this way the chimerism occurs. This kind of chimerism is called microchimerism. 

Twin chimerism or vanishing twin syndrome: 

Twin chimerism is a kind of chimerism where a twin absorbs the other one's cells. In case of pregnancy, with the multiples if one embryo dies in the womb, the surviving embryo may absorb some tissues of it. By this way the surviving twin has two DNA sets in its body and shows chimerism. 

Tetragametic chimerism:

This is a very rare kind of chimerism. When fertilization takes place, two different sperm cells fertilize two different eggs. After that the two fertilized eggs fused together and formed a single embryo. This kind of embryo has crossed cell lines and shows tetragametic chimerism. 

Artificial chimerism or chimerism after transplant:

This is a very common kind of chimerism where Chimeras devebloodlop after having some kind of artificial body parts transplant. When a person receives some kind of body parts transplant like bone marrow transplant, stem a or  transfusion, the receiver person absorbs some tissues from the donor's body and shows chimerism. This is called artificial chimerism. 

Chimerism after blood transfusion:

In the case of an individual who suffers from a traumatic injury and has a large amount of blood loss, blood transfusion is needed. The individual receives white blood cells or leukocytes from a donor one. The donor's genetically distinct blood cells remain in the recipient's body for decades. The donor's cells are tolerated by the recipient's immune system and show chimerism.  About 10% to 15% of people who experience blood transfusion are showing this kind of transfusion associated with microchimerism. 

Chimerism after bone marrow transplant:

In those cases when an individual has some kind of cancer or any other disease which damages the bone marrow, then the individual may require  bone marrow transplant. When a genetically distinct bone marrow is transplanted into the recipient's body it starts to generate blood cells of the donor's origin and the cells have been durably engrafted in the recipient body. It shows artificial chimerism and the recipient becomes Chimera. In some cases full donor chimerism is noticed where 100% of blood cells are of the donor's origin. In some cases partial chimerism is noticed where recipients also have some own blood cells present in the body.

Chimerism after stem cell transplant:

In case of many vital diseases like leukemia, Congenital disorders of blood production, autoimmune diseases, cancers the body is unable to make blood cells and many other immunological proteins. In those cases the patient requires stem cell transplant to replace the damaged stem cells or bone marrow. The stem cells which are to be transplanted come from the donor's bone marrow, peripheral blood or from the umbilical cord. After the transplant the stem cell produces blood cells and many immunological proteins from the donor's origin and that's how it shows chimerism. 


 

Now after the huge development of medical science, artificial chimerism is becoming less common. The transplant organs or transfused blood is treated with radiation, which prevents the permanent incorporation of the donor's cell in the recipient's body. 

Symptoms of chimerism

The symptoms of chimerism vary widely. In some cases it is almost impossible to notice any external effects of chimerism. Also in some cases the symptoms of chimerism are expressed by their unique skin colouration pattern or something like that.

  • Hyperpigmentation or hypopigmentation in a small area of skin or in half of the body is one of the most common signs of chimerism.
  • Two different eye colourations is also a sign of chimerism.
  • Different types of  blood cell population in one body is also an effect of present different DNA sets in one. 
  • Presence of male and female both genital parts also indicates chimerism.
  • Various autoimmune diseases are also related to chimerism.

How to diagnosed chimerism:

As we know that there are not always specific symptoms of chimerism, so the diagnosis is not possible in all cases. The only way to diagnose chimerism is through genetic testing and we all know that without any serious medical situation nobody goes for genetic testing throughout their lives. For this reason, most chimeras  do not even know about their special feature of chimerism. 

Conclusion

Here we are talking about the concept of chimerism. We try to give you an idea about the meaning of chimerism. We state the difference between chimerism and mosaicism. We briefly describe the possible causes, symptoms and diagnosis process of chimerism. Hope this article will be well informative and helpful to you. Thank you very much. 


Piyali Das

Piyali Das