Microchimerism is when one organism contains cells from another organism, acquired through pregnancy, organ transplants, or blood transfusions. It can cause autoimmune disorders, but its effects are still unclear. In some animals, microchimerism can result in a freemartin, a sterile female with partially male characteristics.
Microchimerism is a phenomenon in which the body of one organism contains a small number of cells from another organism. Unlike tetragametic chimerism, microchimerism is acquired. In tetragametic chimerism, however, an inborn trait occurs when two non-identical zygotes or blastocysts fuse before implantation and develop into a single organism composed of two cell populations, each with its own distinct genes. Microchimerism occurs in many species, including humans.
Microchimerism in humans commonly occurs during pregnancy. Cells of the fetus’s immune system can enter the mother through the placenta, where they can survive and continue to reproduce through mitosis. In some cases, the descendants of these fetal cells remain in the mother decades later. The same reverse process can lead to maternal immune cell populations living in the fetus, although this is less common. It can also result from organ transplants and blood transfusions.
Some research indicates that microchimerism can cause some autoimmune disorders, such as lupus. The presence of maternal immune cells has been blamed for some autoimmune diseases of children, such as juvenile dermatomyositis. Some cases of systemic sclerosis, a disease that damages the skin, joints and some internal organs, can be caused by fetal immune cells living in the mother, and some studies have linked the presence of fetal immune cells to breast cancer. Autoimmune disorders are more common in women than in men, and fetal cell effects have been proposed as a possible cause.
However, the implications of research in this area are still ambiguous. The frequent presence of fetal or maternal immune cells in tissues affected by these diseases may indicate that they cause or contribute to the disease, but it could also mean that foreign cells are present because they help fight the disease or mitigate the damage. Therefore, it has also been speculated that some microchimerism may actually be helpful.
In some animals, such as cattle, it is common for the placentas to coalesce in the uterus and share the bloodstream. As a result, cells can easily pass between fraternal twins in utero, resulting in microchimerism. In the case of a male and female fetus united in this way, the exchange of sex hormones during development causes the female to become partially masculinized. This prevents the normal development of the reproductive organs and results in what is known as a freemartin, a sterile female that may have partially male characteristics. The presence of microchimerism is used to confirm that a female is a freemartin, as her blood will contain cells with male genetic material that originated in her twin.
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