Chimera Formation (Animal and Human)

Human Chimeras

In 2002, human chimerism (the condition of being a chimera) was publicized in the popular media with the case of a 52—year—old woman who needed a kidney transplant. To find a potential kidney donor, the woman and her immediate family submitted blood samples for genetic screening. The results were surprising: The tests indicated that the woman did not have biological similarities to two of the three sons to whom she had given birth. To solve this conundrum, doctors examined samples of the woman's mucus, hair, and skin and determined that she was a tetragametic chimera; that is, a chimera formed when two of a mother's eggs are fertilized by two of the father's sperm, and the resulting two embryos fuse to form one person. Tissues (e.g., the skin, muscle, etc.) of tetragametic chimeras either can be composed of cells from both original embryos or can be made up of cells from only one of the embryos. In the case described above, the woman's blood cells were derived from only one of the fused twins, whereas her other tissues were made up of cells from both twins.

With only 30 reported cases, human chimeras appear to be relatively rare. However, because naturally occurring chimeras rarely have identifiable features, the rate of human chimerism may be higher than reported. Genetic testing of several tissue types can usually identify human chimeras, although these tests can be complicated and expensive.

Thus, testing for human chimerism usually only occurs when rare, outward physical symptoms of chimerism are observed. The physical symptoms of chimerism may include ambiguous genitalia, hermaphroditism (having both male and female sex organs), patchy colored skin, or two differently colored eyes.

Recently, in vitro fertilization has been shown to increase the rate of tetragametic chimerism in embryos. This may result from the embryos being grown in close contact before implantation into the mother's uterus or from an increased chance that an egg with two nuclei will be fertilized by two different sperm. Either way, with the increased use of in vitro fertilization, diagnosing chimerism may be increasingly relevant when considering maternity/paternity cases, blood donation, or organ donation.

In addition to tetragametic chimeras, other types of chimeras exist in the human population. These include parthenogenetic chimeras, androgenetic chimeras, microchimerism, and organ transplant patients. A parthenogenetic chimera is formed when an egg that has not undergone meiosis (a cellular process that decreases the egg's genetic material by half) is fertilized by two sperm. In this case, the two sperm provide double the typical dosage of genetic material from the father, which pairs with the doubled genetic material from the mother and results in chimera formation. Only one case of human parthenogenetic chimerism has been reported.

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