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Human Chimeras

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     One of the most astounding things that has come to light in the past few years is the existence of human Chimeras.  In Greek mythology, the Chimera is a monster with the head of a lion, the body of a she-goat, and the tail of a dragon (sometimes it has multiple heads). It is a child of  the fearful creatures, Typhon and Echidna, It terrorized Lycia (in Asia Minor), but was eventually killed by the hero Bellerophon.

 Human tetragametic Chimeras  are people derived from the fusing of two separate fertilized eggs (making a total of four gametes), usually either fraternal or identical twins who fused together in the womb.  The new combination of cells then continued its development to form a person who is a mosaic of cells derived from the original zygotes.  For example, if fraternal twins of different sexes fused, the resulting individual would be a person with some cell lines derived from the female twin and with other cell lines derived from the male twin.  The exact pattern of this mosaic would depend upon which cell lines formed which organs.  For example one Human Chimera formed in this manner who was outwardly a male showed that the left side of his internal reproductive organs had developed an ovary and a fallopian tube.

    Although there are sometimes outward signs of such a condition, for example the girl who had two eyes of different colors, there is often no sign that a person is a tetragametic Chimera.    At least thirty cases have been noted, but many more probably will never be identified, and the condition seems to be more common than once imagined.  One lady found after being tested for a kidney transplant that she didn't appear to be the mother of two of her three sons ( New Scientist : vol. 80, issue 2421).  Apparently this woman was a fusion of two different non-identical twin sisters with some parts of her derived from one twin and other parts from the other twin.      Apparently one of the sons had resulted from an  egg derived from the cells of one twin and the other two sons had  come from eggs derived from the cells of the other twin.  The woman's ovaries were a mixture of cells from the two original twin girls. 

    Several questions come to mind: would you consider the two different sets of brothers half brothers since they had two different genetic mothers?  Also if a person believes that the soul comes into existence at fertilization, would the Chimera have two souls, or just one?

     Amazing as this idea may be, it doesn't end there.  As the author of this article in New Scientist says, "Far from being pure-bred individuals composed of a single genetic cell line, our bodies are cellular mongrels, teeming with cells from our mothers, maybe even from grandparents and siblings. "   In a condition called microchimerism (New Scientist, 24 April 1999, p 4) the following things may happen:

1. There is often a mixing of fetal cells and maternal cells, and as many of 80-90% of mothers carry fetal cells or DNA in their blood while pregnant and perhaps 50% carry these cells for decades after delivery.

2.  Subsequent children may be invaded by these fetal cells from a prior pregnancy.

3.  Twins may exchange cells during development.

4.  Thus people may end up being mixtures of cells from siblings and other generations.

     As the author says, "This discovery raises some puzzling questions. How come the invading cells don't simply get wiped out by the immune system? Do the cells divide inside their new host? And why do mother and child exchange cells at all -is it just an accident, or does it have a purpose?"

     1. Some speculate that the fetal cells encourage the mother's body to tolerate the "foreign" fetus.  It is also possible that the breakdown of this tolerance by the mother to the fetal cells may result in various autoimmune diseases in these women. 

     2.  The invading of the fetus by the maternal cells may also result in tissue repair.

     3.  Unknown maternal cells may also "transdifferentiate" in the fetus to form certain tissues.  This may cause an autoimmune response in the fetus, but the primary function may be to repair damaged tissues in the fetus.  Read this incredible story!  The implications are mind boggling!  Ethical implications of experimentation in this area.  Not on chimerism per se, but interesting article on the implications of Biotechnology.

















    Web Sites

Good Summary

Definition and short discussion

Fetal Microchimerism

Ethical Implications of

creating Chimeras

Creation of Human Chimera Embryos

Chimera Myth

Blaschko's lines

Chimeras and Parthogenesis!