An Overview of Human Animal Chimeras

 The Legend

    The term "chimera" has been known to mankind for centuries.  The word originates from ancient Greek culture, and it was used to name an incredible creature that was said to have had the parts of a lion, a goat, and a serpent.  (Citation 12) Said to be the daughter of the gods Typhon and Echidna,  this beast breathed fire and was one of the most savage creatures in Greek mythology.  Its fame has inspired scientists, musicians, and architects alike to name their masterpieces after this peculiar animal.

 This is a picture of what one artist believed the Chimera of Greek mythology looked like. (Citation 3)

Modern Chimeras

    The mythological idea of a chimera is that of a ferocious monster.  However, as National Geographic explains, "A chimera is a mixture of two or more species in one body," (Citation 10).  With that said, you can see that laboratory-made chimeras are not as troubling as the mythological Greek chimera.

 How Chimeras Are Made

    Chimeras are made by introducing stem cells of one species into the embryos of another species.  Regular cells in an organism have specialized functions, but stem cells are still in the early stages of cell development, and therefore, can be made to do any function that is imposed upon them.  Their ability to do virtually any cell function, and to multiply for extended periods of time, renders them invaluable to scientists for cellular research. 

   According to Lee Silver, "The significance of stem cells lies in the fact that they are, in essence, embryonic or otherwise immature precursors to the cells that function within our mature tissues and organs. The mother of all stem cells is the embryonic stem (ES) cell, which exists naturally in 10-day old embryos. ES cells have the capacity to form everything in the human body. During normal development, ES cells divide repeatedly, giving rise to an array of stem cell types that each have a more restricted capacity to form only one or a few tissues or organs. Human stem cells at many different stages, from embryonic to adult, can now be cultivated in laboratory petri dishes, and chimeras can now be produced by inserting these stem cells into animal fetuses or embryos," (Citation 14).

    In the creation of chimeras, the nucleus or entire stem cell of one species is placed into the egg of another species that no longer as any genetic information in it.  The cell divides, producing two hybrid cells, and once enough hybrid cells divide, tissues are formed that are a combination of the two diffferent species.

  Here is a diagram showing how a human-animal chimera is formed. (Citation 11)

The First Chimera

    The first chimera ever made was not a human-animal hybrid, but a cross between two similar, but not the same, species: "In March 1984, an animal unlike any other ever born, or seen, adorned the cover of Nature, the international journal of science. The journal's audience of scientists was treated to a  photograph of an animal with a head that was mostly goat, an upper torso that was wooly sheep, with other body parts that alternated between the two species types. 'It behaved like a goat,' claimed its creator, Danish embryologist Steen Willadsen, 'but did not quite smell like one, preferring the company of sheep.'  This first geep (as the animal was aptly named) was physically healthy and behaved like any sheep or goat should.  Upon his success, Willadsen created additional geeps over the next several years," (Citation 14).

  Here is a picture of the amazing creation, the Geep. (Citation 1)

The First Human-Animal Chimeras

    The first human-animal chimera was created in 1988 by Irving Weissman and his team of scientists from Stanford University.  Their experiments resulted in mice whose immune systems were identical to those of humans.  They created these in order to better study the immune deficiency disease, AIDS.  (Citation 12) The second recorded human-animal chimera was created in 2003 by Chinese researchers from Shanghai Second Medical University.  This chimera was a mix of human cells and rabbit eggs.  Minnesota researchers at the Mayo Clinic also created a chimera; a pig with human blood. (Citation 10)

The Hopes

    Now you may wonder why on Earth would anyone want to merge two different organisms into one animal, especially a human with a mouse or pig?  The answer is that scientists believe the more humanlike the animal, the better research model it makes for drug testing.  These chimeras may also possibly be used for growing "spare parts," such as livers and other vital organs, for transplant into humans. (Citation 10) Scientists like Irv Weissman hope that chimeric experiments will lead to a better understanding of how the brain works, which would be useful in treating diseases like Alzheimer's or Parkinson's disease. (Citation 12)

    However, there are many controversies about whether or not the creation of chimeras is entirely ethical.  To find out more about the arguments that scientists have made for and against human-animal chimeras, please click on the tab labeled, "The Issues."