Chimerism may occur naturally during pregnancy, when two non-identical twins combine in the womb, at a very early stage of development, to form a single organism. Such an organism is called a tetragametic chimera as it is formed from four gametes - two eggs and two sperm. As the organism develops, the resulting chimera can come to possess organss that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosones and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, though it is considered extremely rare, but since it can only be detected through DNA testing, which in itself is rare, it may be more common than currently believed. As of 2003, there were about 30 human cases in the literature, according to New Scientist.
In biological research, chimeras are artificially produced by mixing cells from two different organisms. This can result in the eventual development of an adult animal composed of cells from both donors, which may be of different species - for example, in 1984 a chimeric geep was produced by combining embryos from a goat and a sheep. A chicken with a quail's brain has been produced by grafting portions of a quail embryo into a chicken embryo.
In August 2003, researchers at the Shanghai Second Medical University in China reported that they had successfully fused human skin cells and rabbit eggs to create the first human chimeric embryos. The embryos were allowed to develop for several days in a laboratory setting, then destroyed to harvest the resulting stem cells. Research using human embryonic material is troubling to many theologians and bioethicists.
Chimeras should not be confused with hybrids, which are organisms formed from two gametes (each from a different species) which formed a single zygote. All cells in a hybrid originate from this single zygote.
Chimeras should also not be confused with mosaics, which are organisms with genetically different cell types, but which again originate from a single zygote.
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