Chimeras, usually confused with hybrids, which are formed from the fusion of gametes from two species that form a single zygote with a combined genetic makeup in order to produce a hybrid organism, or Hybridomas which, as with hybrids, result from fusion of two species' cells into a single cell and artificial propagation of this cell in the laboratory. In a chimera, each cell is from either of the parent species, whereas in a hybrid and hybridoma, each cell is derived from both parent species. Chimeras are formed by taking at least four parent cells (two fertilized eggs or early embryos fused together). It, thus, basically a single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that have originated from different zygotes involved in the process of sexual reproduction.

Fred Gage, a biologist at the Salk Institute created some part-human animals to understand the basic mechanisms underlying the question how human neurons degrade in people suffering from neuro-degenerative diseases for instance, Parkinson's and Alzheimer's. Studying and perturbing brain cells in their natural environment that is inside a functioning brain do provide the best hope and methods for developing therapies to prevent or overcome disease symptoms. But experimentation on human brains is entirely unacceptable, and thus scientists are hoping that if animals are provided with a small percentage of human brain cells, it may provide a substitute for human subjects.

Going back into the history of chimeras, the first chimera to have appeared was 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. Its creator Steen Willadsen , a Danish embryologist, states "It behaved like a goat but did not quite smell like one, preferring the company of sheep." This first geep was physically healthy, long-lived, and even fertile. Additional geeps were created by another scientist Willadsen over the next several years.

Coming to the part human animal chimeras, human stem cells at many different stages, from embryonic to adult, can be cultivated under laboratory conditions and chimeras can now be produced significantly by inserting these stem cells into animal foetuses or embryos. Yair Reisner from Weizmann Institute in Israel introduced human kidney stem cells into mice, and stimulated the cells to multiply and develop into miniature, but fully functional, human kidneys that actually happened to secrete urine. Gage attempted at introducing human brain cells in mice that lead to production of mice that were less than 1 percent human and the human brain cells seemed to have successfully incorporated into the adult mice brain.

In several researches, faulty human heart valves were routinely replaced with ones taken from cows and pigs. The surgery which makes the recipient a human-animal chimera is widely accepted. In the past two years, scientists have successfully attempted at creating pigs with human blood, fused rabbit eggs with human DNA and injected human stem cells to enable paralyzed mice to walk. Scientist, Esmail Zanjani, and his team at the University of Nevada-Reno want to turn sheep into living factories for human organs and tissues and one day, along the way create cutting-edge lab animals to more effectively test experimental drugs. But first, Zanjani must ensure no animal diseases would be passed on to patients.

Back in August 2003, researchers at the Shanghai Second Medical University located in China had successfully demonstrated that they had fused human skin cells and dead rabbit eggs to create the first human chimeric embryos. Also, further, the embryos were allowed to develop for several days in a laboratory setting which were then destroyed to harvest the resulting stem cells. Again in 2007, scientists at the University of Nevada, school of Medicine created a sheep whose blood contained 15% human cells and 85% sheep cells.

Thus we can speculate that the part-human animal chimera are being developed constantly by various researchers at a high pace and they are significantly helping in studying the mechanisms of human cells. The organ transplantation has been the major advantage of this new era of chimera production and is surely helping human kind as long as the ethical boundaries are not disturbed and the red line to the remotely human chimeras is not crossed.

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