Chimera, << ky MIHR uh or kuh MIHR uh >> in genetics, is a single organism made up of a mixture of genetically distinct cells originating from two or more genetically distinct individuals. A chimera differs from a hybrid, offspring produced by mating two different varieties or species. Each cell in a hybrid’s body has a mixture of genetic material from both parents. Each cell in a chimera, on the other hand, has one of two or more different genetic identities. These genetically distinct cells coexist within a single organism. For example, if a chimera includes both a female and a male genetic identity, it may have both female and male cells in each organ and tissue of its body. The term chimera comes from Greek mythology, in which the Chimera was a monster with a mixture of features from different animals. It had the head of a lion, the body of a goat, and the tail of a serpent.
Genetic chimeras may form naturally as a result of fertilization, the process by which male and female sex cells unite to form an embryo (undeveloped young). Such a chimera typically arises when two sperm cells fertilize two eggs, forming two separate embryos. Normally, such an event results in fraternal (nonidentical) twins. Sometimes, however, the cells of the two embryos fuse together early in development. This fusion forms a single embryo—a chimera. The resulting organism will have tissues and organs composed of a mixture of genetically distinct cells descended from each of the original embryos.
Genetic chimeras can also arise from the sharing of blood supply that sometimes occurs between fraternal twins during pregnancy. During early development, blood vessels from twin embryos may join together, enabling cells, including stem cells, from one fetus to transfer to the other. Stem cells are cells that have the ability to develop into any of the different cell types that make up the body (see Stem cell ). This type of genetic chimerism occurs often among marmosets, a kind of monkey that lives in Central and South America that frequently produces twins. Among cattle, this type of genetic chimerism is relatively common when fraternal twins are of different sex, producing female cattle called freemartins. A freemartin appears female but has many male behavioral characteristics due to hormones produced by cells from the male twin. A freemartin is sterile and has masculinized internal reproductive organs. This type of genetic chimera occurs rarely in other animals.
Scientists can create artificial chimeras in the laboratory. For many years, scientists have created chimeric mice in the laboratory by artificially fusing genetically distinct mouse embryos. They use such creatures to study how an organism develops from an embryo, a process called differentiation. Scientists may even produce chimeras made up of cells from two different species. In 1984, scientists fused a sheep embryo with the embryo of a goat to form an interspecies chimera. The resulting animal, called a geep, showed physical characteristics of both a sheep and a goat. For example, some patches of skin on the animal grew wool, as in sheep, while other patches were covered with hair, as seen in goats.
Scientists have also created human-animal genetic chimeras. Scientists can create such chimeras in several ways. For example, they can inject human cells into live animals. In 1988, scientists announced the development of mice with immune system cells composed almost entirely of human cells. Some of the mice were created by injecting human stem cells into the kidneys of mice born with deficient immune systems. The mice then developed functional human immune systems. Scientists can also create human-animal chimeras by transferring animal cells or tissue into human beings. For example, physicians routinely replace faulty human heart valves with heart valves from cattle or pigs. Such patients are in fact a kind of human-animal chimera.
Scientists create human-animal chimeras to study how human cells interact and grow as an embryo develops. Studies involving human-animal chimeras also have applications in medicine. For example, mice with immune systems composed of human cells have been used to study how the AIDS virus affects human beings. In addition, chimeric animals might someday be used to grow human cells and organs for transplant (see Xenotransplant ). However, many scientists and ethicists (people who address questions of right and wrong) worry about the moral, legal, and health implications of creating an animal partially composed of human cells. In some countries, such as the United Kingdom and the United States, no laws forbid the creation of human-animal chimeras. However, scientific organizations in many countries have developed guidelines and regulations for research involving human-animal chimeras. In other countries, including Australia, Canada, and Japan, laws forbid the creation of many types of human-animal genetic chimeras.
