Chimeric Animals What are Chimeras Chimeras are animals

Chimeric Animals

What are Chimeras? • Chimeras are animals composed of cells that originate from two (or more) different species. • In the research lab, chimeras are created by introducing cells from one species into the developing embryo or fetus of another. • The name chimera comes from Greek mythology and describes a creature with the head of a lion, the body of a goat, and the tail of a serpent.

A sheep-goat chimera, created in 1984, had the head of a goat and the woolly coat of a sheep.

Chimeras, Mosaics, Hybrids • Chimeras are different from mosaics- organisms that contain genetically different populations of cells originating from a single zygote. • Chimeras are different from hybrids too, which are organisms containing genetically identical populations of cells originating from a cross of two different species.

Dispermic Chimeras • Two eggs that have been fertilized by two sperm fuse together, producing a so-called tetragametic individual—an individual originating from four gametes. • The different tissues of tetragametic chimeras are made up of cells derived from one or both zygotes. • Indications of tetragametic chimerism include eyes differing in colour, patchwork skin coloration, and ambiguous external genitalia, which is a sign of hermaphroditism (having both male and female reproductive organs). • In most instances, however, there are no observable symptoms of tetragametic chimerism, and the condition is detected only through extensive genetic analysis

Twin Chimeras • When two zygotes do not undergo fusion but exchange cells and genetic material during development, two individuals called twin chimeras, are produced. • one or both of them contain two genetically distinct cell populations. • The most widely known examples of twin chimerism are blood chimeras. • Blood Chimeras are produced when blood anastomoses (connections) form between the placentas of dizygotic twins, thereby enabling the transfer of stem cells between the developing embryos. • When blood chimerism involves male and female twins, female exposure to male hormones results in freemartin syndrome, in which the female is masculinized; this is commonly seen in cattle and rarely in humans.

Other Types of Chimeras • Microchimeras are produced when fetal stem cells or maternal cells cross the placenta or following blood transfusion or organ transplantation. • Parthenogenetic Chimeras be produced when a fertilized egg generated through parthenogenesis fuses with a normal zygote. • Androgenetic Chimeras are made up of cells that contain the normal combination of maternal and paternal chromosomes and cells that contain two sets of paternal chromosomes (paternal isodisomy). – Mammalian androgenetic chimeras generated experimentally rarely survive to birth – In humans, the condition can occur naturally, though it typically ends in embryonic death.

Mouse Embryonic Chimeras

Major Projects Underway Utilizing Human. Animal Chimeras • At the University of Nevada, researchers have added human stem cells to sheep fetuses to create chimeras. Some sheep now have livers with up to 80% human cells that produce the compounds normally made by human livers. • Mayo Clinic researchers in Minnesota are studying how speciallybred pigs could be used as sources of organs for human transplant patients. They have developed a line of pigs that have pig blood cells, human blood cells, and a new kind of blood cells with characteristics of both humans and pigs. • At St. Kitts Biomedical Foundation in the Caribbean, scientists are transplanting immature human brain cells deep into the brains of vervet monkeys. Their goal is to develop a treatment for Parkinson’s disease

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