The chimaeras of nature and their promise to grow human organs | Explained Premium

Animals have long been used to fill the gap between organ donors and recipients. Recent advances in induced pluripotent stem cell technologies have enabled researchers to grow human organs in animals, raising ethical questions. Chimaeras, organisms composed of cells of more than one genotype, exist in nature and have been documented in humans. Bone marrow and organ transplants can also lead to chimerism. Scientists have successfully created a live chimaera in non-human primates, opening new doors for biomedical applications, but ethical issues must be addressed.

At present, more than 3 lakh people are waiting for an organ transplant in India alone; the global number is far higher, with no respite in sight. There is an alarming disparity in the number of organ donors and the number of recipients – and animals have played an important part in filling this gap.

The successful application of animal insulin and the more recent use of animal heart valves in human surgeries have saved human lives. Researchers have also made attempts to grow full human organs inside the bodies of animals using advancements in induced pluripotent stem cells (iPSCs) technology.

At the same time, controversy continues to beset this field, most of it centred on the use of human iPSCs in animal embryos and the creation of chimeric animals, the results of which we are yet to fully comprehend. However, humans are not new to the concept of chimaeras. Mythology abounds with a rich collection of these beguiling beings.

A genetic chimaera is a single organism composed of cells of more than one distinct genotype (or genetic makeup). The animal kingdom has several examples of varying degrees of chimerism. The half-sider budgerigar, a type of common parakeet widely adopted as pets, has different colours on either side of its body due to chimerism. The anglerfish displays an extreme degree of symbiotic chimerism in which the male fish fuses with and is eventually absorbed into the female fish, mixing their genetic makeups into a single animal. Marine sponges are known to have up to four distinct genotypes in a single organism.

Natural chimaeras among humans are well documented in the medical literature. They occur when the genetic material in one cell changes and gives rise to a clonal population of cells different from all the other cells. The fusion of two fertilised zygotes early in the embryonic stage can also lead to a condition in which two genetic makeups coexist in a single individual. Chimerism can also result from twin or multiple pregnancies evolving into a single foetus or a twin foetus being absorbed into a singleton.

Researchers have also documented individuals living with two blood types. In fact, blood-group chimerism during multiple births is relatively common. Most chimaeras are detected during routine blood tests in hospitals or when family members undergo tests ahead of an organ transplant. Pregnant women have been known to harbour the genetic material of her foetus in the bloodstream during the pregnancy. (Such foetal DNA can be used to screen for genetic defects and congenital abnormalities using non-invasive prenatal testing.)

Studies have also recorded a phenomenon called microchimerism, in which traces of the foetus’s genetic material are observed in mothers’ tissues many years after childbirth, resulting in two different genetic materials in a single person.