Chimeras: no, not the mythical kind.

Perhaps when you think of a chimera this image pops into your head:

The image above features a mythical creature called a chimera. Which is a concoction of goat, lion and snake that make up the creature’s body. However these days, the meaning of a chimera is vastly different. Chimeras are organisms that have two different sets of genetic information (DNA). But can this happen to us? The answer: Yes!

Human Chimeras

Human chimeras do exist. One cool example of human chimeras twin chimerism. In the study, one pair of twins (male and female) expressed two blood types: O and A. One of these blood types were found to be inherited genetically, whereas the other was transferred from one twin to another in the mother’s womb.

Here is a table that shows the siblings’ blood types:

Microchimerism:

Microchimerism can be present in humans but their causes often vary between each case. Maureen Knippen from the US FDA defines microchimerism “as the presence of low concentrations of two genetically distinct cell populations in the same indivdual.”. Or put simply, microchimerism occurs when an organism has two different sets of genetic information with one set in a lesser quantity than the other.

Causes

Causes of microchimerism can stem from the trade of cells between the mother and her fetus during pregnancy. For instance, in one study that explored microchimerism in females who had male children, found that 63% of the females tested (57 in total) had male DNA present in their brain.

Another cause of microchimerism is twin-to-twin transfer. Twin-to-twin transfer happens in the womb or in utero in which the twins exchange cells. One way twins are able to exchange this genetic information is through their placenta. A placenta is an organ that nurtures a baby by giving it the necessary oxygen and nutrients. Though not all twins share the same placenta, there is a rare possibility that the placentas connect through placental anastomoses (vessel that connects the two systems) to exchange blood cells.

Lastly, blood transfusions or organ transplants can also cause microchimeras. Take for instance blood transfusions; a process in which a patient receives a donors blood. Because this is often an emergency replacement for lost blood, the chimeric condition of the patient is temporary. If the patient receives a bone marrow transplant from a non-identical donor, the patient will have two blood types until one replaces the other.

Future of Chimeras

Research into chimeras has been a hot topic in science. Scientists have been experimenting with using pigs and lambs to grow human organs for transplant. This seemingly far away science-fiction idea isn’t too distant in our future. In 2019, researchers led by Professor Juan Carlos Izpiúa Belmonte, found a way to grow human cells in monkey embryos. These embryos were previously tried in pig and sheep embryos. This is because pigs and sheep have similar organ sizes to the organs we have. However, the result was unsatisfactory with only 1 in 10 000 of the embryos cells being human. By trying it in monkeys, the hope was to increase the proportion of human cells to animal cells.

In fact, rats and mice have given hope to scientists that organ transplants can work across different species. For instance, one study implanted mice with rat stem cells in hopes of growing a rat pancreas. Stem cells are pluripotent in that they are able to choose what type of cell they are (liver, kidney, stomach, skin etc.). The mice implanted with the stem cells were found to have functioning rat pancreases, however were too small to be transplanted into mice.

With pigs and sheep, the idea is to not grow a pig-human or sheep-human hybrid (which would be cool albeit scary), but instead, like the rats, to grow a specific needed organ ie: a heart or a set of lungs.

Ethicality

Despite these benefits, we must be weary about the ethical issues that come into play. One paper highlights the issues of farming chimeric animals for organs: the risk of human-like consciousness. This has raised an important question surround the legality and morality of the chimeric animals: If conscious, do they receive the same moral status as a human?

However, animal organ harvesting is at its infancy. Though this is a valid concern, scientists are continuing to work on robust methods to ensure these issue will not surface.