Copyright © Karl Dahlke, 2023
For almost all of us, the heart beats on the left, but there are exceptions. One person in 1,200 has their heart reversed, so that it beats on the right. This is called Dextrocardia, which is Latin for heart on the right. Beyond this, one person in ten thousand is born with all their organs reversed, as if through a mirror. This condition, which usually causes no trouble, is called situs inversus, Latin for turned in place. Still, the patient should wear an alert bracelet, so that an emergency medical technician will listen for the heartbeat on the right, and look for the appendix on the left, etc.
Things do get complicated however, if the patient ever needs an organ transplant. It's hard enough finding a compatible donor, and when you do, he will almost certainly be oriented normally, left to right. The heart and liver have asymmetric shapes, and do not easily fit into a cavity that is the mirror image. It's a bit like putting your right hand into a left glove. Furthermore, these organs attach via veins and arteries at particular locations, and the surgeon must improvise when the point of attachment is on the other side, like stretching a cord all the way across the room to plug a toaster into the wall.
The mystery isn't that situs inversus exists, the mystery is why it doesn't happen more often. How does the developing embryo determine left and right with such fidelity?
The order of orientation is front back, up down, left right. The point of attachment between the embryo and the uterus becomes the front of the baby. When the embryo separates from the uterin wall, the umbilical cord retains the connection, still at the front of the fetus. Thus the front back axis is established.
While the embryo is attached to the uterus, gravity determines head and tail. The mother spends most of her day in one orientation relative to the earth. chemical markers separate slowly within the embryo, directed by gravity, whereupon the top becomes the head and the bottom becomes the tail. That establishes the up down axis.
Determining left from right, an event known as symmetry breaking, is still a matter of research. According to studies in mice, cilia play an important role. They twirl clockwise, and that by itself doesn't favor left or right, but they also tilt down towards the tail. Somewhat unexpectedly, this produces a net flow of fluid to the left, which in turn establishes left from right. The heart develops on the left and the liver grows on the right, and it works almost every time.
Since gravity plays a key role, can an embryo develop in space? Could a woman get pregnant on the Space Station or on the way to Mars? Early experiments suggest the answer is no. Mouse embryos were sent up on the Space Shuttle, STS-80, and none of them developed properly, relative to a control group here on earth. In fact they all degenerated. Another experiment on Cosmos 1129 involved mature rats who were free to mate, but produced no pups. Post flight examination found that two of the females had achieved pregnancy, but their embryos appear to have been resorbed. Based on these preliminary results, it looks like the first extraterrestrial pregnancy will take place on Mars, assuming one third G is good enough. Not in my lifetime though. Even if NASA sends humans to Mars by 2040, nobody is willing to risk a tubal pregnancy so far from home.
A chicken egg is another story altogether. There is no point of attachment to determine front from back, and the mother hen turns the eggs from time to time. I have no idea how the embryo develops here on earth, or how it would develop in zero gravity.
Returning to our original theme, assume Jane has situs inversus. Her organs are reflected, but her molecules are not. Most of life's molecules, sugars, proteins, enzymes, vitamins, etc, come in left and right handed versions. Almost all biology on earth uses left handed proteins and right handed sugars. If you eat left handed sugar, it will pass right through you undigested. The chemicals that built Jane as an embryo are the same as those used to build any other embryo; she simply put her heart on the other side. Thus you can take Jane out to a nice restaurant, and she can eat all the same food, and digest it, just like you and me. Things are quite different if she was truly reflected, by a transporter malfunction. A lab could construct reverse sugars and amino acids for her, but it's not clear if she could be kept alive long term.
Situs inversus exists in other animals, including dogs, cats, pigs, horses, rats, fish, etc, at about the same frequency. We would be surprised if it didn't. There's no reason this phenomenon should be uniquely human.