If you put your hand on your chest you’ll probably realize that your heart is a bit left-of-center. Or perhaps not: about one in 10,000 people have their heart on the right. No one’s heart lies exactly on the body’s midline, nor does anyone’s liver (generally on the right). This is, if you think about it, amazing: we’re roughly left-right symmetric, but not quite; the asymmetry isn’t random, with half the population having left-side hearts and half right-side hearts, but rather is well-controlled. Why? What guides the positioning of our organs and nudges them towards a precise asymmetry?
The answer isn’t known in detail — and even if it were, the aim of this series is to highlight questions, not answers — but we know that fluid flows established when you were a tiny embryo are a key aspect of it. Hair-like cellular projections called cilia push liquid leftward rather than rightward in a microscopic, cavern-like organ. Proteins carried along by the fluid initiate the first stages of anatomical left-right asymmetry. (This of course begs the question of where the cilia’s directionality comes from…)
The asymmetry is not, by the way, a topic I elaborate on in my soon-to-be-released popular science book on biophysics, So Simple A Beginning. I thought for #4 in this series I’d deliberately pick something outside its range — biophysics is a vast subject. I do, however, describe the strange ways in which cilia must contort themselves to generate flows — relevant to many phenomena inside you — as well as other aspects of embryonic development such as how you got a nice array of evenly spaced vertebrae by building a clock out of cells. Obligatory links to the book: My description; publisher’s page; Amazon page.
If you do want to learn more about left-right asymmetry, including its manifestations in writing and culture (for example, social attitudes towards left-handers), I highly recommend Chris McManus’ Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures.
Today’s illustration is, of course, two hearts. Drawing one was easy; drawing the other as the mirror-image of the first was surprisingly difficult.
— Raghuveer Parthasarathy; January 17, 2022