biology zoology blog benno meyer rochow situs inversus

When Everything is Back-to-Front and Left is Right and Right is Left

One of my Indian wife’s uncles was an engineer (another uncle was a person who exhibited eyeshine when a light was shone into his eyes at night and the children found that quite scary). Anyway, what was so special about the first uncle is related to the time he spent working in Libya. He got sick once, suffering from chest pain, saw the doctor and got a chest x-ray. When the nurse hung the x-ray photo against a light box, the doctor got mad at her and rebuked her for pinning the negative up back to front and that as a nurse she should know better. Overhearing that, this said uncle intervened and remarked that the poor nurse had actually been right, for he had a condition known as “situs inversus.

What is that: situs inversus? It’s a situation in which the internal organs of a person are reversed, in a mirror-image position so-to-speak: heart and spleen are on the right, liver and appendix on the left. It’s a rare condition that people are born with and it can affect both men and women -and it has occasionally been reported in mice, dogs, cats, sheep, pigs and even according to a 2008 publication by Palmers et al. in a horse. It’s neither painful nor does it affect your intelligence (you can after all become an engineer, see above), but there are some minor disturbing effects like a lot of coughing, bringing up sputum, and recurring inflammations of the sinuses. In the case of men there is that added problem of infertility: the spermatozoans of situs inversus men simply don’t swim; they can’t move.

The inability of the spermatozoa to move, the consistent coughs and recurring sinus troubles are linked, because in all cases cells are involved that are supposed to possess actively beating cilia (= little hairlike projections of cells that are propelling mucus and slime across the cell’s surface). If the cilia don’t move mucus accumulates in the bronchi and can only be cleared by coughing or the mucus clogs up nasal cavities, where it causes inflammation. It was the Japanese researcher Nobutaka Hirokawa, whose discoveries provided some insight as to what could lead to the heart in an embryo to shift to the right rather than left side of the body. It had to have something to do with the behaviour of those cells whose hairlike cilia were not beating normally. But the ciliated cells Hirokawa was observing were those that were located in a place called “Hensen’s node” in a tube very early in a developing embryo, whose front end gives rise to the heart. Initially the tube (called “primitive streak”) is symmetric, but ciliated cells in Hansen’s node shunt so-called signalling molecules that initiate the “construction” of the heart towards the left. The accumulation of these signals on the left of the tube results in the formation of the heart on the left. If the ciliated cells cannot beat at all, there will be equal amounts on the left and right of the tube and if ciliated cells do beat, but much more slowly than normal, then the signalling molecule don’t arrive at the correct time on the left and actually stay behind causing the signal to form a heart to be acted upon on the right side of the tube.

All this may sound a little complicated. But it demonstrates how chemical events (signalling molecules) are connected to physical aspects (the flow created by the beating cilia) and how the latter depend on physiological properties (membrane events) of anatomical structures (cells of Hansen’s node) to determine the fate of an organ. Amazing! It also shows the necessity of collaboration between embryologists, anatomists, physiologists, biophysicists and chemists (and others as well). However, to predict whether a baby may suffer from situs inversus or not and why handedness is not affected by the condition, has not been possible yet. Some mysteries remain, for even in twins one of them may show the condition and the other won’t.

© Dr V.B. Meyer-Rochow and, 2019.
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