Alcohol doesn’t make you invisible
I simply couldn’t believe it when I was little and my grandfather told me that the air was full of bacteria, spores and other microorganisms. If they were there, why could I not even see them when I strained my eyes? Were they so small or transparent or both? Invisibility has fascinated humankind since time immemorial and many fairytales and the popular TV series “The Invisible Man” corroborate that. While humans may never reach the goal of true invisibility through transparency, some body tissues and in fact some whole animals have achieved just that.
The healthy lens in the vertebrate eye is totally transparent, yet it consists of living cells. Finger nails, skin, the wings of many insects and the fins of a number of fish species are also transparent, but when you can virtually look through an entire fish, an insect, a worm or a medusa, you really wonder how these animals can possibly become translucent to such an extent. In the South American glass catfish only its eye, its brain and its gut as well as what’s in it, are visible; the aquatic larva of a mosquito-like carnivorous insect by the name of Chaoborus is so extraordinarily translucent that you could easily overlook it unless it wriggled (and that’s when the fish get it). The all-abundant marine usually just 1 to 2 cm long arrowworms of the phylum Chaetognatha are yet another example of invisible critter and chances are you’d catch some of them in a bucket of sea water anywhere in the world without noticing them on account of their transparency. Deep sea jellyfish, certain squid and some shrimp, all several centimetres in length, have also produced species that one can see right through.
Transparency, especially in water, is of course a form of camouflage and helps prey as well as predators to remain undetected as long as possible. Three main requirements have to be met by the tissues to become transparent:
- There must be a minimum of blood vessels and pigmentation present
- The number of intracellular organelles like mitochondria for instance must be low
- A relatively orderly, repetitive and constant structural unit is needed.
Since muscle tissue is indeed very regular and repetitive when examined under the microscope, it can be made transparent more easily than, for example, nerve cells, but muscle fibres do need some blood supply. The latter does not hold true for the cellular organization of the lens in the eye, for it receives its nutrients and oxygen through diffusion from cell to cell, so that a blood supply is unnecessary. The nervous system, however, because of its whitish lipid content and a far less orderly and repetitive organization than that present in muscle and lens tissues cannot be made transparent.
Transparency, as useful as it may be, it does not come free: it is maintained by active metabolic processes and it may not cover all wavelengths equally. A dead glass catfish turns white and an arrowworm stuffed into a bottle of alcoholic fixative quickly turns opaque. I demonstrated this effect of alcohol to little children when I was in Papua New Guinea, hoping they’d understand that alcohol and health don’t go together very well. Come to think of it, that’s probably the reason why I never saw that invisible TV character drink a glass of wine! Well, he sure wasn’t a James Bond.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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