Turning invisible: lessons from cephalopod cells

Ellie Bennett looks at the way in which cephalopod cells might hold the key to turning our dreams of invisibility into a reality, albeit at the cellular level. 

Doryteuthis opalescens, the opalescent squid. The females of this species have a unique ability to switch their cells between translucent and opaque states. Image credit: Joshua Sera, Flickr, CC BY-SA 2.0.

There are few other classes of animal that conjure up such feverish images of deep-sea monsters or otherworldly aliens than that of the cephalopods. Translating from Greek into ‘head-feet’, this class of marine creatures include the octopus, squid, cuttlefish and nautilus. Whilst in reality, they are a far cry from the fabled kraken or the ‘heptapod’ aliens in the movie ‘Arrival’, they still possess some decidedly unusual features and continue to fascinate scientists. 

Their stand-out characteristics include their tentacles, an ability to squirt ink when threatened and perhaps most intriguing of all, their intelligence. However, the one cephalopod characteristic that humans seem to envy the most (unless there exists a large cohort of people who wish they had tentacles) is the way in which their skin can change colour, texture and even transparency. Humans have long been fascinated by the possibility that we could one day turn ourselves invisible, but we’ve never been able to figure out how. Even where animals exist with transparent body parts or tissues, these are statically transparent and aren’t able to turn opaque and switch back and forth between the two. The ability to transition between opaque and transparent is known as dynamic transparency and is a highly elusive trait in the animal kingdom.

The one exception to this is a species of squid known as Doryteuthis opalescens. Both the males and females of this species are largely transparent, although always visible on the males back is an opaque white strip, which is actually his testis. To defend herself from aggressive males, the female D. opalescens is able to turn the transparent skin on her back into an opaque strip in order to appear male. 

This squid has inspired a group of scientists at the University of California to see if it’s possible to turn human cells ‘invisible’. The research, led by Atrouli Chatterje, managed to engineer human embryonic kidney (HEK) cells to switch between transparent and opaque states. To do this, they borrowed the protein responsible for forming the female D. opalescens ‘testis’, which is known as ‘reflectin’. In the squid, it would normally be found in the skin, inside cells known as leucophores. When the female squid is relaxed and unthreatened by randy males, the reflectin proteins in the leucophores remain separated, allowing light to pass through them so that the skin appears transparent. As soon as the female is threatened, the reflectins clump together and form spherical structures which change their refractive index to cause them to scatter more light, turning the cells opaque.

Amazingly, the scientists were able to get HEK cells to not only express reflectin in significant enough quantities, but also support them to form structures very similar to the size and appearance of those formed in the squid leucophores. By exposing the reflectin-expressing HEK cells to different salt concentrations they were able to stimulate them to transition between transparent and opaque. We should note that HEK cells are naturally transparent to begin with so it wasn’t strictly a case of turning something invisible. However, similar methods and technology could be utilised to do the opposite with something naturally opaque. 

Image credit: M.J.A. Verbiest, Flickr, CC BY 2.0.

Chatterje reminds us that this research is “decidedly in the realm of science”, and so are the more immediate applications of these findings. One of the most useful for future scientific research would be the ability to turn not just single cells, but whole living tissues, transparent. This would make it much easier to see the processes and interactions between the cells that make up a piece of functioning tissue, or even an entire organ, in real-time. There would be no need for dissection and so the tissue could be kept alive without disruption. 

Whilst probably getting ahead of ourselves and certainly of Chatterje’s research, we could ponder a reality in which whole organisms could be made transparent. In a futuristic hospital, a doctor may be able to inject us with a substance that temporarily invades our skin cells, turning them translucent, allowing early stage tumors or other insidious diseases to be detected. This would be invaluable for doctors who often have to rely on patients’ self-reported symptoms or various physiological tests to figure out what might be wrong. 

But as far as humans turning invisible or transparent becoming a reality, the scientific consensus is not in our favour. Research conducted into ‘invisibility cloaks’, by Andrea Alù at the University of New York found that it would be impossible to conceal something as big as a human. In fact, they found that the upper size limit would be 500 nanometers, so impossibly tiny. Of course, Chatterje’s research shows that transparency can be achieved on a cellular level but this would need to be engineered in numerous different cell types and would need to be easily reversible. A cloak made of some sort of high-tech, light-bending material could be more practical, but as Alù’s research suggests, not physically possible. 
So whilst the natural world continues to taunt us, with beating organs visible through the belly of a glass frog, or the golden tortoise beetle flushing from metallic gold to red as it copulates with a mate, dreams of invisibility remain as just that: the stuff of imagination and clever CGI tricks on the big-screen. But we should take some small comfort in the idea that scientists are out there making small steps to use these light manipulating properties to advance our understanding of the machinery of cells and tissues, at the very least to better understand those fascinating heads-with-feet, under the sea.

Written by Ellie Bennet and edited by Ailie McWhinnie.

Ellie’s thoughts… This article probably puts a bit of a dampener on those who dream to one day turn themselves invisible and probably get up to some mischief if nothing else. I felt this way too, at first. But actually the more realistic small-scale applications for turning something invisible could be really beneficial for the human race in terms of scientific research and medicine. Turning a whole human transparent could be useful too – for reasons mentioned in the article, but I can only really think of the more sinister applications; warfare, espionage and even hunting for sport.

Find me on… Twitter @ellieb2302 and Linkedin @Eleanor Bennett

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