The organism being held via two micropipettes to elongate the “neck”
Elliot Flaum and Manu Prakash/Stanford University
Imagine if your neck was so extendible that your head could reach your local shop while you sat on the sofa. That would be the human equivalent of what one single-celled predator can do – and now the long-standing mystery of how it can extend its “neck” more than 30 times the length of its “body” has been solved.
The cell membrane of this organism is folded up into a series of pleats that can only unfold and refold in one way, Eliott Flaum at Stanford University and her colleague Manu Prakash have discovered, allowing it to extend and refold without ending up in a tangled mess. “We figured most of this out by playing with a piece of paper,” says Prakash.
Lacrymaria olor is a single-celled organism, or protist, that lives in fresh water and hunts its prey with its extraordinarily extendible neck-like protrusion. Its name means “tear of a swan”, after its swan-like neck and tear-shaped body.
While cell membranes are highly flexible, they aren’t elastic and cannot stretch. So how L. olor extends its neck to such a great length has been a mystery since it was first seen under a microscope in the 16th century. “We compared this to many other organisms, and it’s orders of magnitude larger in this extension,” says Prakash. “That’s the puzzle.”
He and Flaum became intrigued when they saw L. olor in samples they collected from a swamp six or seven years ago, and they set out to solve the mystery. Flaum used a number of different techniques to image the outer structure of L. olor and its inner cytoskeleton, made of structures called microtubules. “We tried many different ways of looking at it to understand what was happening,” she says.
This revealed that the cell membrane of L. olor is folded into 15 pleats, and each pleat spirals around the cell, forming a helical structure. Prakash calls this folding pattern “curved crease origami”, or “Lacrigami”.
But how does L. olor unfold and refold this huge area of cell membrane without getting into a tangle? What Prakash and Flaum worked out is that because of the way the pleats are stabilised by bands of microtubules connected to them, an entire crease cannot unfold all at once. Instead, only one point of a crease can unfold or refold at any time.
As these points move in parallel up each of the 15 creases, the cell membrane unfolds in an orderly manner, extending the neck. Reversing the process shortens the neck.
“Instead of randomly folding, like if you were to crumple paper, it has a guiding rail for it to fold back in the same way every time,” says Flaum.
The folding and unfolding is powered by the beating hairs, or cilia, that cover the entire surface of the cell, says Prakash. It requires energy to unfold as well as refold, unlike a spring, but needs very little because cell membranes bend easily.
As far as he knows, no one has discovered this folding pattern before. “When we figured this out, I’d always thought that somebody playing with paper would have discovered this origami,” says Prakash. “It’s quite simple.” Anyone can make it with a piece of paper and bit of tape, he says.
“The clever origami design of its neck makes cilia effective for high-speed, long-range hunting,” write Leonardo Gordillo and Enrique Cerda at the University of Santiago in Chile in an accompanying article. “The origami-like protrusion mechanism identified by Flaum and Prakash has the potential to inspire new strategies in soft-matter engineering.”
Indeed, Prakash and Flaum are now working on developing medical robots based on “Lacrigami”. “If you had a small micro-robot in a very small space, and it could suddenly extend, that would be hugely useful in microsurgeries,” he says. “But we did this work because it’s just beautiful and a mystery to solve. We had no thought of it being useful in any way.”
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