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Cellular machinery and decision-making
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- The Story: In 1906, zoologist Herbert Spencer Jennings published "Behavior of the Lower Organisms", a book that contained a provocative idea: microbes can change their minds! He probed the idea whether individual cells could make conscious decisions.
- Details: His subject was a single cell bristling with beating hairs called "Stentor". These trumpet-shaped predators are so large fish can eat them and humans can see them, and so brazen they can catch and eat rotifers — proper animals with hundreds of cells and a simple brain. In an experiment, Jennings decided to annoy it and see what happened. When confronted with a stream of irritating carmine powder expertly aimed at their mouths by his steady hand, Stentor would first bend away, then reverse the beating of its hairs (called cilia) to expel the powder, then contract and finally detach.
- Learning: He noted that the order of behaviors varied somewhat with different stimuli (he tried other chemicals) and steps were sometimes omitted. He came to the conclusion that the behavior consisted in ‘trying’ successively different reactions, till one is found that affords relief. So, stentors could confront a stimulus with one behaviour, and then choose a costlier approach if the irritant persisted. However, other scientists tried and failed to elicit the same reactions. So the idea was dumped.
- Rebirth of the experiment: In 2011, Jeremy Gunawardena at Harvard Medical School discovered the experiment and its rejection, and decided that it deserved another look. He discovered the 1967 team had not used the correct species of Stentor. The one they had chosen, Stentor coeruleus, strongly prefers to swim, unlike Jennings’s Stentor roeselii, which prefers to not. So he became fascinated by what replicating the experiment might reveal about what single cells are capable of. The Harvard team managed to track down the correct species in an English golf course pond, construct their own “Device for Irritating Stentors” and discovered something extraordinary.
- In their setup, Stentor did not respond to carmine powder the way Jennings had ealrier described.
- When faced with barrages of 21st-century plastic microbeads, individual Stentor roeseli behaved consistent with Jennings’s description, and in one remarkable way that Jennings did not observe in 1906.
- If Stentor really can “decide,” it certainly isn’t the only way the ciliates, the group of shaggy microbes to which Stentor belongs, resemble us. A ciliate operates like an animal at the scale of a single huge cell, and the resemblance can be startling.
- For example, some glue bundles of their cilia into structures called cirri and can use them as legs, mouths, paddles or teeth. The cirri are wired by nervelike neurofibrils. If the fibrils are cut, the cirri fall limp.
- Some ciliates pack tiny tethered darts they can fire to attack prey, deter predators or simply drop anchor.
- Like sea stars, ciliates can regenerate entire bodies within a day or two from tiny pieces if those pieces contain both a bit of the cell’s cilia-studded armour and a bit of nucleus, the cell’s genetic heart.
- One ciliate called Diplodinium lives in the rumen of cows and other hoofed animals, a special environment known to harbour all kinds of strange things, about half of which by mass may be ciliates.
- Diplodinium contains neurofibrils, cirri, musclelike striated contractile fibers called myonemes, a “backbone” made of stacked plates, a mouth, an esophagus that contracts with the help of a ring tethered to its exterior, and an anus. But all this is just a single cell!
- Extreme: The ciliates have taken the biology of the solo cell to its extreme limit. In this new study, the scientists found that Stentor indeed switched behaviours in response to repeated puffs of beads, and the order of operations was generally consistent with Jennings’s description. Detachment was always preceded by contraction, and mathematical analyses revealed cilia alternation or bending were far more likely to appear before contraction than after. It also looks like stentors have personalities!
- The big question: So, does Stentor possess something like a capacity to make decisions? Experiments suggest so. The choice between contracting or detaching was consistent with the probability of a fair coin toss, so it was perfectly random. The problem is that no known cellular mechanism can produce this result.
- Summary: What cells are capable of because they are only cells may need revision now. The capabilities of free-living cells may well exceed our imaginations, trapped in a body made of trillions of tightly wound-together cells!
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