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The appendix may be a “safe house” for helpful gut bacteria, researchers at Duke University proposed in 2007. After severe diarrhoea flushes out the intestine, bacteria sheltering there could help to re-seed it.

William Parker, R. Randal Bollinger et al., Duke University (2007)

HealthScience

The story behind the card

Charles Darwin saw the appendix as a shrinking leftover of a larger digestive pouch that plant-eating ancestors needed, and for over a century it was treated as useless. The Duke team, led by the surgeon Randal Bollinger and the scientist William Parker, pointed out that the appendix is a narrow side pocket off the large intestine, lined with immune tissue and coated in a film of bacteria, and so well placed to keep a reserve away from the main flow. Some hospital studies have linked a missing appendix to more repeat infections with Clostridioides difficile, a bacterium that can cause severe diarrhoea after antibiotics, though others found no link. A 2013 study across mammals found that the appendix evolved independently at least 30 times and was rarely lost once it appeared, which suggests it is useful.

Türkçe oku
Duke Üniversitesi araştırmacıları 2007'de, apandisin yararlı bağırsak bakterileri için bir sığınak olabileceğini öne sürdü. Şiddetli bir ishal bağırsağı boşalttığında, orada saklanan bakteriler bağırsağa yeniden yayılabilir.

Charles Darwin apandisi, bitkiyle beslenen atalarımızın ihtiyaç duyduğu daha büyük bir sindirim kesesinin küçülmüş kalıntısı sayıyordu; apandis yüz yılı aşkın bir süre işe yaramaz bir organ olarak görüldü. Cerrah Randal Bollinger ile bilim insanı William Parker'ın yönettiği Duke ekibi ise başka bir şeye dikkat çekti: Apandis kalın bağırsağın yanında dar bir cepti; içi bağışıklık dokusuyla kaplıydı ve üzerinde bir bakteri tabakası bulunuyordu. Bu yüzden ana akıştan uzakta bir yedek bakteri deposu olmaya çok uygundu. Bazı hastane çalışmaları, apandisi alınmış kişilerde, antibiyotik kullanımından sonra ağır ishale yol açabilen Clostridioides difficile bakterisiyle enfeksiyonun daha sık tekrarladığını buldu; başka çalışmalar ise böyle bir bağ bulmadı. 2013'te memelileri karşılaştıran bir çalışma, apandisin birbirinden bağımsız olarak en az 30 kez ortaya çıktığını ve bir kez oluştuktan sonra nadiren kaybolduğunu gösterdi. Bu da işe yarayan bir organ olduğunu düşündürüyor.

Sources and further reading

Background reading: Appendix (anatomy)
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The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

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The estimate comes from Ron Sender, Shai Fuchs and Ron Milo of the Weizmann Institute of Science in Israel. Their reference man is 20 to 30 years old, 170 cm tall and weighs 70 kg. The ten-to-one figure, they found, went back to a single rough calculation from 1972 that had been repeated for decades. Almost all the bacteria live in the colon, the last part of the gut, and together they weigh only about 0.2 kg. Among human cells, red blood cells dominate the count, making up about 84 per cent, while fat and muscle cells, far fewer but much larger, make up most of the body's cell mass. The balance also shifts through the day: a single trip to the toilet empties a quarter to a third of the colon's contents.

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Ron Sender and Ron Milo combined the number, mass and lifespan of every major cell type in the body. Short-lived cells dominate the count. Red blood cells live about four months, and neutrophils, the most common white blood cells, only days, so the bone marrow must make vast numbers of them every day. By mass, the largest shares come from blood cells and from the cells lining the gut, which are shed and replaced within days. At this pace, a number of cells equal to all the roughly 30 trillion human cells in the body is replaced in about three months. That does not mean the whole body is renewed: most nerve cells in the brain last a lifetime, and heart muscle cells are replaced only very slowly.

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Scientists call this link the gut–brain axis. The gut has its own network of about 500 million nerve cells, the enteric nervous system, sometimes called the “second brain”, which can run digestion largely by itself. It is connected to the brain mainly through the vagus nerve, a long nerve running from the brainstem down to the abdomen, and most of its fibres carry messages upwards, from the gut to the brain. More than 90 per cent of the body's serotonin, a chemical messenger, is found in the gut. Gut bacteria play a part too: in a 2004 Japanese study, mice raised entirely free of germs overreacted to stress. Stress can speed up or slow down the gut, one reason it often worsens irritable bowel syndrome, a common gut disorder. Most research on the microbes' role has been in animals, and human evidence is still limited.

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The word placebo is Latin for “I shall please”. One of the first clues that the effect is physical came in 1978, when Jon Levine and colleagues studied patients recovering from wisdom tooth removal. They gave them a placebo and naloxone, a drug that blocks the effect of endorphins, the body's own painkilling chemicals. In patients who had responded to the placebo, naloxone increased the pain, which suggested that the placebo relief came from endorphins. In 2004 Tor Wager and colleagues used brain scans to show that a placebo painkiller reduced activity in pain-processing regions during a painful stimulus. Expectation works the other way too: in the nocebo effect, from the Latin for “I shall harm”, expecting harm makes symptoms worse. None of this means the pain was imaginary, or that placebos can replace effective treatment.

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