雅思阅读 094: Why Mammals Limit New Brain Cells

Adapted from 'Songbirds reveal the dark side of making new brain cells as adults', Scientific American, April 2026.

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Every day the human body replaces billions of cells: red blood cells last about four months, skin cells about a month, and intestinal lining cells just a few days. But there is one part of the body where humans and other mammals do not generate new cells: the brain.

Aging and damaged neurons can cause memory problems and limit the brain ability to recover from illness. Some scientists have proposed that if we could activate neurogenesis, the creation of new neurons, some of these changes might be reversed. But a new study suggests neurogenesis may be more destructive than previously thought, adding weight to the theory that the brain apparent limitation is actually an evolved protection.

Birds, reptiles and fish all have widespread neurogenesis throughout their forebrains throughout life. It is really in mammals where this ability is restricted.

In a new paper published in Current Biology, researchers analyzed the brains of Zebra Finches, small songbirds that undergo neurogenesis throughout life. Using an electron microscope, they watched how new neurons reach their destination.

Previously, researchers had assumed neurons might follow structures called glial scaffolds that guide them to the right place. But the team observed that new neurons tunnelled straight through older neural pathways. The new brain cells were also more rigid than mature neurons.

Because adult brains are done growing, they lack room for new structures. The tunneling was not a complete surprise. Still, understanding the destructive side of neurogenesis could help explain why mammals limit this ability in adults.

As new neurons move through the brain, they seem to push or deform the tissue. They might alter circuits, breaking connections that are the basis of stored memories. Humans and other mammals may have evolved to limit adult neurogenesis in order to preserve important long-term memories.

Questions 1-5

Choose the correct letter, A, B, C or D.

1. How long do skin cells last? A. Four months B. About a month C. A few days D. A year
2. What is neurogenesis? A. Death of old neurons B. Creation of new neurons C. Freezing of brain tissue D. A type of bird song
3. Which animals have widespread neurogenesis? A. Mammals only B. Humans only C. Birds, reptiles and fish D. Primates only
4. What did new neurons do in Zebra Finch brains? A. Followed glial scaffolds B. Tunneled through older pathways C. Died immediately D. Grew into full neurons
5. Why might mammals limit adult neurogenesis? A. To save energy B. To preserve long-term memories C. To allow faster aging D. To produce more glial cells

Questions 6-10

Do the following statements agree with the information? Write TRUE, FALSE or NOT GIVEN.

6. Red blood cells last about four months.
7. The study was conducted on human brains.
8. New neurons were softer than mature neurons.
9. Mammals and birds have similar brain organization.
10. The researchers used an electron microscope.

Questions 11-13

Complete the summary. Choose ONE WORD ONLY.

Previously scientists assumed neurons followed glial 11 to reach their destination. But the new neurons were more 12 than mature cells and pushed through tissue, potentially breaking stored 13.

Answers

1. B 2. B 3. C 4. B 5. B
2. TRUE 7. FALSE 8. FALSE 9. FALSE 10. TRUE
3. scaffolds 12. rigid 13. memories

Key Vocabulary

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