Going against what we thought was true, a controversial new Nature study has shown that nerve cell regeneration in a part of our brains called the dentate gyrus seems to halt when we hit the age of 13, suggesting that once we lose these cells to things like disease and aging, they do not come back.
The dentate gyrus is part of the hippocampus and is important in the formation of memories. Meanwhile, the hippocampus is an area of the brain central to emotion, memory, and the autonomic nervous system, which controls unconscious bodily functions like digestion.
Some previous studies have found that hippocampal cell regeneration does decline with age, while others have suggested that the cells regenerate well into adulthood, with one study even claiming that the human hippocampus gains 700 new neurons each day. However, the team behind the new study point out various methodological issues in previous research, such as the type of marker proteins used, that likely led to misreported results.
What’s more, many previous studies have focused on non-human animals, particularly rodents, which although mammals, are quite different from us.
To avoid the problem of using potentially misrepresentative non-human animals, as well as the ethical implications of studying live human brains, the researchers used a total of 59 human brain samples that had either been removed post-mortem or during surgery. These samples ranged in age from a 14-week-old fetus to a 77-year-old man.
To investigate whether new cells were being formed in the dentate gyrus, the researchers looked for both young neurons and progenitor neurons. Progenitor cells are cells that can differentiate into a specific type of cell, similar to stem cells but with a more specific predetermined ending. Antibody markers were used to highlight the presence of immature neurons and progenitor cells.
Contrary to recent research, the team found that the number of developing neurons in the dentate gyrus reduces with age, coming to a complete halt around the age of 13. Brain samples from people aged 18 to 77 showed no signs of immature neurons in this area of the brain.
The researchers also found similar results in the brains of macaque monkeys, suggesting that a lack of nerve cell regeneration, or neurogenesis, within the hippocampus could be a feature of the primate brain.
The researchers also note that this phenomenon has been proposed for aquatic mammals, such as whales, dolphins, and porpoises, which like many primates exhibit intelligent, complex behaviors and have evolved large brains.
It is unclear exactly why these animals might experience a lack of hippocampal neurogenesis in adulthood, which has mainly been reported in various species of rodents, but it could be linked to having a large brain. Perhaps future research will tell us why.
