For the past two decades, scientists believed that adults are capable of creating hundreds of new brain cells or neurons per day.
However, recent evidence challenges this long-standing theory by suggesting that neurogenesis stops by age 13.
In a controversial new study, a team led by University of California San Francisco's Arturo Alvarez-Buylla spent five years collecting brain tissue samples from a total of 59 donors. They were either obtained from the deceased or from epileptic patients who underwent brain surgery.
The donors also came from different age groups. Some tissues belonged to fetuses, while others were from seniors between 60 and 77 years of age.
Using fluorescent antibodies, the team labeled specific proteins in brain cells at various stages of maturity. Then with a specialized electron microscope, they searched for the presence of young or immature neurons in each sample.
Neurogenesis In Hippocampus Slows Down After Birth
Based on their observations, scientists discovered that human brains are abundant with progenitor and neural stem cells in early childhood. At birth, an individual has around 1,618 immature neurons in every millimeter of brain tissue.
Surprisingly, these cells did not go on to multiply and develop into a layer made of NSCs. From ages one to seven, neurogenesis dropped significantly by 23-fold. Production then stopped completely upon reaching adulthood.
Among all the samples studied, the oldest brain tissue that still contained a few immature brain cells belonged to a 13-year-old donor. In contrast, none were found in an 18-year-old's.
Gerd Kempermann, co-author of the study and neuroscientist at Germany's Technical University of Dresden, cautions that their findings are not so conclusive. It is entirely possible that samples from older donors have new neurons which were undetected because of the fluorescent antibodies' limitations.
Alvarez-Buylla's team only began tagging the samples obtained from deceased donors within 48 hours since the time of death. How soon the brain tissues were processed affected not only the quality of the tissue but also the marker's accuracy. Despite this, the study's lead author stands strongly by the results.
"We have done our homework and studied many samples of different ages," says Alvarez-Buylla in a report.
The team's findings have been published March 7 in the International Journal of Science: Nature.
Adult Neurogenesis Proposed In The 1960s, Not A New Theory
Joseph Altman, a biologist, proposed the theory of adult neurogenesis in the 1960s. It states that the human adult brain manufactures new brain cells in the dentate gyrus, a subregion of the hippocampus that is critical for memory.
Unfortunately, Altman was not able to conduct further investigation to prove his theory because of all the disbelief it received. The scientific community only accepted his theory in the 1990s when technology allowed the artificial simulation of neurons.