New neurons continuously develop in the healthy human brain up to the ninth decade of life, according to a paper published online this week in Nature Medicine. The study also finds that the formation of new neurons drops considerably in patients with Alzheimer’s disease.

The majority of neurons in the brain are already in place by the time of birth. The formation of new neurons in adult brains (also known as adult neurogenesis) can occur in certain regions, such as the hippocampus. Previous research has established that this phenomenon occurs in rodents and other vertebrate species. Earlier work has also suggested that new neurons are incorporated into human tissue. However, the degree of adult neurogenesis in the human brain has been questioned by recent research.

Maria Llorens-Martin and colleagues analysed tissue samples from 58 human participants. They found that, although there is some degree of decline associated with age, adult neurogenesis in the hippocampus of the human brain can be observed across the lifespan. The authors also found that adult neurogenesis decreases sharply during the course of human Alzheimer’s disease.

The authors note that discrepancies between their results and previous research that did not detect human adult neurogenesis may result from differences in the methodologies used or the quality of tissue specimens examined. This new study evaluates how factors such as tissue fixation techniques or delays in the time between tissue acquisition and processing can affect the quality of histological staining, which is critical for the detection of new neurons.