A Lack of New Neurons in the Adult Hippocampus

What's the science?

Throughout fetal development and into childhood, many new neurons are being made in the brain -- this process plays an important role in learning and memory. In adults, things slow down, but it has been generally accepted that new neurons are still being made in a particular brain region called the hippocampus. Understanding whether or not new neurons are made in adults is critical because neurogenesis represents an important target for neurological diseases. This week in Nature, Sorrells and colleagues assessed neurogenesis across the lifespan in humans. 

How did they do it?

They assessed the young neurons and progenitor cells (cells that would eventually become neurons) in the hippocampus of 59 individuals. These individuals ranged in age between 14 weeks gestation and 77 years. Brain tissue was examined either a) after death or b) following resection due to epilepsy. Using light and electron microscopy, and staining, they imaged and counted rapidly dividing cells (Ki-67+ progenitor cells), and young neurons (DCX+PSA-NCAM+ neurons).

What did they find?

Many progenitor cells and young neurons were found during fetal development stages, but few were found after 7 years of age. There were no rapidly dividing Ki-67+ cells located in the subgranular zone of the hippocampus (where progenitor cells usually are) in brains older than 22 weeks gestational age. In surgical resections from epileptic patients, young neurons were found at 10 months gestational age (in the dentate gyrus region of the hippocampus), but few were found by 7 years and none were found in adults.

What's the impact?

This study suggests that the generation of new neurons in the hippocampus of adults may be rare or non-existent in humans, contrary to commonly held beliefs. Researchers are now examining other species with little neurogenesis during the adult years, in order to find clues about why new neurons might not be generated in adults, despite their role in learning and memory.

S.F. Sorrells et al., Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults. Nature (2018). Access the original scientific publication here.