Human Adult Neurogenesis: Evidence and Remaining Questions

Abstract

Renewed discussion about whether or not adult neurogenesis exists in the human hippocampus, and the nature and strength of the supporting evidence, has been reignited by two prominently published reports with opposite conclusions. Here, we summarize the state of the field and argue that there is currently no reason to abandon the idea that adult-generated neurons make important functional contributions to neural plasticity and cognition across the human lifespan.

Figure 1. Multiple lines of evidence in support of adult hippocampal neurogenesis in humans

Data from rodents suggest a particular and specific function for adult-generated neurons of the dentate gyrus, which would be of great relevance to human cognition in health and disease (green box). Three birthdating studies confirm the idea that adult hippocampal neurogenesis exists in humans (dark green box, top), and a much larger set of studies based on ex vivo analyses of precursor cells and marker expression provide supportive evidence (light green box, bottom). Sorrells et al. (2018) have questioned the validity of marker studies (red X), but there is little general support for that claim. The other lines of evidence are untouched by their argumentation.

Figure 2. Consequences of species differences in the course of neurogenesis

Besides methodological considerations, a hypothetical concept of a temporal decoupling of the stages of adult neurogenesis and species differences in marker expression, although largely speculative at this time, might explain part of the discrepancies between rodent and human data. The point is that alternative hypotheses are possible that are consistent with the available data.