For most of the twentieth century, neuroscience taught that you were born with all the neurons you would ever have, and that neuronal loss was permanent. That view was wrong. The hippocampus continues to make new neurons throughout adult life — a process called adult neurogenesis, occurring in the dentate gyrus, a sub-region of the hippocampus.
Neural progenitor cells in the dentate gyrus divide, differentiate into granule neurons, migrate a short distance into the granule cell layer, extend axons and dendrites, and integrate into existing hippocampal circuits. The rate varies — younger people, more enriched environments, and aerobic exercise increase it; chronic stress, aging, and depression suppress it.
The functional significance of adult neurogenesis is still being worked out, but several findings have converged. New neurons appear to support pattern separation — the ability to distinguish similar but distinct experiences, which is essential for episodic memory and for accurate contextual learning. Animals with suppressed hippocampal neurogenesis show difficulty distinguishing very similar contexts, which has implications for both memory function and anxiety (where overgeneralization of fear from one context to similar contexts drives symptoms).
Several classes of antidepressants enhance hippocampal neurogenesis on a timeline that matches their clinical effects. SSRIs increase neurogenesis over four to six weeks of treatment, a timeline that maps onto the patient's subjective improvement. Ketamine increases neurogenesis within days, again matching its rapid clinical effect. Tricyclic antidepressants and MAOIs also increase neurogenesis over weeks. Electroconvulsive therapy strongly increases neurogenesis. Aerobic exercise increases it, as does environmental enrichment.
Animal experiments have produced an interesting causal claim: if hippocampal neurogenesis is blocked (by radiation or genetic methods), the behavioral antidepressant effects of SSRIs are reduced or abolished. This does not prove the mechanism in humans, but it makes the case that neurogenesis is part of the therapeutic effect, not a side observation.
Clinically, this gives a tangible biological substrate for why antidepressants take weeks to work, why exercise helps depression, why environmental enrichment in inpatient settings matters, and why ECT remains one of the most effective treatments for severe depression. They all converge on the same hippocampal regrowth.
Hold the picture. The dentate gyrus is one of the most neurogenic regions of the adult brain. Chronic stress turns it off. Effective treatment turns it back on. The brain you are working with is more plastic than the textbooks of a generation ago described.