Executive Summary:
Adult neurogenesis – the birth of new neurons in a mature brain – occurs mainly in the hippocampal dentate gyrus (DG) and subventricular zone (SVZ) of many mammals. In rodents, neurogenesis is wellestablished and contributes to learning, memory, and mood regulation. In humans, evidence has been controversial: earlier studies found abundant new neurons in adult hippocampus, while more recent analyses report very few new neurons in healthy adults. Cutting-edge single-cell sequencing now reveals rare neural stem cells and immature neurons in the adult human DG, suggesting that some level of hippocampal neurogenesis persists. Key regulators include local neural stem cells (type‐1/2 cells), proliferation markers (e.g. Nestin, DCX, NeuN), and signaling factors (BDNF, Notch, miRNAs) that govern each developmental stage. Adult neurogenesis is influenced by exercise, environment, stress, hormones and disease. It has practical implications for cognitive aging, mood disorders, and neurodegeneration: for example, exercise and learning stimulate neurogenesis, while stress and depression suppress it. Major controversies remain about the extent and function of human adult neurogenesis. Some researchers argue it is “extremely rare”, whereas others report robust generation of new neurons that decline in Alzheimer’s disease. The field is rapidly evolving with new molecular methods. This blog reviews mechanisms, evidence, and open questions about adult neurogenesis, with implications for brain health.
Introduction
Adult neurogenesis was first demonstrated in rodents in the 1960s and later confirmed in the adult human hippocampus. In mammals, two main niches harbor neural stem cells (NSCs): the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone lining the lateral ventricles. In the DG, dormant NSCs (type1 cells) intermittently divide to produce transit-amplifying progenitors (type2/3), which differentiate into immature neurons that migrate into the granule cell layer. These new granule neurons then mature and integrate into hippocampal circuits involved in memory.
Figure: Schematic of the hippocampal dentate gyrus. Neural stem cells in the subgranular zone (SGZ, green) proliferate and differentiate into new granule neurons in the granule cell layer. Adult hippocampal neurogenesis (AHN) is the process of generating these new granule neurons in the adult brain. The hippocampus (part of the limbic system) is critical for memory formation, spatial navigation and mood regulation. In rodents, AHN contributes to pattern separation and cognitive flexibility, as well as modulating stress responses.
Adult neurogenesis beyond the hippocampus has been reported in some species (e.g. rodents show additional neurogenesis in the olfactory bulb and possibly striatum), but these are generally much weaker or absent in adult humans. For example, one study showed ongoing neurogenesis in the adult human striatum (via ^14C dating), but such findings are still debated. In humans, the consensus is that the hippocampus and SVZ are primary neurogenic zones. Recent single-cell analyses of human post-mortem hippocampus have indeed identified rare NSCs, neuroblasts and immature neurons in adults, though at much lower levels than in young rodents. The functional relevance of these new cells in humans remains an open question.
Mechanisms of Adult Neurogenesis
Neurogenic Niches and Lineage
The adult hippocampal DG contains the main neurogenic niche. Here, radial glia-like NSCs (type-1) in the SGZ give rise to proliferative progenitors (type-2) which differentiate into immature neurons (type-3) that eventually become mature granule cells. This lineage progression is regulated by transcription factors and signaling pathways. Marker proteins characterize each stage: for example, type-1 NSCs express Sox2 and GFAP; type-2 progenitors express DCX (doublecortin) and Nestin; immature neurons express DCX and Prox1; and mature neurons express NeuN and Calbindin.
Figure: Molecular regulators of adult hippocampal neurogenesis. Distinct cell types (NSC, progenitor, immature neuron) are defined by characteristic markers (e.g. GFAP, DCX, NeuN) and influenced by growth factors (BDNF, FGF) and miRNAs. These regulatory molecules control cell proliferation, differentiation, migration and survival. For instance, BDNF/TrkB signaling promotes survival of newborn neurons, while Notch signaling maintains stemness of NSCs. MicroRNAs (e.g. miR-124) also modulate neuronal differentiation. Disrupting these pathways in rodents alters neurogenesis and associated behaviors, underscoring their importance.
The SVZ niche (adjacent to lateral ventricles) also harbors NSCs in many mammals. In rodents, SVZ progenitors migrate along the rostral migratory stream to the olfactory bulb, replacing interneurons. However, in adult humans this pathway is less active – most SVZ NSCs differentiate into astrocytes with age, and few new neurons are produced. Thus, adult neurogenesis is largely confined to hippocampus in humans.
Cellular and Epigenetic Regulation
Neural stem cell activation is influenced by cell-intrinsic mechanisms (transcription factors like SOX2, epigenetic modifications) and cell-extrinsic factors. The local microenvironment (“niche”) includes endothelial cells, astrocytes, and extracellular matrix factors that provide cues. For example, increased neuronal activity or learning tasks enhance neurogenesis via release of neurotrophins. Conversely, stress hormones (e.g. corticosteroids) suppress NSC proliferation. Exercise robustly stimulates hippocampal neurogenesis: running increases levels of BDNF and IGF-1 in the brain, leading to higher progenitor proliferation. Indeed, enriched environments or voluntary wheel running can double the number of new neurons in the rodent dentate gyrus (one study found up to 100% increase with exercise). Such environmental effects highlight the plasticity of adult neurogenesis.
Species Differences
Much of our detailed mechanistic knowledge comes from rodent studies. In young adult mice or rats, thousands of new hippocampal neurons are generated daily. In humans, estimates vary widely. Some studies (using nuclear ^14C dating or advanced imaging) suggest around ~700 new hippocampal neurons are added daily in young adults, declining steeply with age. Others argue that classic markers like DCX/PSA-NCAM may label non-neuronal cells in humans, making counts uncertain. What is clear is that human hippocampal neurogenesis does decline dramatically with age and is severely reduced in Alzheimer’s disease. The existence of even a small pool of new neurons in humans remains an active area of research.
Evidence and Findings
Animal Studies
In rodents, the reality of adult neurogenesis is well-established. Landmark studies showed that experimental manipulations modulate neurogenesis: for example, hippocampal lesions, stress, or disease models reduce it, while antidepressants and learning tasks enhance it. Ablating neurogenesis (e.g. via irradiation) impairs certain hippocampus-dependent tasks, suggesting functional roles. Table 1 (below) summarizes key studies in humans and rodents for comparison.
Study / Context
Subjects
Method
Main Finding
Sorrells et al. (2018)
Adult humans
Immunostaining (DCX, PSA-NCAM)
Very few if any new neurons in adult hippocampus
Moreno-Jimenez et al. (2019)
Adult humans
Immunohistochemistry (DCX, NeuN)
Many new neurons in healthy adult DG; reduced in Alzheimer’s
Lazarov et al. (2026)
Adult humans
Single-cell multi-omics (snRNA-seq + ATAC)
Identified NSCs, neuroblasts and immature granule cells in adult DG
Van Praag et al. (2005)
Adult mice
Running wheel + BrdU labeling (rodent)
Exercise increases hippocampal neurogenesis and learning
In rodents, visualizing new neurons often uses thymidine analogs (BrdU) or cell-cycle markers (Ki67) plus neuronal markers. For example, Van Praag et al. (2005) showed that wheel running for several weeks nearly doubles new neuron survival in aged mice. Other studies labeled doublecortin (DCX) to mark immature neurons; for instance, DCX+ cells are abundant in the adult rodent DG. Figure panels and publications from these studies illustrate the distinct cell types involved.
Figure: Adult mouse dentate gyrus stained for neurogenic markers. Green = neuronal progenitor lineage; red = doublecortin (DCX, labels immature neurons); white = GFAP (astrocytes/NSCs). In rodents, DCX reliably marks young neurons; its presence demonstrates ongoing neurogenesis. Such images confirm robust neurogenesis in adult mammalian hippocampi.
Human Studies and Controversies
Human adult neurogenesis has been much debated. Early landmark work by Eriksson et al. (1998) used ^14C dating to show new neurons in adult human hippocampus. However, later studies have conflicted. Sorrells et al. (2018) used immunostaining on post-mortem hippocampi and concluded that young neurons essentially vanish by early adulthood – “neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans”. In contrast, Moreno-Jiménez et al. (2019) reported abundant DCX+ immature neurons in healthy adults (even up to age 80), with a sharp decline in Alzheimer’s patients.
The latest evidence comes from single-cell/multi-omic profiling. Lazarov et al. (2026) analyzed 355,000 nuclei from adult human DG (normal, aged, and Alzheimer’s) and identified clear transcriptional profiles of NSCs, neuroblasts, and immature granule neurons. This confirms that, although sparse, neurogenic cells do exist in adult human hippocampus. They also showed the number of these cells drops with aging and disease, consistent with prior findings.
A further controversy concerns markers: many human studies use DCX or PSA-NCAM to label new neurons. However, some have pointed out that DCX can stain cells that appear morphologically mature in adult hippocampus. For example, a 2010 study noted that DCX+ cells in adult human DG often lack juvenile features, casting doubt on their “newborn” identity. Thus, some DCX+ cells in humans might be atypical interneurons or glia. Better markers and standardized protocols are needed to resolve these discrepancies.
Figure: Simplified diagram of hippocampal circuitry (Cornu Ammonis regions, Schaffer collaterals) showing typical location of newborn dentate granule neurons (left). The entire hippocampal loop (right) is critical for memory; adult-born neurons (green arrow) integrate into CA3 and CA1 regions. (Adapted from schematics of hippocampal pathways.) Human hippocampal neurogenesis, though limited, could still have functional impact on this circuit.
Functional Roles and Implications
In rodents, new neurons have been linked to specific functions. They are thought to enhance pattern separation (distinguishing similar memories) and to aid in mood resilience. For example, blocking neurogenesis can impair performance in certain spatial memory tasks and diminish responses to antidepressants. The most well-known finding is Santarelli et al. (2003): chronic SSRI antidepressants increased hippocampal neurogenesis in mice, and if neurogenesis was experimentally ablated, the behavioral (antidepressant-like) effects were abolished. This suggests a mechanistic link between adult neurogenesis and antidepressant efficacy.
Physical exercise is another modulator: it not only boosts neurogenesis but also improves cognition and mood. In humans, several studies have shown that aerobic exercise can increase hippocampal volume on MRI in older adults, possibly reflecting enhanced neurogenesis. Exercise elevates levels of BDNF and IGF-1, key neurogenic factors. Conversely, chronic stress and high cortisol lower neurogenesis, which may underlie stress-induced memory deficits.
Figure: Conceptual diagram of adult neurogenesis functions. New neurons in the hippocampus (dorsal region) are implicated in cognitive processes like memory, while those in the ventral hippocampus affect mood and anxiety. Neurogenesis in the SVZ (olfactory bulb) influences smell and reproductive behaviors. Neural progenitors also modulate hypothalamic control of metabolism. Clinically, enhancing adult neurogenesis is of great interest. Potential applications include recovery from brain injury, slowing cognitive aging, and treating depression. For instance, stem-cell therapies or drugs targeting NSCs are being investigated to promote endogenous regeneration. Likewise, lifestyle interventions (cognitive training, enriched environment) that boost neurogenesis could improve brain health. However, translation from rodents to humans is complex: human brains differ in scale and physiology, and it is not yet clear how much boosting neurogenesis would affect human cognition.
Controversies and Open Questions
Despite progress, major questions remain. The extent of human adult neurogenesis is unsettled: while multiomic data confirms the presence of neurogenic cells, their absolute number and functional significance are unclear. Why do rodents show robust AHN but primates much less? Some have hypothesized an evolutionary trade-off: humans may rely more on synaptic plasticity or glial support than on generating new neurons.
The function of any new human neurons is also unknown. Do they really contribute to memory or mood as in rodents, or are they vestigial? Non-human primate studies suggest limited neurogenesis confined to early life. It’s possible human AHN is too sparse to impact cognition significantly. On the other hand, new single-cell tools are revealing that these cells may interact with larger cell networks in subtle ways that we have yet to understand.
Methodological issues are another open question. Post-mortem human tissue can suffer from variable fixation times, affecting immunostaining. Some argue that improved tissue preservation (e.g. very short post-mortem interval) may reveal more neurogenesis. New imaging techniques (MRI with specific tracers) are being developed to detect neurogenesis in living humans.
Finally, the roles of neurogenesis outside the hippocampus (e.g. striatum, hypothalamus) are emerging areas. For example, a recent study (Lazarov et al., 2026) hints at neurogenic changes in hypothalamic areas controlling metabolism. Whether such findings will hold up and what they imply for diseases like obesity or depression is an active field.
1960s: Altman & Das (rats)
1998: Eriksson (human)
2001: Van Praag – exercise ↑ neurogenesis (rodents)
2018: Sorrells – no adult human neurogenesis
2019: Moreno-Jimenez – abundant AHN in adults
2026: Lazarov – human NSCs via single-cell
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Figure: Timeline of key discoveries in adult neurogenesis. Early rodent studies (Altman, 1960s) paved the way for human evidence (Eriksson 1998). The field surged with findings that exercise boosts neurogenesis (Van Praag 2001), but later human studies conflicted (Sorrells 2018 vs. Moreno-Jimenez 2019). The latest single-cell evidence from Lazarov et al. (2026) further refines our understanding of human neurogenic potential.
Conclusion
Adult neurogenesis remains a fascinating and evolving topic. In rodents, it is a clear and functionally important phenomenon; in humans, it is present but much scarcer, leading to intense debate. The weight of recent evidence suggests that adult-born neurons do exist in the human hippocampus, albeit at low levels that decline with age and disease. These new neurons likely contribute to hippocampal plasticity, but their precise roles in human cognition and pathology are still speculative. If harnessed therapeutically, modulating neurogenesis could offer novel approaches for brain repair, cognitive enhancement, and mental health.
Ongoing research will resolve current controversies by using more sensitive detection methods (e.g. single-cell genomics, in vivo imaging) and by linking neurogenic changes to behavioral outcomes. Key open questions include understanding inter-individual variability (why do some healthy elderly maintain more neurogenesis?), the interplay with gliogenesis, and how systemic factors (inflammation, metabolism) influence NSCs. As we chart the “maps of regeneration” in the adult brain, each new neuron discovered may open a path to understanding the resilient and adaptable nature of the human mind.
Practical Implications: Insights into adult neurogenesis highlight lifestyle interventions (exercise, enriched learning) that may boost brain plasticity. They also guide stem-cell and drug research for neurodegenerative diseases. For example, clinical trials are exploring whether hippocampal neurogenesis can be increased in mild cognitive impairment. In psychiatry, neurogenesis provides a framework for understanding how antidepressants or electroconvulsive therapy exert long-term effects on the brain.
Controversies: The chief controversies center on measurement: different labs report vastly different levels of human AHN due to methodological differences. There is also debate over the use of rodent models to infer human neurogenesis. Finally, while many functional roles have been proposed (memory encoding, anxiety regulation), these are mostly based on rodent knock-out studies and may not translate directly to humans.
Assumptions: This review assumes that hippocampal neurogenesis is the most relevant form of adult neurogenesis for humans. We also assume that rodent mechanisms (e.g. marker usage, pathways) are broadly similar to humans, recognizing potential species differences. Our discussion emphasizes recent findings (up to 2026) including state-of-the-art single-cell data.
Suggested image credits: All embedded figures are from open-access sources as cited. (Sources include Nature and Wikimedia Commons entries under Creative Commons licenses.)
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