🧠 neuroscience

The dentate gyrus grows throughout life despite turnover of developmentally-born neurons

This study demonstrates that the rat dentate gyrus undergoes net growth throughout adulthood due to the addition of adult-born neurons, a process that is partially offset by the concurrent turnover and loss of developmentally-born neurons.

Original authors: Ciric, T., Cahill, S. P., Lin, T., Choi, S.-a., Snyder, J. S.

Published 2026-02-18

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CC BY 4.0

Original authors: Ciric, T., Cahill, S. P., Lin, T., Choi, S.-a., Snyder, J. S.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: A Brain That Keeps Growing (But Also Renovates)

Imagine your brain's dentate gyrus (a specific part of the hippocampus responsible for memory) as a bustling, ever-changing city.

For decades, scientists have been arguing about whether this city just stays the same size once you reach adulthood, or if it keeps building new neighborhoods. Some thought it was a static city where no new houses were built after childhood. Others suspected that new houses (neurons) were being built, but they weren't sure if the city was actually getting bigger or if the new houses were just replacing old ones.

This study, conducted on rats, finally settled the debate with a clear answer: The city is growing, but it's also undergoing a massive renovation project.

The Three Key Findings

1. The Construction Crew is Still Working (Adult Neurogenesis)

Think of adult-born neurons as a construction crew that never clocks out. The researchers found that from the time a rat is a young adult (2 months old) until it is quite old (18 months), this crew is constantly building new "houses."

The Math: They estimated that over this time, about 670,000 new neurons were added to the population. That's a huge number—roughly 30% of the entire population of that brain region!

2. The Old Houses Are Being Demolished (Neuronal Turnover)

Here is the twist: While the construction crew is busy building new houses, the city isn't just adding them to the map. It's also tearing down some of the old houses (neurons born when the rat was a baby).

The Math: About 30,000 of these "developmentally-born" neurons died off during the same period.
The Location: The researchers found that the "demolition crew" was mostly targeting the older, established neurons, while the new construction was happening in the same area.

3. The Net Result: A Bigger City

So, if you add 670,000 new houses and lose 30,000 old ones, what happens?

The Result: The total population of the city grew by 385,000 houses.
The Takeaway: Even though the city was losing some old residents, the influx of new ones was so strong that the total population increased by about 20%. The dentate gyrus gets bigger as the animal ages.

Why Did Scientists Miss This Before?

You might wonder, "If the city is growing, why didn't we know this sooner?" The authors explain two main reasons using a "Blind Spot" analogy:

Looking at the Wrong Time: Many previous studies started looking at the city when the rats were already middle-aged (5-6 months old). By then, the construction crew had slowed down. It's like trying to measure a skyscraper's growth by only visiting the construction site after the top 10 floors are already finished. You miss the most active building phase.
The "Net Zero" Illusion: Because the city was both building new houses and demolishing old ones, the total number looked like it was staying the same. It was a "net zero" illusion. The researchers had to count the new bricks and the demolished walls separately to see the true growth.

What Does This Mean for Us?

This isn't just about rats; it changes how we think about memory and mental health.

Memory Storage: Imagine your brain is a hard drive. If the drive keeps getting bigger (growing new neurons), it has more space to store new memories throughout your life.
Making Room for New Memories: The fact that old neurons are dying off suggests the brain might be "clearing cache." Just like a computer deletes old, unused files to make room for new data, the brain might be removing old, less relevant memories to make space for new learning.
Mental Health: Conditions like depression are often linked to a shrinking hippocampus. This study suggests that the problem might not just be a lack of new growth, but an imbalance where too many old neurons are dying off, or the new ones aren't arriving fast enough to replace them.

The Bottom Line

Your brain's memory center isn't a static museum; it's a living, breathing city. It is constantly building new neighborhoods while gently renovating the old ones. This constant cycle of growth and turnover is likely how we keep our minds flexible, capable of learning new things, and able to store a lifetime of memories.

1. Problem Statement

The magnitude and functional significance of adult hippocampal neurogenesis have been debated since its discovery. While theoretical models suggest that cumulative adult neurogenesis could add 40–50% of the dentate gyrus (DG) granule cell population over a lifespan, empirical studies often fail to detect an increase in total neuron numbers during adulthood.

The Discrepancy: Previous studies suggesting no net growth may suffer from low statistical power, small sample sizes, or the examination of older age windows where neurogenesis has naturally declined.
The Hypothesis: The authors propose that net growth is masked by the simultaneous turnover of developmentally-born neurons (DBNs). In normal aging, mature DBNs die off, potentially canceling out the addition of new adult-born neurons (ABNs) in total cell counts.
Objective: To quantify the dynamics of DG neuron addition (ABNs), loss (DBNs), and total population changes from young adulthood (2 months) to old age (18 months) in rats to determine if the DG undergoes net growth.

2. Methodology

The study utilized a longitudinal cross-sectional design with a large cohort of male Long-Evans rats.

Animal Model & Labeling:
- Rats were injected with CldU (50 mg/kg, IP) on Postnatal Day 6 (P6) to permanently label neurons born during the peak of DG development (DBNs).
- Rats were sacrificed at 2, 4, 6, 12, and 18 months of age.
- Note: An IdU injection was planned 4 weeks prior to sacrifice to label adult-born cells, but immunostaining failed; therefore, adult neurogenesis was quantified using the proliferation marker Ki67 instead.
Tissue Processing & Staining:
- Ki67 Immunohistochemistry: Used to quantify proliferating cells (proxy for adult neurogenesis) in the subgranular zone (SGZ) and deep granule cell layer (GCL).
- CldU Immunohistochemistry: Used to quantify surviving developmentally-born neurons (DBNs) throughout the GCL.
- Cresyl Violet Staining: Used to quantify the total number of granule neurons using unbiased stereological methods.
- Caspase-3 Staining: Used to identify apoptotic (dying) cells. Cells were co-stained with DAPI to confirm pyknotic morphology.
Quantification:
- Stereology: The Optical Fractionator method was used for Cresyl violet and CldU counts to estimate total cell numbers ( $N = \sum Q^- \times \frac{1}{ssf} \times \frac{1}{asf} \times \frac{1}{tsf}$ ).
- Ki67 Counts: Manual counting of proliferating cells in a 1-in-12 series of sections.
- Apoptosis Analysis: Caspase-3+ cells were mapped within the GCL (normalized 0–100 scale, where 0 = SGZ/hilus and 100 = molecular layer) to distinguish between the death of immature ABNs (deep) and mature DBNs (superficial).
Statistical Analysis:
- Ki67 data was fitted with a cubic polynomial to estimate cumulative addition.
- CldU and Cresyl violet data were fitted with linear regression to estimate rates of loss and net growth, respectively.

3. Key Results

A. Adult Neurogenesis (Addition)

Ki67+ Cell Decline: Proliferating cells declined exponentially with age, plateauing by 12 months.
Cumulative Addition: Integrating the cubic fit of Ki67 data from 2 to 18 months, the authors estimated that 670,000 new neurons are added to the DG during this period. This represents approximately 30% of the total DG population.

B. Developmentally-Born Neuron Loss (Turnover)

DBN Decline: CldU+ cells (born at P6) showed a significant linear decline with age.
Rate of Loss: Approximately 1,900 DBNs are lost per month.
Total Loss: Over the 2–18 month interval, roughly 30,000 DBNs were lost, representing a 19–20% loss of the developmentally-born cohort.
Location of Death: Caspase-3 analysis revealed that while many dying cells were immature (deep GCL/SGZ), a significant portion of dying cells in 2- and 12-month-old rats were located in the superficial GCL, consistent with the death of mature, developmentally-born neurons.

C. Net Population Growth

Total Neuron Count: Cresyl violet staining revealed a significant linear increase in total granule cell numbers with age.
Net Growth: The DG gained approximately 385,000 cells (a 20% increase) between 2 and 18 months.
The Discrepancy Explained: The net growth (385k) is significantly lower than the total addition (670k). The difference (~285k) is attributed to the loss of DBNs and potentially other turnover mechanisms, confirming that neuronal loss masks the full magnitude of neurogenic growth.

4. Key Contributions

Resolution of the "No Growth" Paradox: The study provides direct evidence that the DG does grow throughout adulthood in rats, correcting previous literature that suggested static cell numbers.
Quantification of Turnover: It establishes that adult neurogenesis is accompanied by the active turnover of mature, developmentally-born neurons, a factor previously underappreciated in mammalian models.
Methodological Rigor: By combining cumulative proliferation markers (Ki67), birthdating (CldU), and total cell counts (Cresyl violet) in a single large cohort, the authors disentangled addition from loss, overcoming the limitations of previous "snapshot" studies.
Refined Estimates: The study estimates a daily addition rate of ~800 cells (aligning with historical Bayer data) but clarifies that this growth is partially offset by the loss of ~1,900 DBNs per month.

5. Significance and Implications

Memory Dynamics: The findings support theories of hippocampal memory storage where net growth increases storage capacity for new episodic memories, while the selective turnover of DBNs may facilitate the clearing of outdated or irrelevant memory traces (similar to mechanisms observed in avian song systems).
Psychiatric Disorders: The interplay between neurogenesis and neuronal turnover may be critical in understanding hippocampal volume changes in conditions like depression and schizophrenia, where atrophy is observed. The study suggests that volume changes may not solely reflect cell loss but could involve disrupted turnover dynamics.
Translational Relevance: While human data is limited, the authors suggest that if humans exhibit similar turnover rates, net growth might be harder to detect or occur only in early adulthood. Understanding these dynamics is essential for interpreting hippocampal plasticity in response to therapies (e.g., antidepressants, electroconvulsive therapy).

Conclusion: The dentate gyrus is a dynamic structure that undergoes substantial net growth in adulthood. This growth is the result of high rates of adult neurogenesis that are partially, but not fully, counterbalanced by the physiological death of developmentally-born neurons.