The hippocampus is a seahorse-shaped structure located in the medial temporal lobe of the brain, forming a critical part of the limbic system. It plays essential roles in memory consolidation, spatial navigation, and emotional processing. The hippocampus is particularly notable for its involvement in neurodegenerative diseases, especially Alzheimer's Disease, where it is one of the first brain regions to show pathological changes and functional decline.
The hippocampus is a pair of seahorse-shaped structures in the medial temporal lobes, critical for:
The hippocampus receives major input from the entorhinal cortex, which itself receives processed information from virtually all associational cortical areas, making the hippocampus a critical hub for integrating diverse sensory and cognitive information[1].
The hippocampus consists of several distinct subregions, each with unique cellular composition and connectivity:
Dentate Gyrus: Contains the granule cells that give rise to mossy fiber axons. The dentate gyrus is one of two brain regions that maintain adult neurogenesis throughout life. The hilus (polymorphic layer) lies between the granule cell layer and CA3[2].
CA3 (Cornu Ammonis 3): Receives input from dentate gyrus granule cells via mossy fibers. CA3 pyramidal neurons have extensive recurrent collateral connections with other CA3 neurons, forming an auto-associative network critical for memory storage and pattern completion. CA3 also projects to CA1 via Schaffer collateral axons[3].
CA2: A small region between CA3 and CA1 that shows resistance to excitotoxicity and certain neurodegenerative processes. CA2 pyramidal cells project to the lateral septum and receive input from hypothalamic oxytocin-producing neurons.
CA1 (Cornu Ammonis 1): The primary output region of the hippocampus, receiving input from CA3 via Schaffer collaterals and directly from the entorhinal cortex via the perforant path. CA1 pyramidal neurons are exquisitely vulnerable to ischemia, excitotoxicity, and tau pathology in Alzheimer's Disease[4].
Subiculum: The major output structure of the hippocampus, projecting to the entorhinal cortex, hypothalamus, amygdala, and septal nuclei.
The hippocampal formation includes:
The hippocampus contains multiple neuron types:
The hippocampus is essential for multiple forms of memory[5]:
The hippocampus generates characteristic oscillations critical for information processing:
The CA1 region exhibits particular vulnerability to various insults:
The classic hippocampal circuit proceeds:
This trisynaptic circuit provides a sequential processing pathway for cortical information.
The hippocampus is one of the earliest and most severely affected regions in AD[6]:
Hippocampal involvement in PD includes[7]:
The hippocampus is both cause and victim of epileptic activity:
The dentate gyrus maintains neurogenesis throughout adulthood, a unique feature among brain regions[8]. This process:
| Factor | Effect on Neurogenesis |
|---|---|
| Exercise | Increase |
| Environmental enrichment | Increase |
| Learning | Increase |
| Chronic stress | Decrease |
| Aging | Decrease |
| Inflammation | Decrease |
| Seizures | Increase (aberrant) |
The study of Hippocampus has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
BANKSY separated CA3 neurons from SSC neurons and fornix from thalamic oligodendrocytes. Nonspatial and MERINGUE merged these subtypes. BANKSY identified region-specific cell types with subtly distinct transcriptomes. Validated with scRNA-seq using DEGs and guilt-by-association genes.
Model System: Mouse hippocampus VeraFISH data
Statistical Significance: Multiple comparisons across clustering resolutions (16-20 clusters)
Identified 13 astrocyte subpopulations organized into Type 1 (telencephalic) and Type 2 (non-telencephalic), each containing GFAP low (grey-matter) and high (white-matter) populations. Type 1 included cortical populations: WIF1+ grey-matter, TNC+ white-matter, and LMO2+ interlaminar astrocytes. Additional regional heterogeneity within both types. Topic analysis revealed expression gradients (HGF in cortical grey-matter), hippocampus-specific TBX5+ astrocytes, and potential reactive astrocyte markers (NR4A1, CRYAB, CHI3L1, SERPINA3, CCL2).
Model System: Astrocytes from human brain (155,025 cells, 13 clusters)
Statistical Significance: N/A
Emotional Dm (remembered > forgotten for emotional pictures) was greater than neutral Dm in amygdala (basolateral aspect), hippocampus (head), and anterior parahippocampal gyrus (entorhinal cortex), but not in posterior PHG. Memory performance showed retention advantage for emotional pictures (pleasant: 52%, unpleasant: 53%) vs neutral (38%).
Model System: 16 young healthy female adults (mean age = 25 ± 4.6 years)
Statistical Significance: Picture type effect: F(2,15) = 41.21, p < 0.0001; Emotional Dm > Neutral Dm: amygdala F = 5.09* (BLA), hippocampus head F = 6.67* (L), anterior PHG F = 6.63*, entorhinal ctx F = 6.43*
Dolcos et al., (2004)
Double dissociation found: emotional Dm was greater in anterior MTL (hippocampus head, entorhinal cortex) while neutral Dm was greater in posterior MTL (hippocampus tail, parahippocampal cortex).
Model System: 16 young healthy female adults
Statistical Significance: Hippocampus double dissociation: F(1,15) = 5.7, p < 0.05; PHG double dissociation: F(1,15) = 11.25, p < 0.005; entorhinal vs posterior PHG: F(1,15) = 10.9, p < 0.005
Dolcos et al., (2004)
NicheCompass was the only method that could identify four separable cortical layer niches and preserve their spatial adjacency. Showed superior overall score outperforming GraphST (second-ranked). Achieved highest spatial conservation and niche coherence scores. Successfully recovered anatomical subcomponents including Isocortex, Hippocampus, Thalamus.
Model System: Mouse hippocampus tissue
Statistical Significance: Quantitative metrics show NicheCompass ranked first in both spatial conservation and niche coherence categories
Tau pathology appears in hippocampus, parahippocampus, and entorhinal cortex in early dementia stages. Increased tau in inferior temporal lobe associated with worse memory. CSF tau levels correlated with tau imaging in 6 brain regions consistent with Braak staging. Test/retest reproducibility ~4-5%. ~10% year-over-year increase in mean cortical SUVR in high amyloid burden subjects. Increased tau accompanied by lower MMSE performance.
Model System: Human subjects from Harvard Aging Brain Study (75 older subjects)
Statistical Significance: Statistically significant correlations between CSF tau and tau PET in entorhinal/parahippocampal regions, inferior temporal, middle temporal, and superior temporal cortices
Hartmuth C. Kolb, José Ignacio Andrés (2017)
First regions to show atrophy are superior temporal region and hippocampus, followed by amygdala, remaining temporal regions, insular and supramarginal regions, then precentral and postcentral regions and posterior lobe. Topological profile explains 82% of variance vs 51% for single best descriptor (network proximity).
Model System: Human subjects from ADNI (AD/MCI/HC diagnosis)
Statistical Significance: p<0.01 for most network metrics; inverse degree for AD p=0.048
Early involvement of insula, superior and middle temporal lobes, middle frontal region, and putamen. Subsequently amygdala, hippocampus and nucleus accumbens affected, then inferior temporal and more frontal regions, followed by parietal lobe and cingulate. Topological profile explains 88% of variance vs 64% for single best descriptor (cortical proximity).
Model System: Human subjects from Rotterdam Scan Study (community-dwelling aging individuals)
Statistical Significance: p<0.01 for all network metrics
The amygdala shows early NFT accumulation (from stage II), particularly in the inferior-medial domain. There is strong connectivity between amygdala and anterior hippocampus, entorhinal cortex, and locus coeruleus. The amygdala may serve as an alternative pathway for NFT spreading in the MTL. Early amygdala involvement is linked to neuropsychiatric symptoms in AD.
Model System: Human subjects (young individuals for functional connectivity; various Alzheimer cohorts)
Statistical Significance: Not applicable for review paper; referenced studies reported various p-values (e.g., FWE P < 0.05 for functional connectivity)
Amygdala shows high tau-PET signal in early stages. Earliest tau-PET uptake in amygdala ~10 years before AD diagnosis. Amygdala, ERC, and hippocampus show largest annual increase in tau-PET uptake in Aβ-positive cognitively unimpaired individuals.
Model System: Human subjects (cognitively unimpaired, MCI, AD patients)
Statistical Significance: Not specified in detail in this review
Significant amygdala volume loss in converters. Greatest atrophy in inferior-medial domain. Amygdala atrophy is an early event, occurring after ERC but before hippocampus.
Model System: Human subjects (controls, MCI, AD patients)
Statistical Significance: Various significance levels reported in original studies
Model successfully reproduced Braak stages of Alzheimer's disease progression: initial seeding in entorhinal cortex (stages I-II), progression to hippocampus and brain stem, then to limbic system (stages III-IV), and finally to cortical association areas and primary sensory-motor regions (stages V-VI)
Model System: 2D finite element model from T2-weighted MRI of 32-year-old male brain (sagittal slice)
Statistical Significance: N/A - computational model
Significant atrophy first occurs in hippocampus, precuneus, and inferior parietal cortex, followed by temporal neocortical regions. Other parietal and frontal areas become involved subsequently, while primary cortices are only affected in late stages. Clinical events (MCI and AD diagnosis) occur after significant atrophy is detectable in hippocampus.
Model System: Familial Alzheimer's disease cohort: 9 mutation carriers (presymptomatic, MCI, AD), 25 age-matched controls
Statistical Significance: Posterior distributions on event ordering computed via MCMC
DSP-4-treated mice display five-fold increase in Aβ plaque burden and increased average plaque size; LC-lesioned animals exhibit increased levels of APP C-terminal cleavage fragments; decreased expression and activity of Aβ-degrading enzyme neprilysin; impaired long-term synaptic plasticity in hippocampus leading to spatial memory deficits
Model System: APP23, APP V717F, APP/PS1 mice with DSP-4 LC lesioning
Statistical Significance: Not explicitly reported
Increased NE levels in brain; improved cognitive functions; reduced neuronal cell death; decreased astrocyte activation; increased BDNF and NGF levels in cortex and hippocampus; upregulated neprilysin expression leading to increased Aβ degradation and reduced plaque burden
Model System: 5xFAD transgenic mice
Statistical Significance: Not reported
Increased NE turnover; upregulated tyrosine hydroxylase protein expression and activity; increased LC cell count and volume; induced neuronal maturation in LC; restored BDNF levels in hippocampus; reduced Aβ deposition throughout brain
Model System: 5xFAD transgenic mice
Statistical Significance: Not reported
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Kempermann G, et al. Human adult neurogenesis: Evidence and remaining questions. Cell Stem Cell. 2018;23(1):25-30. DOI:10.1016/j.stem.2018.04.004
Rolls ET, Kesner RP. A computational theory of hippocampal function, and tests of the theory. Prog Neurobiol. 2006;79(1):1-48. DOI:10.1016/j.pneurobio.2006.04.005
Palop JJ, Mucke L. Amyloid-beta-induced neuronal dysfunction in Alzheimer's disease: from synapses toward neural networks. Nat Neurosci. 2010;13(7):812-818. DOI:10.1038/nn.2583
Eichenbaum H. Hippocampus: cognitive processes and neural representations that underlie declarative memory. Neuron. 2004;44(1):109-120. DOI:10.1016/j.neuron.2004.08.028
Jack CR Jr, et al. Hypothetical model of dynamic biomarkers of the Alzheimer's pathological cascade. Lancet Neurol. 2010;9(1):119-128. DOI:10.1016/S1474-4422(0970299-6
Goldman JG, et al. Hippocampal volume loss in Parkinson's disease with mild cognitive impairment. Mov Disord. 2020;35(3):488-495. DOI:10.1002/mds.27938
Sorrells SF, et al. Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults. Nature. 2018;555(7696):377-381. DOI:10.1038/nature25975