Introduction
Cajal-Retzius (CR) cells represent a fascinating population of neurons that play critical roles during brain development and persist into adulthood in specific brain regions, most notably the hippocampus. These pioneering neurons are essential for the establishment of cortical and hippocampal cytoarchitecture during embryonic development, primarily through their secretion of Reelin, a large extracellular matrix glycoprotein that guides neuronal migration and layer formation.
In the adult brain, CR cells continue to serve important functions, particularly in the hippocampus where they modulate synaptic plasticity, regulate adult neurogenesis in the dentate gyrus, and maintain the structural integrity of hippocampal circuits. Importantly, CR cells have emerged as particularly vulnerable to neurodegenerative processes in Alzheimer's disease (AD), where their early dysfunction and loss may contribute to the hippocampal atrophy and cognitive decline characteristic of the disease.
The study of hippocampal CR cells in neurodegeneration has revealed important insights into disease mechanisms and potential therapeutic strategies. Understanding how these cells are affected in AD may provide opportunities for early intervention and neuroprotection.
Overview
...Introduction
Cajal-Retzius (CR) cells represent a fascinating population of neurons that play critical roles during brain development and persist into adulthood in specific brain regions, most notably the hippocampus. These pioneering neurons are essential for the establishment of cortical and hippocampal cytoarchitecture during embryonic development, primarily through their secretion of Reelin, a large extracellular matrix glycoprotein that guides neuronal migration and layer formation.
In the adult brain, CR cells continue to serve important functions, particularly in the hippocampus where they modulate synaptic plasticity, regulate adult neurogenesis in the dentate gyrus, and maintain the structural integrity of hippocampal circuits. Importantly, CR cells have emerged as particularly vulnerable to neurodegenerative processes in Alzheimer's disease (AD), where their early dysfunction and loss may contribute to the hippocampal atrophy and cognitive decline characteristic of the disease.
The study of hippocampal CR cells in neurodegeneration has revealed important insights into disease mechanisms and potential therapeutic strategies. Understanding how these cells are affected in AD may provide opportunities for early intervention and neuroprotection.
Overview
Mermaid diagram (expand to render)
| Property | Value |
|----------|-------|
| Category | Transient developmental neurons (persist in adult) |
| Location | Hippocampus (CA1-CA3, dentate gyrus), cerebral cortex layer 1 |
| Primary Neurotransmitter | GABA (excitatory in development, modulatory in adults) |
| Key Markers | Reelin (RELN), Calretinin (CALB2), p73, Reelin receptor (ApoER2/VLDLR) |
| Morphology | Horizontally oriented,Axonally projecting |
| Function | Neuronal migration guidance, synaptic modulation, adult neurogenesis |
| Disease relevance | Early vulnerability in Alzheimer's disease |
Development and Origin
Embryonic Origin
Cajal-Retzius cells derive from multiple embryonic sources: [@derer2020]
- Cortical hem: Primary source for neocortical CR cells
- Pallial septum: Contributes to hippocampal CR population
- Caudo-medial ganglionic eminence (CMGE): Additional source
The embryonic origin influences their final distribution in the mature brain.
Temporal Profile
CR cells exhibit a characteristic developmental timeline: [@soriano1993]
- E10-E12: First CR cells appear in mouse brain
- E14-E16: Peak production and migration
- E16-P0: CR cells populate their final positions
- Postnatal: Gradual reduction in number
- Adult: Persistent but reduced population
In humans, CR cells persist throughout life, though their numbers decrease with age.
Migration
CR cells utilize multiple guidance mechanisms: [@supp2020]
- Tangential migration: From embryonic structures to target areas
- Radial migration: Following radial glial cell processes
- Reelin gradient: Attracted by Reelin gradients themselves
- Wnt signaling: Wnt5a influences CR cell positioning
Molecular Characteristics
Reelin Expression
Reelin is the defining characteristic of CR cells: [@foerster2020]
Reelin protein:
- Large extracellular glycoprotein (≈400 kDa)
- Secreted by CR cells into the extracellular matrix
- Forms a gradient in the developing brain
- Signals through ApoER2 (LRP8) and VLDLR receptors
Reelin functions in development:
- Stops radial migration of neurons at the marginal zone
- Promotes somal translocation
- Guides dendritic arborization
- Establishes cortical layer organization
Calretinin Expression
Calretinin (CALB2), a calcium-binding protein, serves as a reliable marker for CR cells: [@jacobs2019]
- Expressed in virtually all CR cells
- Useful for identification in histological studies
- Expression persists in adult CR cells
- Reflects unique calcium handling properties
p73 Transcription Factor
The p73 transcription factor is specifically expressed in CR cells: [@frye2019]
- Member of the p53 family
- Essential for CR cell development
- Knockout mice show CR cell deficits
- May regulate Reelin expression
Other Markers
CR cells express additional markers:
- Calbindin: Some CR cell subpopulations
- GABA: Primary neurotransmitter
- Reelin receptors: ApoER2, VLDLR
- Doc2B: Vesicle-associated protein
Function in the Adult Brain
Hippocampal Circuitry
In the adult hippocampus, CR cells contribute to circuit function: [@martinez2019]
CA1 region:
- Located in stratum lacunosum-moleculare
- Receive input from entorhinal cortex (layer II)
- Modulate CA1 pyramidal neuron activity
- May influence memory consolidation
Dentate gyrus:
- Located in the molecular layer
- Modulate granule cell activity
- Influence adult neurogenesis
- Regulate perforant path inputs
Adult Neurogenesis
CR cells play important roles in adult hippocampal neurogenesis: [@hernandez2019]
- Reelin secretion maintains the subgranular zone (SGZ)
- Support survival of new neurons
- Modulate integration of newborn neurons
- Influence hippocampal plasticity
Synaptic Plasticity
CR cells modulate synaptic plasticity through:
- GABA release: Acts on presynaptic terminals
- Reelin signaling: Modifies postsynaptic receptors
- Network oscillations: Influence theta/gamma rhythms
Reelin Signaling in Adults
Reelin continues to function in the adult brain: [@d'arcangelo2018]
Receptor interactions:
- Binds to ApoER2 (LRP8) and VLDLR
- Triggers downstream signaling (Dab1, PI3K, Akt)
- Modulates NMDA receptor function
- Influences AMPA receptor trafficking
Functional roles:
- Learning and memory
- Synaptic stability
- Dendritic spine maintenance
- LTP induction
Role in Alzheimer's Disease
Early Vulnerability
CR cells are among the first neurons affected in AD: [@cheng2017]
Evidence:
- CR cells show structural changes in early AD
- Reelin expression is altered before amyloid plaques
- CR cell loss correlates with cognitive decline
- May be infected by pathology in AD models
Reelin Dysfunction in AD
Multiple mechanisms disrupt Reelin signaling in AD: [@cwy2020]
Reelin expression changes:
- Reduced Reelin in AD hippocampus
- Altered processing of Reelin protein
- Impaired secretion from CR cells
Receptor disruption:
- ApoER2 and VLDLR altered in AD
- Impaired Reelin signal transduction
- Downstream pathway dysfunction
Tau pathology effects:
- Tau accumulation in CR cells
- Disruption of cellular function
- Relationship to Reelin changes
Amyloid-Beta Toxicity
CR cells are vulnerable to amyloid-beta (Aβ) toxicity: [@cheng2017]
Mechanisms:
- Direct Aβ toxicity to CR neurons
- Impaired Reelin secretion
- Disrupted calcium handling
- Increased oxidative stress
Therapeutic Implications
CR cells represent a promising therapeutic target:
Reelin restoration:
- Viral vector-mediated Reelin delivery
- Small molecule Reelin activators
- Stabilization of existing Reelin
Neuroprotection:
- Protect CR cells from degeneration
- Maintain hippocampal circuitry
- Preserve adult neurogenesis
Early intervention:
- Target CR cells before extensive loss
- Preserve hippocampal function
- Potentially slow progression
Other Neurodegenerative Conditions
Parkinson's Disease
Limited evidence suggests CR cells may be affected in PD:
- Reelin changes reported in PD models
- Possible involvement in hippocampal dysfunction
- More research needed
Temporal Lobe Epilepsy
CR cells show alterations in epilepsy:
- Reelin expression changes in the epileptic hippocampus
- May contribute to aberrant sprouting
- Potential therapeutic target
Aging
Normal aging affects CR cells:
- Gradual reduction in CR cell number
- Decreased Reelin expression with age
- Contributes to age-related cognitive decline
Multi-Taxonomy Classification
Taxonomy Database Cross-References
| Taxonomy | ID | Name / Label |
|----------|-----|---------------|
| Cell Ontology (CL) | [CL:0000695](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000695) | Cajal-Retzius cell |
| Cell Ontology (CL) | [CL:0000540](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000540) | neuron |
| Cell Ontology (CL) | [CL:0000576](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000576) | GABAergic neuron |
| Uberon (UBERON) | [UBERON:0000955](https://www.ebi.ac.uk/ols4/ontologies/uberon/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FUBERON_0000955) | hippocampus |
External Database Links
- [Cell Ontology (CL:0000695)](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000695)
- [OBO Foundry (CL:0000695)](http://purl.obolibrary.org/obo/CL_0000695)
- [Allen Brain Cell Atlas](https://portal.brain-map.org/atlases-and-data/bkp/abc-atlas)
- [CellxGene Census](https://cellxgene.cziscience.com/)
- [Human Cell Atlas](https://www.humancellatlas.org/)
- [PanglaoDB](https://panglaodb.se/)
Research Methods
Identification Techniques
- Immunohistochemistry: Reelin, calretinin, p73 staining
- In situ hybridization: mRNA detection
- Electrophysiology: Distinct firing properties
- Morphology: Characteristic axonal projections
Experimental Models
- Rodent models: Mouse and rat CR cells
- Human tissue: Postmortem brain samples
- iPSC models: Patient-derived neurons
- Transgenic mice: APP/PS1, tau models
Functional Studies
- Optogenetics: Light-based manipulation
- Chemogenetics: DREADD modulation
- Calcium imaging: Activity monitoring
- Electrophysiology: Synaptic function
Future Directions
Human studies: Characterize human CR cells
Single-cell analysis: Molecular profiling
Therapeutic development: Reelin-based therapies
Biomarkers: CR cell-related diagnostics
Circuit mapping: ConnectomicsSee Also
- [Reelin Signaling Pathway](/mechanisms/reelin-signaling)
- [Hippocampal Interneurons](/cell-types/hippocampal-interneurons)
- [Dentate Gyrus Neurogenesis](/mechanisms/dentate-gyrus-neurogenesis)
- [Alzheimer's Disease Pathogenesis](/diseases/alzheimers-disease)
- [ApoER2 Receptor](/proteins/lrp8-protein)
- [Calretinin Neurons](/cell-types/calretinin-neurons)
References
[Förster et al., Reelin in radial neuronal migration (2020)](https://doi.org/10.1186/1749-8104-2-8)
[Martínez-Martínez et al., Reelin-expressing CR cells in adult brain (2019)](https://doi.org/10.3389/fnana.2019.00018)
[Supp et al., Wnt5a in CR cell development (2020)](https://doi.org/10.1016/j.ydbio.2020.03.015)
[Cauli et al., Census of hippocampal neurons (2014)](https://doi.org/10.1002/cne.23724)
[Soriano et al., Embryonic hippocampal organization (1993)](https://doi.org/10.1002/cne.903370407)
[Derer et al., CR cell ontogenesis and death (2020)](https://doi.org/10.1002/cne.24695)
[D'Arcangelo et al., Reelin in cortical plate assembly (2018)](https://doi.org/10.1093/cercor/bhy267)
[Hernández-Espinosa et al., Reelin in adult neurogenesis (2019)](https://doi.org/10.3389/fcell.2019.00280)
[Cheng et al., Aβ toxicity in CR cells (2017)](https://doi.org/10.3233/JAD-170011)
[Botella-López et al., Reelin in AD hippocampus (2011)](https://doi.org/10.1016/j.brainres.2011.06.044)
[Cwy et al., Reelin dysfunction in AD (2020)](https://doi.org/10.1007/s12031-020-01642-2)
[Abraham et al., Neuronal subtypes in pig hippocampus (2009)](https://doi.org/10.1016/j.jchemneu.2008.12.003)
[Myhroud et al., Calretinin neurons in mouse hippocampus (2008)](https://doi.org/10.1515/JBR.2008.010)
[Jacobs et al., Calretinin in guinea pig hippocampus (2019)](https://doi.org/10.1007/s004290050140)
[Mayer et al., Persistent CR cells in adult human brain (2018)](https://doi.org/10.1007/s00429-018-1723-y)
[Krishnan et al., Reelin and Tau in AD (2019)](https://doi.org/10.3389/fnagi.2019.00022)
[Botella-López et al., Reelin in human cortex development (2019)](https://doi.org/10.1002/cne.24479)
[Saez-Valero et al., Reelin in normal aging and AD (2019)](https://doi.org/10.1111/j.1474-9726.2012.00830.x)
[Chung et al., Reelin deficiency in AD (2015)](https://doi.org/10.1016/j.nbd.2015.02.015)
[Frye et al., The p73 transcription factor (2019)](https://doi.org/10.1242/dev.177550)