Cortical Pyramidal Neurons (Layers 2/3)

Cortical Pyramidal Neurons (Layers 2/3)

<table class="infobox infobox-celltype">
<tr>
<th class="infobox-header" colspan="2">Cortical pyramidal neurons (layers 2/3 </th>
</tr>
<tr>
<td class="label">Allen Atlas ID</td>
<td><a href="https://brain-map.org/atlases-and-data/rnaseq" target="_blank">CS202210140_3050</a></td>
</tr>
<tr>
<td class="label">Lineage</td>
<td>Neuron > Glutamatergic > Cortical > Superficial Layer</td>
</tr>
<tr>
<td class="label">Markers</td> [@external]
<td>CUX1, CUX2, SATB2, RORB (low), LAMP5</td>
</tr>
<tr>
<td class="label">Brain Regions</td>
<td>[Neocortex (layers 2/3)](/brain-regions/cortex ), [Entorhinal Cortex](/brain-regions/entorhinal-cortex)</td>
</tr>
<tr>
<td class="label">Vulnerable In</td> [@allen]
<td>[Alzheimer's Disease](/diseases/alzheimers-disease), [Frontotemporal Dementia](/diseases/ftd)</td> [@cux]
</tr> [@allena]
<tr>
<td class="label">Cell Ontology ID</td>
<td>[CL:CL:0000598](https://purl.obolibrary.org/obo/CL_0000598), [CL:CL:1001571](https://purl.obolibrary.org/obo/CL_1001571), [CL:CL:4023041](https://purl.obolibrary.org/obo/CL_4023041)</td>
</tr>
<tr>
<td class="label">Cell Ontology ID</td>
<td>[CL:0000598](https://purl.obolibrary.org/obo/CL_0000598), [CL:1001571](https://purl.obolibrary.org/obo/CL_1001571), [CL:4023041](https://purl.obolibrary.org/obo/CL_4023041)</td>
</tr>
</table>

Cortical Pyramidal Neurons (Layers 2/3)

Introduction

...

Cortical Pyramidal Neurons (Layers 2/3)

<table class="infobox infobox-celltype">
<tr>
<th class="infobox-header" colspan="2">Cortical pyramidal neurons (layers 2/3 </th>
</tr>
<tr>
<td class="label">Allen Atlas ID</td>
<td><a href="https://brain-map.org/atlases-and-data/rnaseq" target="_blank">CS202210140_3050</a></td>
</tr>
<tr>
<td class="label">Lineage</td>
<td>Neuron > Glutamatergic > Cortical > Superficial Layer</td>
</tr>
<tr>
<td class="label">Markers</td> [@external]
<td>CUX1, CUX2, SATB2, RORB (low), LAMP5</td>
</tr>
<tr>
<td class="label">Brain Regions</td>
<td>[Neocortex (layers 2/3)](/brain-regions/cortex ), [Entorhinal Cortex](/brain-regions/entorhinal-cortex)</td>
</tr>
<tr>
<td class="label">Vulnerable In</td> [@allen]
<td>[Alzheimer's Disease](/diseases/alzheimers-disease), [Frontotemporal Dementia](/diseases/ftd)</td> [@cux]
</tr> [@allena]
<tr>
<td class="label">Cell Ontology ID</td>
<td>[CL:CL:0000598](https://purl.obolibrary.org/obo/CL_0000598), [CL:CL:1001571](https://purl.obolibrary.org/obo/CL_1001571), [CL:CL:4023041](https://purl.obolibrary.org/obo/CL_4023041)</td>
</tr>
<tr>
<td class="label">Cell Ontology ID</td>
<td>[CL:0000598](https://purl.obolibrary.org/obo/CL_0000598), [CL:1001571](https://purl.obolibrary.org/obo/CL_1001571), [CL:4023041](https://purl.obolibrary.org/obo/CL_4023041)</td>
</tr>
</table>

Cortical Pyramidal Neurons (Layers 2/3)

Introduction

Cortical Pyramidal Neurons (Layers 2 3) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

Layer 2/3 (L2/3) pyramidal neurons are the principal intracortical communication neurons of the neocortex. They form the densest layer of cortical excitatory neurons, with axons that project laterally within the same cortical area and to distant ipsilateral and contralateral cortical regions via the corpus callosum (callosal/commissural projections). L2/3 neurons are central to higher-order cortical processing including sensory feature integration, associative learning, and the formation of cortical representations. [@l23_review_2019]

In the entorhinal-cortex, layer 2 pyramidal and stellate neurons form the perforant pathway to the hippocampus and are among the earliest and most severely affected neuronal populations in alzheimers.1,10</a> Profound loss of layer 2 entorhinal neurons occurs even in very mild disease stages (Braak stage I–II), preceding widespread cortical involvement by years. [@braak_stages_1991]

<!-- taxonomy-enrichment -->

<!-- multi-taxonomy-enrichment -->

Multi-Taxonomy Classification

Taxonomy Database Cross-References

| Taxonomy | ID | Name / Label |
|----------|----|---------------|
| Cell Ontology (CL) | [CL:0000598](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000598) | pyramidal neuron |

Morphology & Electrophysiology

• Morphology: pyramidal neuron (source: Cell Ontology)
• Morphology can be inferred from Cell Ontology classification

PanglaoDB Marker Cross-References

• Unknown (PanglaoDB):

External Database Links

• [Cell Ontology (CL:0000598)](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000598)
• [OBO Foundry (CL:0000598)](http://purl.obolibrary.org/obo/CL_0000598)
• [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/)

Taxonomy & Classification

| Database | ID | Name | Confidence |
|----------|----|------|------------|
| Cell Ontology | [CL:0000598](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000598) | pyramidal neuron | Exact |
| Cell Ontology | [CL:1001571](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_1001571) | hippocampal pyramidal neuron | Exact |
| Cell Ontology | [CL:4023041](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_4023041) | L5 extratelencephalic projecting glutamatergic cortical neuron | Exact |

PanglaoDB Marker Cross-References

• Unknown (PanglaoDB):

External Database Links

• [Cell Ontology (CL:0000598)](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000598)
• [OBO Foundry (CL:0000598)](http://purl.obolibrary.org/obo/CL_0000598)
• [Allen Brain Cell Atlas](https://portal.brain-map.org/atlases-and-data/bkp/abc-atlas)
• [CellxGene Census](https://cellxgene.cziscience.com/)
• [PanglaoDB](https://panglaodb.se/)

Molecular Identity

L2/3 pyramidal neurons are defined by expression of the transcription factors CUX1 and CUX2, which are required for superficial layer identity and dendritic branching. SATB2 directs callosal axon projection identity, distinguishing L2/3 neurons from deep-layer subcortical projection neurons that express CTIP2/BCL11B. Additional markers include low-level RORB expression (distinguishing them from L4 neurons with high RORB), LAMP5, and RASGRF2. [@cux2_2015]

Single-cell RNA sequencing from the Allen Cell Type Atlas and other large-scale efforts has revealed substantial transcriptomic diversity within the L2/3 population, with distinct subtypes showing areal specialization. In the entorhinal cortex, L2 neurons include both reelin-positive stellate cells and calbindin-positive pyramidal cells, each with different projection targets and disease vulnerability profiles. [@allen_cell_atlas_2020] A 2024 multi-region single-cell atlas of the aging human brain further demonstrated region-specific transcriptomic signatures in L2/3 neurons that correlate with differential vulnerability to Alzheimer's pathology across cortical areas. [@human_atlas_2024]

Transcriptomic Subtypes

Recent single-nucleus RNA sequencing studies have identified multiple L2/3 subtypes:

• L2/3 IT (intratelencephalic) neurons: The canonical cortico-cortical projection type, expressing CUX2, LAMP5, and RASGRF2
• Entorhinal L2 stellate cells: Express RELN (reelin) and NDNF; these project via the perforant pathway to dentate gyrus and CA3
• Entorhinal L2 pyramidal cells: Express CALB1 (calbindin) and project to hippocampal-ca1 via the temporoammonic pathway
• Area-specific subtypes: Prefrontal, temporal, and sensory cortex L2/3 neurons show distinct transcriptomic profiles reflecting regional functional specialization

Morphology and Electrophysiology

L2/3 pyramidal neurons have medium-sized triangular somata (12–20 μm diameter), a prominent apical dendrite extending to layer 1 where it branches into apical tufts receiving feedback from higher cortical areas, and extensive basal dendrites in layers 2/3. Their axons branch locally within the same layer and send long-range horizontal projections spanning several millimeters, interconnecting columns with similar stimulus selectivity — the anatomical basis for cortical feature maps. Callosal axons cross the corpus callosum to innervate corresponding regions of the contralateral hemisphere. [@cortical_circuitry_2018]

Electrophysiologically, L2/3 neurons are regular-spiking with higher input resistance and lower rheobase compared to cortical-pyramidal-layer5, making them more excitable to synaptic input. They show prominent spike-frequency adaptation and sparse coding properties, with only ~5–10% of neurons active during any given stimulus in sensory cortex. This sparse coding enables high-capacity information storage with minimal metabolic cost.

Dendritic Integration

L2/3 pyramidal neurons integrate inputs from multiple sources along their dendritic tree:

• Apical tuft (layer 1): Receives top-down feedback from higher cortical areas and thalamocortical matrix inputs
• Apical oblique dendrites (layer 2/3): Receive local recurrent excitatory connections
• Basal dendrites (layer 2/3): Receive feedforward input from layer 4 and local interneuron inhibition
• Perisomatic region: Receives pv-interneurons basket cell inhibition controlling spike timing

Role in Cortical Processing

L2/3 neurons serve as the primary substrate for intracortical and corticocortical computation. In sensory cortices, they receive processed input from layer 4 (thalamocortical recipient layer) and perform feature integration — combining simple stimulus features into complex representations. They are critical for experience-dependent plasticity: L2/3 is the primary site of synaptic potentiation during perceptual learning, and spine turnover in L2/3 dendrites reflects ongoing memory formation. [@spine_turnover_2010]

In prefrontal-cortex, L2/3 neurons encode abstract rules, working memory content, and categorical decisions.

Entorhinal-Hippocampal Circuit

In the entorhinal-cortex, L2 neurons play a unique role as the gateway to the hippocampal memory system: [@entorhinal_l2_2016]

• Stellate cells in medial entorhinal cortex (MEC) L2 express grid cell firing patterns and project via the perforant pathway to dentate-granule-cells and CA3
• Pyramidal cells in MEC L2 project via the temporoammonic pathway to proximal hippocampal-ca1
• Lateral entorhinal cortex (LEC) L2 neurons convey object and contextual information to the hippocampus, projecting to distal CA1
• This entorhinal-hippocampal circuit is essential for episodic memory encoding, spatial navigation, and memory consolidation

The precise topographic organization of these projections means that loss of specific L2 entorhinal neuron subtypes produces distinct patterns of hippocampal disconnection and memory impairment.

Vulnerability in Neurodegenerative Disease

Alzheimer's Disease

L2 neurons of the entorhinal-cortex are among the earliest and most severely affected neurons in alzheimers. Profound neuronal loss occurs even in very mild disease stages (Braak stage I–II), preceding widespread cortical involvement by years. [@braak_stages_1991] This early vulnerability disconnects the hippocampus from cortical input, directly causing the episodic memory deficits that define clinical Alzheimer's onset.

Multiple factors contribute to their selective vulnerability: [@entorhinal_l2_2016]

**apoe or tau]/proteins/tau inclusions (depending on the pathological subtype), leading to cortical thinning that predominantly affects superficial layers. The pattern of L2/3 involvement differs between FTD subtypes: behavioral variant FTD primarily affects frontal L2/3 neurons, while semantic dementia preferentially involves anterior temporal L2/3 neurons.

Chronic Traumatic Encephalopathy

In cte, tau-protein pathology characteristically begins in L2/3 at the depths of cortical sulci, reflecting biomechanical vulnerability to repetitive traumatic brain injury. This sulcal depth pattern distinguishes CTE from AD, where tau begins in entorhinal L2.

Therapeutic Relevance

The early and selective vulnerability of entorhinal L2 neurons makes them a critical target for early alzheimers intervention:

• Anti-amyloid antibodies: lecanemab and donanemab aim to reduce amyloid burden before irreversible entorhinal neuron loss occurs
• Anti-tau therapies: Targeting the entorhinal-hippocampal axis with tau immunotherapy and antisense oligonucleotides to halt trans-synaptic propagation of pathological tau through the perforant pathway
• Entorhinal cortex biomarkers: Entorhinal cortex volume measured by high-resolution MRI has emerged as a sensitive biomarker for preclinical Alzheimer's Disease
• Neural circuit restoration: Deep brain stimulation of the entorhinal cortex has shown promise in enhancing hippocampal-dependent memory in animal models

Brain Atlas Resources

• [Allen Brain Atlas](https://brain-map.org)
• [Allen Cell Type Atlas](https://portal.brain-map.org/atlases-and-data/rnaseq)
• [Allen Human Brain Atlas](https://human.brain-map.org)
• [Allen Mouse Brain Atlas](https://mouse.brain-map.org)
• [BrainSpan Developmental Transcriptome](https://www.brainspan.org)

Background

The study of Cortical Pyramidal Neurons (Layers 2 3) 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.

• alzheimers
• hippocampal-ca1
• cortical-pyramidal-layer5
• pv-interneurons
• dentate-granule-cells
• cte

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)

References

[Cubelos et al., CUX2 controls excitatory neuron numbers across cortical areas (2015)](https://doi.org/10.1093/nc/npv007)

[Alcamo et al., Satb2 regulates callosal projection neuron identity (2008)](https://doi.org/10.1016/j.neuron.2008.09.001)

[Khan et al., Selective vulnerability of entorhinal cortex layer 2 neurons in early Alzheimer's disease (2016)](https://doi.org/10.1016/j.neurobiolaging.2016.07.018)

[Braak & Braak, Neuropathological stageing of Alzheimer-related changes (1991)](https://doi.org/10.1007/BF00308809)

[Allen Institute Cell Type Atlas (2020)](https://celltypes.brain-map.org/)

[Single-cell multi-region atlas of the aging human brain (2024)](https://doi.org/10.1038/s41586-024-08700-2)

[Yang et al., Experience-dependent spine dynamics in L2/3 pyramidal neurons (2010)](https://doi.org/10.1016/j.neuron.2010.11.003)

[Lodato & Shetty, Cortical interneurons in development and disease (2019)](https://doi.org/10.1016/j.tins.2019.01.002)