<table class="infobox infobox-cell">
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<th class="infobox-header" colspan="2">Cortico-Basal Degeneration Neurons</th>
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<td class="label">Taxonomy</td>
<td>ID</td>
</tr>
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<td class="label">Cell Ontology (CL)</td>
<td>[CL:0000646](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000646)</td>
</tr>
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<td class="label">Database</td>
<td>ID</td>
</tr>
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<td class="label">Cell Ontology</td>
<td>[CL:0000646](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000646)</td>
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Corticobasal degeneration (CBD) is a progressive 4-repeat (4R) tauopathy characterized by asymmetric cortical and basal ganglia neuronal loss, producing the clinical syndrome of corticobasal syndrome (CBS) with limb apraxia, alien limb phenomenon, cortical sensory loss, and asymmetric rigidity[@armstrong2013]. CBD shares the 4R tau signature with progressive supranuclear palsy but targets a distinct pattern of neuronal populations, with greater cortical involvement and a unique astrocytic plaque pathology that distinguishes it neuropathologically[@dickson2002].
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<table class="infobox infobox-cell">
<tr>
<th class="infobox-header" colspan="2">Cortico-Basal Degeneration Neurons</th>
</tr>
<tr>
<td class="label">Taxonomy</td>
<td>ID</td>
</tr>
<tr>
<td class="label">Cell Ontology (CL)</td>
<td>[CL:0000646](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000646)</td>
</tr>
<tr>
<td class="label">Database</td>
<td>ID</td>
</tr>
<tr>
<td class="label">Cell Ontology</td>
<td>[CL:0000646](https://www.ebi.ac.uk/ols4/ontologies/cl/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FCL_0000646)</td>
</tr>
</table>
Corticobasal degeneration (CBD) is a progressive 4-repeat (4R) tauopathy characterized by asymmetric cortical and basal ganglia neuronal loss, producing the clinical syndrome of corticobasal syndrome (CBS) with limb apraxia, alien limb phenomenon, cortical sensory loss, and asymmetric rigidity[@armstrong2013]. CBD shares the 4R tau signature with progressive supranuclear palsy but targets a distinct pattern of neuronal populations, with greater cortical involvement and a unique astrocytic plaque pathology that distinguishes it neuropathologically[@dickson2002].
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The cerebral cortex is the primary site of neurodegeneration in CBD. The perirolandic cortex — including the primary motor cortex (Brodmann area 4), premotor cortex (area 6), and supplementary motor area — shows the most severe neuronal loss, consistent with the prominent motor and praxis deficits[@kouri2011]. Cortical thinning is characteristically asymmetric, more pronounced contralateral to the clinically more affected limb.
Layer III and V pyramidal neurons are preferentially vulnerable, particularly large-caliber projection neurons in the motor strip that give rise to the corticospinal tract. These neurons develop ballooned (achromatic) neurons — a pathological hallmark of CBD in which the perikaryon swells, loses Nissl substance, and accumulates phosphorylated neurofilament[@gibb1989]. Ballooned neurons are not exclusive to CBD (they also appear in Pick's disease and argyrophilic grain disease) but their concentration in the perirolandic cortex is diagnostically suggestive.
The posterior parietal cortex, particularly the superior parietal lobule and intraparietal sulcus, also shows significant neuronal loss, explaining the cortical sensory loss, visuospatial deficits, and ideomotor apraxia. In the CBS variant with progressive aphasia, the left perisylvian cortex (inferior frontal and superior temporal regions) is preferentially affected[@lee2011].
Within the basal ganglia, the substantia nigra pars compacta shows severe dopaminergic neuron loss, accounting for the levodopa-unresponsive parkinsonism. The globus pallidus — both internal (GPi) and external (GPe) segments — undergoes significant neuronal depletion with tau-positive NFTs in surviving neurons[@schneider1997]. The caudate nucleus and putamen show moderate neuronal loss with thread-like tau pathology.
The subthalamic nucleus (STN) is affected but typically less severely than in PSP, which helps distinguish the two tauopathies on neuropathological examination[@josephs2006].
The ventrolateral thalamus, which receives GPi output and projects to the motor cortex, shows neuronal loss and gliosis. Brainstem involvement is generally less prominent than in PSP but includes neuronal loss in the red nucleus, pontine nuclei, and periaqueductal gray. The dentate nucleus of the cerebellum may show moderate neuronal loss with grumose degeneration[@sakurai2000].
Like PSP, CBD is defined by 4R tau accumulation, but cryo-electron microscopy has revealed that CBD tau filaments adopt a unique C-shaped fold distinct from the PSP fold[@zhang2020]. This structural difference in the tau protofilament core implies that distinct molecular seeds may initiate and propagate the two diseases, despite sharing the same 4R tau isoform composition.
The [MAPT H1 haplotype](/genes/mapt) is the strongest genetic risk factor for CBD (OR ~3.5-5.0), as in PSP. The H1c sub-haplotype particularly increases exon 10 inclusion, promoting 4R tau overproduction[@kouri2015]. Unlike PSP, where globose tangles predominate, CBD neurons develop pretangles and small, flame-shaped neurofibrillary tangles with extensive neuropil thread pathology.
The astrocytic plaque is the defining neuropathological feature of CBD. Unlike the tufted astrocytes of PSP (which have a tuft-like morphology concentrated at the cell body), astrocytic plaques show tau accumulation in the distal processes of astrocytes, producing an annular or ring-like pattern of tau immunoreactivity[@komori1999]. Astrocytic plaques are most abundant in the frontoparietal cortex and striatum. Their formation disrupts astrocyte-neuron metabolic coupling, impairing lactate shuttle provision and glutamate buffering that neurons depend on for survival[@forrest2019].
Oligodendrocytes in CBD develop coiled bodies — comma-shaped tau inclusions within the cytoplasm — that are particularly abundant in the white matter underlying affected cortical regions. Coiled bodies are associated with white matter rarefaction and axonal degeneration, contributing to disconnection between cortical and subcortical structures[@arima1997].
The vulnerability of layer III and V pyramidal neurons in CBD likely reflects their high metabolic demands and long-range projection architecture. These neurons maintain extensive dendritic arbors requiring sustained mitochondrial energy production and express high levels of 4R tau to stabilize their complex axonal cytoskeleton. Both features increase vulnerability to tau misfolding and bioenergetic failure[@murray2011].
The mechanism underlying the characteristic asymmetry of CBD remains poorly understood. Hypotheses include asymmetric initiation of tau seeding (a stochastic "first hit" in one hemisphere), lateralized differences in neuronal vulnerability or clearance capacity, and network-level spreading patterns along asymmetric connectomic pathways[@whitwell2010]. The lateralization is maintained throughout disease progression, suggesting an early, fixed pathological template.
CBD neurons show evidence of impaired protein quality control at multiple levels. The ubiquitin-proteasome system shows reduced activity, evidenced by ubiquitin-positive inclusions. Autophagy-lysosomal pathway dysfunction, with accumulation of p62 and reduced TFEB activity, impairs the clearance of tau aggregates. Endoplasmic reticulum stress markers (GRP78, CHOP) are elevated in affected cortical neurons[@piras2016].
Activated microglia surround neurons with tau pathology, particularly in the cortex and substantia nigra. Complement pathway activation (C1q, C3) tags synapses for microglial phagocytosis, contributing to synapse loss before frank neuronal death. Reactive astrogliosis compounds the problem, as tau-bearing astrocytic plaques lose their normal homeostatic functions[@ishizawa2001].
The Armstrong criteria for neuropathological diagnosis of CBD require: (1) tau-positive neuronal and glial lesions in cortical and subcortical regions, (2) astrocytic plaques, and (3) predominance of 4R tau[@armstrong2013a]. Ling and colleagues proposed a staging system with early involvement of the frontal cortex and striatum (stage 1), subsequent spread to parietal cortex, thalamus, and subthalamic nucleus (stage 2), followed by temporal cortex, brainstem, and dentate nucleus involvement (stage 3)[@ling2016].
Fluid biomarkers for CBD include elevated plasma and CSF neurofilament light chain (NfL), which reflects the intensity of axonal degeneration. CSF total tau and phospho-tau levels are variably elevated. Tau PET imaging with [^18F]PI-2620 and [^18F]flortaucipir shows asymmetric cortical and basal ganglia signal, though off-target binding limits specificity[@brendel2020]. Structural MRI reveals asymmetric frontoparietal atrophy with contralateral predominance, while FDG-PET shows corresponding hypometabolism[@whitwell2017].
Current therapeutic strategies relevant to CBD neuronal protection include: