Hippocampal dentate gyrus mossy cells represent a unique and critical neuronal population within the hippocampal formation that plays essential roles in memory encoding, pattern separation, and circuit regulation. These large glutamatergic neurons are located in the hilus (polymorphic layer) of the dentate gyrus and form a powerful excitatory feedback circuit with dentate granule cells and interneurons. Mossy cells are increasingly recognized as vulnerable in both Alzheimer's disease and temporal lobe epilepsy, making them an important therapeutic target. Understanding their normal function and pathological alterations provides crucial insights into hippocampal dysfunction in neurodegenerative processes.
Introduction
Hippocampal dentate gyrus mossy cells are a unique neuronal population in the dentate gyrus that play critical roles in hippocampal circuitry, memory encoding, and are vulnerable in neurodegenerative diseases. [@amaral2007] These large glutamatergic neurons are located in the hilus (polymorphic layer) of the dentate gyrus. They receive inputs from dentate granule cell axons (mossy fibers) and project back to granule cells and interneurons, forming a powerful excitatory feedback circuit. [@jinde2012]
The dentate gyrus serves as the gateway to the hippocampus, filtering and orthogonalizing cortical inputs before they reach the CA3 region. Mossy cells are positioned to modulate this filtering process, regulating the flow of information through the hippocampal circuit. Their strategic location and extensive connectivity make them essential for proper hippocampal function. [@treves2008]
Neuroanatomy
Location and Morphology
Cell body: Located in the hilus/polymorphic layer of the dentate gyrus
Cell size: Large cell bodies (20-30 μm diameter) among the largest in the dentate gyrus
Dendrites: Highly spiny, extending into the molecular layer and receiving input from granule cells
Axon: Large, thin unmyelinated axons (mossy fibers) that project to CA3 pyramidal neurons
Synaptic contacts: Receive mossy fiber inputs from granule cells, project to granule cells and interneurons
Circuitry
The mossy cell axon gives rise to extensive collaterals that innervate both granule cell bodies and interneurons in the dentate molecular layer. This dual targeting enables mossy cells to simultaneously excite granule cells while activating inhibitory interneurons, creating a complex gain control mechanism. [@yeckel1999]
Gain control: Regulate excitability of dentate granule cells through feedback excitation
Pattern separation: Contribute to orthogonalization of similar memory representations
Feedforward inhibition: Coordinate activity between granule cells and interneurons
Memory encoding: Support hippocampal-dependent learning and memory
Reward prediction: Encode reward prediction error signals in behavioral contexts[@howe2021]
Electrophysiology
Firing pattern: Regular spiking, adapting
Resting membrane potential: ~-65 mV
Input resistance: High (~150 MΩ)
Action potential threshold: ~-50 mV
Action potential duration: Broad spikes (1-2 ms)
Accommodation: Moderate frequency adaptation
Synaptic Properties
Excitatory inputs: Powerful mossy fiber input from granule cells
Inhibitory inputs: GABAergic input from local interneurons
Plasticity: LTP and LTD at mossy fiber-mossy cell synapses
Neuromodulation: Cholinergic and serotonergic modulation
Role in Neurodegenerative Diseases
Alzheimer's Disease
Mossy cells demonstrate significant vulnerability in Alzheimer's disease:
Early loss: Mossy cell degeneration occurs in early AD stages
Correlates with memory deficits: Mossy cell loss correlates strongly with episodic memory impairment
Hyperexcitability: Mossy cell loss contributes to dentate gyrus hyperexcitability and network dysfunction
Tau pathology: Mossy cells accumulate neurofibrillary tangles in AD
Therapeutic target: Preserving mossy cells may improve hippocampal function[@myers2013]
Studies in 5xFAD and 3xTG mouse models demonstrate mossy cell loss precedes granule cell degeneration, suggesting selective vulnerability. The calretinin-positive mossy cell population shows particular susceptibility to amyloid-beta toxicity. [@sato2015]
Temporal Lobe Epilepsy
Mossy cells are critically involved in epileptogenesis:
Early loss: Mossy cell death is an early event in hippocampal sclerosis
Aberrant sprouting: Loss of mossy cells promotes aberrant sprouting of granule cell axons
Circuit remodeling: Mossy cell loss disrupts the normal excitatory/inhibitory balance
Hyperexcitability: Reduced mossy cell input leads to granule cell disinhibition
Status epilepticus: Mossy cells are selectively vulnerable even in acute seizures[@jinde2012]
The pattern of mossy cell loss in human temporal lobe epilepsy follows a characteristic topography, with the suprapyramidal blade showing earlier and more severe degeneration. [@engel2020]
Traumatic Brain Injury
Excitotoxic vulnerability: Mossy cells are highly vulnerable to excitotoxic damage
Secondary degeneration: Progressive mossy cell loss occurs weeks after initial injury
Cognitive deficits: Mossy cell loss contributes to post-traumatic memory impairment
Normal Aging
Progressive decline: Mossy cells undergo age-related degeneration even in the absence of disease
Cognitive impact: Mossy cell loss contributes to age-related memory decline
Vulnerability factors: Enhanced susceptibility to oxidative stress and mitochondrial dysfunction[@butler2019]
Molecular Mechanisms of Degeneration
Excitotoxicity
NMDA receptor overactivation: Excessive glutamate leads to calcium overload