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Mossy fiber (hippocampus)

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Subject: Fascia dentata, Mossy fiber (cerebellum), Wim Crusio, Neuroanatomy
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Mossy fiber (hippocampus)

Diagram of a Timm-stained cross-section of the mouse hippocampus. The hippocampal subregion CA3-CA4 is indicated in black, stippled, and hatched areas. Black areas: suprapyramidal (SP), intra- and infrapyramidal (IIP) and hilar (CA4) mossy fiber terminal fields originating from the dentate gyrus. Stippled area: strata oriens (OR) and radiatum (RD). Hatched area: stratum lacunosum-moleculare (LM). CA1, subregion of the hippocampus without mossy fibers; FI, fimbria hippocampi; FD, fascia dentata; OL and ML, outer and middle molecular layers of the fascia dentata; SG, supragranular layer; GC, granular cells.[1]

In the hippocampus, granule cells of the dentate gyrus form distinctive unmyelinated axons that project along the mossy fiber pathway to the Cornu Ammonis area 3 (CA3). The axons emerge from the basal portions of the granule cells and pass through the hilus (or polymorphic cell layer) of the dentate gyrus before entering the stratum lucidum of CA3. Granule cell synapses tend to be glutamatergic (e.g. excitatory), though immunohistological data has indicated that some synapses contain neuropeptidergic elements including opiate peptides such as dynorphin and enkephalin. Moreover, there is growing evidence for co-localization of GABAergic and glutamatergic neurotransmitters within mossy fiber terminals, though the functional consequence of this co-localization is unclear.[2]

The pathway was so named by Ramon y Cajal because the axons display varicosities all along their lengths, giving them a "mossy" appearance. These mossy fibers form multiple synapses with the elaborate dendritic spines of CA3 pyramidal cells in stratum lucidum. These complex spines are known as "thorny excrescences". It has also been shown that the axons of granule cells synapse with a wide variety of inhibitory GABAergic interneurons in the hilar region of the dentate gyrus before continuing on to innervate pyramidal cells in the CA3 region. A single mossy fiber projection may make as many as 37 contacts with a single pyramidal cell, but innervates only about a dozen different pyramidal cells. In contrast, a single CA3 pyramidal cell receives input from about 50 different granule cells. It has been shown in rodents that the size of the mossy fiber projections can show large interindividual variations, which are to a large part heritable.[3] In addition, these variations show strong correlations with different types of behavior, mainly, but not exclusively, spatial learning.[4]

The synapses of the mossy fibers contain zinc, which can be stained specifically with a Timm staining.[5]

See also

Mossy fiber (cerebellum), which shares little similarity with its hippocampal namesake.

References

  1. ^ Sluyter, Frans; Jamot, Laure; Bertholet, Jean-Yves; Crusio, Wim (2005-04-22). "Prenatal exposure to alcohol does not affect radial maze learning and hippocampal mossy fiber sizes in three inbred strains of mouse".  
  2. ^ Sandler, R; Smith, AD (1991). "Coexistence of GABA and glutamate in mossy fiber terminals of the primate hippocampus: An ultrastructural study".  
  3. ^  
  4. ^ Crusio WE, Schwegler H (April 2005). "Learning spatial orientation tasks in the radial-maze and structural variation in the hippocampus in inbred mice". Behav Brain Funct 1 (1): 3.  
  5. ^ Danscher, G; Zimmer, J (1978). "An improved Timm sulphide silver method for light and electron microscopic localization of heavy metals in biological tissues".  
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