Abstract
The circuitry of the dentate gyrus (DG) of the hippocampus is unique compared to other hippocampal subfields because there are two glutamatergic principal cells instead of one: granule cells, which are the vast majority of the cells in the DG, and the so-called “mossy cells.” The distinctive appearance of mossy cells, the extensive divergence of their axons, and their vulnerability to excitotoxicity relative to granule cells has led to a great deal of interest in mossy cells. Nevertheless, there is no consensus about the normal functions of mossy cells and the implications of their vulnerability. There even seems to be some ambiguity about exactly what mossy cells are. Here we review initial studies of mossy cells, characteristics that define them, and suggest a practical definition to allow investigators to distinguish mossy cells from other hilar neurons even if all morphological and physiological information is unavailable due to technical limitations of their experiments. In addition, hypotheses are discussed about the role of mossy cells in the DG network, reasons for their vulnerability and their implications for disease.
Highlights
NEURAL CIRCUITSReviewed by: John Lisman, Brandeis University, USA Michael Frotscher, Albert Ludwigs University, Germany
Many decades later, electron microscopy was used to describe mossy fiber boutons in more detail, and showed that they are complex, large terminals, densely packed with synaptic vesicles (Blackstad and Kjaerheim, 1961; Laatsch and Cowan, 1966)
The hilus was described by other terms: area H5 (Rose, 1926), CA4 (Lorente De Nó, 1934), or the polymorphic zone, and it was debated if the area between the granule cell layer and area CA3 should be addressed as a single area or multiple subregions
Summary
Reviewed by: John Lisman, Brandeis University, USA Michael Frotscher, Albert Ludwigs University, Germany. Large mossy fiber boutons of granule cells are abundant in the hilus, where they contact thorny excrescences on the proximal dendrites and somata of a subset of hilar neurons. “hilus” has replaced these terms for the most part, the term “polymorphic zone” is still applicable to the dentate gyrus (DG) because the hilus is a polymorphic layer if one defines the DG as a structure composed of a molecular layer, cell layer, and polymorphic layer Another aspect of nomenclature that is important relates to the terms for the different parts of the hippocampus: septal vs temporal poles. A section cut in the horizontal plane in the dorsal part of the brain is best
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