Electron microscopic studies of the dentate gyrus of the rat. I. Normal structure with special reference to synaptic organization

Abstract

The fine structure of the dentate gyrus of the rat has been studied in glutaraldehyde perfused material. With the exception of the short axon association cells, the major components of each of the three layers of the gyrus have been identified, and a detailed account is given of the synaptic organization of the dentate granule cells. The granule cells have most of the features typical of small neurons and although they contain no large aggregates of granular endoplasmic reticular they exhibit a variety of cisternal specializations. In the granule cell layer numerous axo‐somatic and axo‐dendritic synapses have been observed, and on occasion a single presynaptic fiber has been seen to contact the soma of one cell and a proximal dendrite of another. In the inner part of the molecular layer the majority of synapses are upon the main dendritic shafts of the granule cells but many also contact short dendritic spines. In the outer two‐thirds of this layer most presynaptic fibers end upon long dendritic spines some of which are characterized by a prominent projection from the subsynaptic surface into the presynaptic process. The unmyelinated axons of the granule cells establish many en passant contacts with dendrites and spines of CA 4 pyramids before ending in large presynaptic bags which may contain a number of large agranular vesicles (up to 2,000 Å in diameter) and vesicles with dense cores.