A light and electron microscopic analysis of the mossy fibers of the rat dentate gyrus

Abstract

The axon collaterals of dentate granule cells have been analyzed with the aid of a computerized microscope, following intracellular injections of horseradish peroxidase in hippocampal slice preparations. The axon of each granule cell gives rise to approximately seven primary collaterals; these collaterals usually divide into secondary and tertiary branches, which form an extensive plexus within the hilar region of the dentate gyrus. Individual axon collaterals vary greatly in length, but most have been found to be between 100 and 300 μm long. On average, the summed lengths of the collaterals (exclusive of the parent mossy fiber) are approximately 2,300 μm. Except for an occasional collateral that is given off by a mossy fiber in the proximal part of field CA₃ of the hippocampus, the collaterals of the granule cell axons are confined to the hilar region; they are rarely seen in the granule cell layer itself and have never been observed in the molecular layer. In the longitudinal dimension of the dentate gyros, most of the collaterals are contained within a zone about 400 μm wide. The distribution of the collaterals within the hilar region is correlated with the location of the granule cell body. Those that arise from cells near the tip of the suprapyramidal blade tend to be confined to the region above field CA₃; those from cells nearer the crest and from the infrapyramidal blade ramify widely throughout the hilus. Two types of varicosities are present on the collaterals. Numerous small (∼2 μm), round varicosities are distributed unevenly along the collaterals; in electron micrographs these varicosities can be seen to make asymmetric synaptic contacts with dendritic shafts. On average, each granule cell collateral plexus his about 160 of these varicosities. The second type of varicosity is irregular in shape and ranges from 2 to 4 μm in diameter; there is usually only one sucil varicosity per collateral. In all respects except size, these varicosities rE semble the expansions found on the parent mossy fibers. Mossy fiber trajectories in the proximal part of field CA₃ were studied after extracellular injections of HRP into localized regions of the granule cell layer. Granule cells at different locations around the blade send their mossy fibers to different depths within the pyramidal cell layer in the proximal part of field CA₃. However, further distally, mossy fibers from all parts of the granule cell layer contribute to the suprapyramidal bundle that occupies the stratum lucidum.