The three-dimensional organization of dentate granule cell dendritic trees has been quantitatively analyzed with the aid of a computerized microscope system. The dendrites were visualized by iontophoretic injection of horseradish peroxidase into individual granule cells in the in vitro hippocampal slice preparation. Selection criteria insured that the analyzed cells were completely stained and that only neurons with two or fewer cut dendrites in the distal portion of the molecular layer were analyzed. Twenty-nine of the 48 sampled granule cells had no cut dendrites. The granule cells had between one and four primary dendrites. Granule cell dendritic branches were covered with spines and most extended to the hippocampal fissure or pial surface. The mean total dendritic length was 3,221 microns with a range from 2,324 microns to 4,582 microns. The dendrites formed an elliptical plexus with the transverse spread averaging 325 microns and the spread in the septotemporal axis averaging 176 microns. On individual neurons, the maximum branch order ranged from four to eight and the number of dendritic segments ranged from 22 to 40. Approximately 63% of the dendritic branch points occurred in a zone that included the granule cell layer and the inner one-third of the molecular layer. The dendritic tree was organized so that, on average, 30% of the length was in the granule cell layer and proximal third of the molecular layer, 30% was in the middle third, and 40% was in the distal third. Comparisons were made between the dendrites of granule cells in the suprapyramidal and infrapyramidal blades of the dentate gyrus. Suprapyramidal cells had a significantly greater total dendritic length than infrapyramidal cells, their transverse spread was higher, and they had a greater number of dendritic segments. When neurons in the suprapyramidal blade were further subdivided on the basis of somal position within the depth of the cell body layer, superficial neurons were found to have a greater number of primary dendrites, more elliptical trees, and larger transverse spreads of their dendrites. There were no significant differences in dendritic segment number or total dendritic length between superficial and deep cells.

Figure 1

Figure 2

Figure 3

Computer-generated dendritic tree diagrams from six different granule cells, three of which were located in the suprapyramidal blade (A-C) and three in the infrapyramidal blade (D - F). The three dimensional lengths (in mm) of each segment are given. Scale bar = 100 mm.

Figure 4

Computer-generated plots of reconstructions of the dendritic the total dendritic trees from granule cells located at various positions around the transverse axis of the dentate gyrus. Each neuron was from a different animal. The total dendritic length of each neuron is indicated. Note that the total dendritic lengths of the neurons in the suprapyramidal blade are greater than those in the infrapyramidal blade. CA₃ : field CA₃ of hippocampus.

Figure 5

Micrographs showing multiple labeling of granule cells from a single HRP injection. In A and C, two neurons were filled, while in B and D, three neurons were labeled. Close examination of each of these preparations indicates that a dendrite from one filled cell runs directly over (or under) the filled soma of another cell; we assume that it is at such sites that the HRP labeling has spread from the injection and into the adjoining neuron(s). This is especially clear in panel C where one of the dendrites from the labeled cell at the bottom can be seen to course over the filled soma of the top cell (open arrow). In many of the micrographs, an axon (asterisk) can be seen exiting from the granule cell body. Scale bar = 100 mm.

TABLE 1. Dendritic Parameters for Entire Population of Granule Cells¹