Synapses in the granule cell layer of the rat dentate gyrus: serial-sectioning study

Synaptic contacts on the granule cell somata as well as on their axon initial segments in the dentate gyrus of one juvenile 5-week-old rat and one adult 12-week-old rat were analyzed in an electron microscopic serial-sectioning study. In the dentate gyrus of the juvenile rat, somata of 17 granule cells were nearly completely reconstructed from a series of 183 serial sections, and the axon initial segments of 15 of these granule cells were traced in various lengths. On the other hand, in the dentate gyrus of the adult rat, somata of 31 granule cells were almost completely reconstructed from a series of 238 serial sections, and the axon initial segments of 23 of these granule cells were traced in various lengths. Both symmetrical and asymmetrical synapses were observed on the somata, whereas almost all synapses on the axon initial segments were of symmetrical type and asymmetrical synapses on the axon initial segments were rather exceptional. Although we confirmed two conclusions from previous random-section studies to some extent, that is, the superficial-to-deep gradient of synaptic densities on granule cell somata and the presence of a substantial number of asymmetrical synapses on granule cell somata (about 23% of total somatic synapses), the present serial-sectioning study clearly revealed that granule cells vary greatly with regard to the number of synapses on their somata (15-186 in a 5-week-old rat and 9-144 in a 12-week-old rat) and axon initial segments. The granule cells also differed in the proportion of somatic asymmetrical synapses to total synapses they received (0-44% in a 5-week-old rat and 0-60% in a 12-week-old rat). The results of the present study indicated that, when a relatively small number of granule cells are analyzed, one should take the heterogeneity of synaptic contacts on granule cells in number and type into consideration.