A quantitative analysis of the laminar distribution of synaptic boutons in field CA3 of the rat hippocampus.

We analyzed the laminar distribution of synaptic boutons in field CA3 of the rat hippocampus using a large montage electron micrograph. The size of boutons and synaptic vesicles was measured using a computer-assisted digitizing system. In all, 3353 synaptic boutons were observed in a 15 microm x 100 microm strip. Of these, 86.3% contained spherical vesicles (S-boutons), 12% contained flat vesicles (F-boutons), and 1.7% were mossy terminals (M-boutons). S-boutons were distributed widely in the strata moleculare (st. Mol), radiatum (st. Rad), and oriens (st. Ori), but there were only a few in the strata lucidum (st. Luc) and pyramidale (st. Pyr). The upper portions of both the st. Rad and Ori contained slightly fewer boutons. In terms of the location of synaptic contacts, 83% of all S-boutons were found on the dendritic spines and the rest were on the dendritic shafts. S-boutons on the dendritic shafts were observed more frequently in the st. Mol than in the other strata. According to the morphometry of the size of synaptic vesicles, S-boutons with small vesicles (mean vesicle area <1109 nm(2)) were located exclusively in the st. Mol, S-boutons with medium-sized vesicles (mean vesicle area 1109-1482 nm(2)) were observed in all strata, and S-boutons with large vesicles (mean vesicle area >1482 nm(2)) were distributed in the st. Luc and Ori, but not in the st. Mol. F-boutons were predominantly distributed in the upper half of the st. Mol and in the area around the st. Pyr, although they were observed in all strata. In the st. Mol, all the F-boutons were in contact with dendritic shafts, while near the st. Pyr, F-boutons were found exclusively on somata, the proximal parts of the dendritic shafts, and the initial segments of axons. The average F-bouton was smaller in the st. Mol (0.23 microm(2)) than near the st. Pyr (0.39 microm(2)). In this synapto-architectural study of the hippocampal CA3 region using large montage electron micrographs, we observed (1) an intimate relationship between synapse distribution and the dendritic structure of pyramidal neurons, (2) the distribution of different types of boutons containing vesicles of various size, and (3) two different plausible foci of postsynaptic inhibition where F-boutons were distributed densely, and (4) estimated the input ratios of pyramidal neurons.