Quantitative analysis of synapse structure with a semiautomatic interactive computer system: A study on identified pyramidal tract neurons in the sensory-motor cortex of cat
Abstract
Pyramidal tract (Pt) neurons in the sensory-motor cortex of cat were labeled by injection of HRP into the spinal cord. Ultrastructural and quantitative analysis of the synaptic covering of their soma and apical dendrite (up to 100 μm from soma) was undertaken. We intercalated a visual-manual treatment between composite electron micrographs and a fully automated computer system and developed specific programs for evaluation of the morphometric data. Programs are included. A total of 412 synaptic boutons were examined that were found in contact with large Pt neurons. The mean linear percentage of the surface area covered by boutons was 26.2 ± 8.4% and the mean contacting length and cross-sectional area of the bouton profiles were 1.28 ± 0.58 μm and 0.89 ± 0.59 μm2, respectively. All types of boutons with active zones accounted for 41.2% of the total. The distribution of two types of bouton (S- and F-type boutons, showing asymmetric and symmetric contacts, respectively) was examined quantitatively. The mean proportion of F-type boutons was 89.1% with a soma and S-type boutons contacting apical dendrites was 10.9%. In addition, GABAergic boutons were identified with the soma by immunocytochemistry with antibodies against glutamic acid decarboxylase. They formed symmetric synaptic contacts with the Pt cells that were identical to those formed by F-type boutons. The quantitative analysis revealed that synaptic clefts are narrower and synaptic vesicles are smaller in symmetric F-type boutons than in S-type boutons forming asymmetric contacts. These data establish that at least three parameters (postsynaptic density, synaptic cleft, and size of vesicles) can be utilized singly or in combination to identify GABAergic inhibitory synapses in neocortex.
