Some electron microscope findings of the claustrum of the cat.

The claustrum of the cat is basically composed of 4 classes of neuron. Class I is a rather small neuron (less than 10 mu in diameter) and is provided with rather few cytoplasmic organelles, while Class IV, large (25-30 mu), neuron possesses an abundance of them. Class II, medium-sized (15-20 mu), neuron and fusiform Class III neuron (about 15 X 25 mu) exhibit an intermediate form of the former two classes. Perikaryal and proximal dendritic surface of Class IV neurons are covered with many boutons, while neurons of the other three classes have few of such terminals on their perikaryal surface. The dendritic profiles of rather reduced diameter bear a few axonal terminals. Five types of terminals are recognized: (1) Type A terminal, small in profile, with uniform sized round vesicles (about 300 A in diameter) making asymmetrical synapses; (2) Type B terminal of small size, containing round vesicles of various diameters (300-400 A) and forming asymmetrical synaptic contacts; (3) Type C, also small in profile, being filled with pleomorphic vesicles and making symmetrical synapses; (4) Type D large terminal with numerous pleomorphic vesicles, many mitochondria and a number of glycogen-like granules, forming symmetrical synapses; and (5) Type E terminal containing numerous granular vesicles (800-1,000 A). The Type C and some of Type A terminals usually synapse upon the cell bodies and proximal dendrites, while most of Type A, B, D and E make synapse upon medium to small dendritic profiles and spines. The problem whether or not the cytoarchitecture of the claustrum at the light microscope level is identifiable at the ultrastructural level as well, and the possible origin of various types of terminals above described, were discussed briefly.