title: Identification of eight types of synapses in the pallidum externum and internum in squirrel monkey (Saimiri sciureus).
abstract: The electron microscopic study of the neuronal structures of the pallidum externum (Pa.e) and internum (Pa.i) in squirrel monkey (Saimiri sciureus) has revealed in both segments a frequent large type of nerve cell and a rare small one, probably an interneuron. With respect to the small cells, only one axosomatic synapse inserts on the slender rim of cytoplasm; the clear nucleus is always invaginated and contains a small nucleolus. Eight types of synapses can be differentiated in Pa.e and Pa.i. The axodendritic type I P with pleomorphic vesicles and an inconspicuous symmetric contact to a central dendrite, make up 65% of all synapses in Pa.e and 79% in Pa.i. These synapses are commonly arranged in a rosette pattern around the dendrite. Two subtypes of the type I P are found in both segments--the one with invaginated boutons (subtype I Piv) and the other (subtype I Pol) oligovesicular. The type III SR synapses are characterized by small round or slightly oval vesicles and a strongly asymmetric contact (12% in Pa.e and 8.1% in Pa.i). The predominantly axosomatic type II EF synapses are characterized by elongated vesicles as well as by encapsulation in thin astrocytic processes (5% in Pa.e, 8.5% in Pa.i). The boutons of type IV Sph characterized by large uniformly spherical vesicles undergo symmetric synaptic contacts (in Pa.e 6% and in Pa.i 0.5%). The type V Osl stiletto-shaped bouton with ovoid vesicles undergoes a symmetric contact with a dendrite (in Pa.e 5% and in Pa.i 1.1%). Two types of dendrodendritic and dendrosomatic synapses are found: the type VI DE containing sparsely distributed elongated vesicles suspended on neurofilaments, undergoing a symmetric contact, and the type VII DC with clustering mostly round vesicles near the presynaptic component of the asymmetric contact. The rare axodendritic type VIII My arising from a myelinated axon, exhibits small elongated vesicles and symmetric contact. The synaptic types I, III, IV, and VI are similar to the same types in the substantia nigra, whereas types II, V and VII resemble the equivalent types in the motor thalamic VA and ventro-oralis anterior nuclei.
mesh_terms: D000818:Animals; D002427:Cebidae / Q000033:anatomy & histology*; D002540:Cerebral Cortex / Q000648:ultrastructure*; D008854:Microscopy, Electron; D012453:Saimiri / Q000033:anatomy & histology*; D013569:Synapses / Q000648:ultrastructure*