Cytoarchitecture and ultrastructure of the avian ectostriatum: Afferent terminals from the dorsal telencephalon and some nuclei in the thalamus

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Abstract
The cytoarchitecture, ultrastructure, and afferent terminals in the ectostriatal complex of the Japanese quail were examined. The complex consists of the central core (Ec) and peripheral belt (Ep). Terminals in the complex were categorized into three main groups according to the shape of synaptic vesicles: S(spherical), P(pleomorphic), and F(flat). S terminals are further classified into three types: Ss, small terminals which have densely packed vesicles and a long active zone and are presynaptic to large spines; Sm, medium-sized to large terminals which have a relatively short active zone and contact dendritic spines, trunks, and somata; Sl, large terminals which have many mitochondria and cored vesicles and form synapses only with somata. Some of the Sm terminals are derived from myelinated axons. The Sl terminals are frequently combined with gap junctions as so-called mixed synapses. The P terminal occasionally surrounds an axon hillock, making symmetric synaptic contacts. The F terminals often cover a wide area of the soma. A few gap junctions are also recognized between adjacent somata. Afferent sources of the ectostriatal complex were examined by means of horseradish peroxidase (HRP) retrograde transport. Many large HRP-labeled cells were recognized in the nucleus rotundus (Rt). HRP-labeled cells were seen in the nucleus triangularis (Tr), nucleus dorsolateralis posterior thalami (DLP), and a few labeled cells were scattered in the hyperstriatum ventrale (HV). Substantial numbers of Sm terminals in Ec degenerated after destruction of Rt; they made synaptic contacts with dendritic trunks (71.8%) and small spines (28.2%). Degenerating and intact Sm terminals were found to form synapses with the same dendritic trunk by the reconstruction of serial thin sections. Among the 167 Sm terminals, 20 terminals (12.0%) degenerated after lesions in Rt. The Sm terminals in Ec degenerated after destruction of Tr and the terminals formed synapses with somata as well as dendritic trunks and spines. After lesions of the dorsal telencephalon including HV, degenerating fibers sparsely entered the ectostriatal complex associated with the Ss terminal degeneration. DLP seemed to project mainly on the medial area of posterior Ep. The terminals from DLP made asymmetric synaptic contacts with dendritic spines, trunks, and somata. Some of the terminals from DLP were identified as the Sl type, but other S types could not be identified.

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