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Ultrastructure of the marginal division in the rat striatum.

The discovery of a new subdivision in the striatum of the rat, the marginal division, has recently been reported. The marginal division is located at the caudal extent of the striatum, surrounding the rostrolateral border of the globus pallidus, and has different cellular morphology, immunohistochemistry and an efferent projection pattern from those of the main body of the striatum. In the present study, the ultrastructural organization of the marginal division was investigated. Most neuronal somata in the marginal division were fusiform in shape and had a large pale oval nucleus without in-foldings. There were four types of synapses in the marginal division: axo-somatic, axo-dendritic, axo-spinous and axo-axonic. Both symmetric and asymmetric synapses were observed on the somata, dendrites, or dendritic spines. Most of the symmetric synapses contained pleomorphic vesicles, whereas the asymmetric ones contained mainly round vesicles. Individual axo-axo-spinous synapses, which were first described in the striatum, were also observed in the marginal division. These ultrastructural characteristics distinguish the marginal division from the rest of the striatum.