Electron-microscopic examination of serial sections from the dorsal lateral geniculate nucleus of the Rhesus monkey reveals the existence of axonal initial segments belonging to interneurons (I-cells). These initial segments are recognized by their characteristic subaxolemmal undercoating and by containing fascicles of microtubules. They originate either from I-cell presynaptic dendrites, or occasionally from I-cell perikarya via an axon hillock-like process. The perikarya as well as the axon hillocks have both presynaptic and postsynaptic sites, a main identification feature of I-cells. An unusual property of the I-cell axon initial segment is the presence of varying amounts of vesicles which look similar to the synaptic vesicles seen in the presumed I-cell axon terminals. Additionally, the initial segment may also exhibit presynaptic sites, a feature which has not been reported so far for initial segments of other more classical types of neurons. The electron-microscopic identification of I-cell initial segments gives strong support to the notion, drawn originally from Golgi studies, that most or all of the Golgi type II interneurons in the dorsal lateral geniculate nucleus of the monkey are not amacrines but axonal cells, and that the action of these neurons may be dual, namely through a conventional axonic mechanism as well as through the presynaptic dendrites.
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