The fine structure of the intermediolateral nucleus of the spinal cord of the monkey (Macaca fascicularis).

An electron microscopical study of the intermediolateral nucleus of the monkey has shown that the neurons could be classified into at least three main types. The perikarya and proximal dendrite of Types I and II neurons were almost completely covered with astrocytic processes. Both these types of neurons were characterized by prominent profiles of rough endoplasmic reticulum arranged in parallel stacks in Type I and in small aggregates in Type II. The Golgi apparatus was well developed in both types. In contrast, Type III neurons, which were consistently smaller than the other two types, were only partially covered by astrocytic processes, their meagre RER consisted of single short strands of ribosome-studded cisterns randomly dispersed in the cytoplasm and their Golgi apparatus was poorly dveloped. The nuclear outline of all three types of neuron was irregular, but that of Type III neurons was deeply indented. Axosomatic synapses were rarely observed in Types I and II neurons but were common in Type III neurons. Axon terminals in the intermediolateral nucleus could also be divided into three main classes according to their synpatic vesicular morphology. R-boutons contained round agranular vesicles, DCV-boutons contained a mixture of agranular and large dense-cored vesicles and F-boutons contained flattened or ellipsoidal vesicles. All three classes of axon terminals formed simple axodendritic synapses and also took part in the formation of glia-encapsulated glomerular synapses in which the most common arrangement was that of a central dendrite which was synaptically contacted by axon terminals. F-boutons formed axosomatic synapses with all three types of neurons, R-boutons synapsed commonly on Type III neurons and only occasionally on Type II neurons, while DCV-boutons were rarely seen to form axosomatic synapses. No axo-axonal synapses were observed.