The ultrastructure and synaptic organization of the anterodorsal nucleus (AD) of the thalamus were investigated under normal and experimental conditions. The large glomeruli are composed of an extensive central dendrite, probably arising from a projecting neuron, and of various terminal boutons. Besides the typical small bouton (RS) filled with round vesicles, two specializations of the large bouton (RL) containing round vesicles are found. The larger one (RL1) is characterized by a looser arrangement of synaptic vesicles and many mitochondria; it undergoes dark degeneration following unilateral lesions of the mamillary body in both AD. The intermediate type (RL2) is distinguished by a denser arrangement of vesicles; it undergoes dark degeneration following lesions of the midline nuclei of the thalamus. The two types of terminal boutons (F1and F2) which contain flattened or pleomorphic vesicles do not degenerate following lesions of the mamillary body and midline nuclei; they belong to Golgi type II interneurons.
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