A correlative HRP, Golgi, and EM study of the intrinsic organization of the feline dorsal column nuclei.

The retrograde transport of horseradish peroxidase (HRP), Golgi impregnations, and electron microscopic (EM) observations have been employed to investigate the intrinsic organization of one cytoarchitectonic subdivision of the feline dorsal column nuclei (DCN): the “clusters” region. Previous studies have demonstrated that neurons arranged in typical cell clusters in the dorsal two-thirds of the feline DCN project to the ventrobasal complex (VB) of the thalamus. Following injections of HRP in the VB of adult cats, over 90% of the neurons in this region contain detectable reaction product. These thalamic projecting neurons (TPN) are typically round, range from 20 to 35 μm in diameter, and have round nuclei and abundant cytoplasm. In Golgi preparations, TPN are identified by their characteristic arrangement in the cell clusters. Observations of their cytological characteristics in gold-toned preparations in both 1-μm-thick sections, and in thin sections at the EM level, supplement the data from HRP material. The dendrites of TPN in the clusters region characteristically converge in the perikarya-free center of each cluster, and appendages originate from both proximal and distal portions of the major dendrites of these neurons. These appendages vary in morphology from short-stalked, occasionally multi-lobed, bulbous thorns to long-stalked processes terminating in a single or multilobed swelling. Other appendages often display beaded swellings along their length (moniliform appendages). Neurons in the clusters region unlabelled by HRP injected in the VB have cytological charateristics quite distinct from those of TPN in the same region. Typically located at the periphery of the cell clusters, unlabelled neurons have fusiform perikarya, range in diameter from 8 to 12 μm along their short axis, and have highly indented nuclei and sparse cytoplasm. In Golgi preparations, small fusiform neurons located at the periphery of cell clusters typically have two major dendritic trunks originating from opposite poles of their perikarya. Observations in 1-μm-thick plastic sections of re-embedded Golgi-impregnated neurons of this type support their identifications as neurons which are unlabelled after HRP injection in VB. Dendritic appendages are present also on this type of neuron, although they do not appear to be as frequent as in the case of the TPN. Although the axon of the small, fusiform neurons have not been impregnated beyond the initial segment in the present Golgi material, thus precluding the classification of this type of neuron as a Golgi type II neuron, a combination of the HRP and Golgi observations suggest that these neurons are distinct from TPN, and are possibly interneurons. In addition to the previously described synaptic complexes of primary afferent terminals associated with, or postsynaptic to, small boutons with flattened vesicles (F-boutons, of presumed interneuronal origin), EM observations of serial sections demonstrate triadic synaptic arrangements similar to those encountered in other CNS nuclei. In such instances, a primary afferent terminal is presynaptic to a pale profile containing polymorphicvesicles (P-bouton), and both boutons are presynaptic to a dendrite. In serial sections, P-boutons are demonstrated to be of dendritic origin, while F-boutons are observed to arise frommyelinated fibers. By comparison with Golgi observations, P-boutons are believed to corre-spond to the dendritic appendages of TPN, suggesting that TPN, as well as interneurons, contribute to the intrinsic synaptology of the cell clusters region.