Thalamic projecting neurons in the feline nucleus cuneatus. A combined horseradish peroxidase and high voltage electron microscopic study.

The retrograde transport of horseradish peroxidase (HRP) has been employed to identify thalamic projection neurons (TPN) in the feline nucleus cuneatus by means of light microscopy and high voltage electron microscopy. Forty-eight hours after injection of HRP in the contralateral ventrobasal complex of the thalamus, labelled neurons at levels caudal to the obex are concentrated in the cell clusters of the dorsal two-thirds of the nucleus. In plastic sections, labelled TPN are identified by the presence of HRP-positive granules in the perinuclear cytoplasm. TPN are typically about 25 micrometers in diameter, have a round nucleus with a smooth contour and abundant cytoplasm. In contrast, neurons unlabelled after thalamic injection are located at the periphery of clusters of TPN. Unlabelled neurons are characterized by their fusiform shape (hence, round when encountered in cross-section), small diameter (10-15 micrometers), a nucleus with an irregular or highly indented contour, and sparse cytoplasm. At the ultrastructural level, TPN are identified by the presence of HRP-positive, membrane-bound, dense bodies in the perinuclear cytoplasm. Furthermore, the presence of such dense bodies in cross-sections of dendrites allows their identification as processes of TPN. The perikarya of adjacent neurons in a cluster are often closely apposed and separated by an extracellular space of 20 to 25 nm. Adjacent to such sites of apposition, small boutons are often presynaptic to one or both of the neurons. The possible functional implications of such an arrangement are discussed.