A newly recognized element in the monkey dorsal lateral geniculate nucleus exhibiting both presynaptic and postsynaptic sites.

The dorsal lateral geniculate nucleus (LGNd) of four normal monkeys (Macaca mulatta) and of two other such animals with total unilateral ablation of the visual cortices (4-6 days survival) were examined in serial thin sections with the electron microscope. In these materials we have observed a new neuropil component which has the cytologic characteristics of principal cell (P-cell) dendrites, i.e. large and dark mitochondria, smooth endoplasmic cisterns and filamentous, non-synaptic contacts with retinal terminals. In addition, these elements contain large round synaptic vesicles and can be seen forming asymmetric synapses exclusively with presynaptic dendrites belonging to interneurons (I-cells). Occasionally, a reciprocal synapse is formed between the two profiles. The novel elements are postsynaptic to various vesicle-containing profiles, i.e. axonal boutons of presumably retinal and cortical origin, and I-cell presynaptic dendrites. They are found more frequently in the specimens with cortical ablations, although their number is still much lower than that of the other classic components of the neuropil. Measurements made on X 80 000 electron micrographs of spheroid vesicles within presumptive retinal terminals, cortical endings and the new profile described in this report, result in mean diameters of 38.6 nm, 33.3 nm and 44.3 nm, respectively. The differences between the means are statistically significant. Although the profile with large dark mitochondria and large round vesicles may represent a dendrite of a different I-cell type, or a recurrent axon collateral of a P-cell, it appears more probable that it is a presynaptic dendrite of a P-cell. The infrequent but consistent occurrence of these elements suggests that at least some P-cells can develop presynaptic sites on their dendrites, a property which contributes to the synaptic complexity of the LGNd.