The structure of the dorsal lateral geniculate nucleus in the mouse. A Golgi and electron microscopic study

Two types of neurons have been distinguished in Golgi and electron microscopic preparations of the dorsal lateral geniculate nucleus of young mice. In addition to a thalamo striate relay cell (TSR neuron) with brush like dendritic arbors and a thick, unbranched axon, a small to medium size cell (PA neuron) of oval or spindle shaped body and few, long and seldom branched dendrites is frequently identified in the Golgi preparations. This second type of cell may exhibit none, one or several sparsely branched axon like processes which terminate in the vicinity of the cell body. The dendrites of the PA neurons show characteristically large, spheroidal processes (p) 1-3μ in diameter issuing forth singly, in clusters or as a 'string of beads' from delicate, often long, pedicles attached to the dendritic shafts. Profiles comparable to these processes and in apparent continuity with PA dendrites have been identified with the electron microscope and show synaptic vesicles and a system of sacs of smooth E.R. The portions of the dendrites from which these vesicle containing processes issue also show clusters of vesicles, ribosomes and an orderly array of microtubules. Golgi impregnated axons are followed from the optic tract and seen terminating as irregular enlargements (2-6μ) on proximal dendrites of relay cells and on distal dendrites of the PA neuron. The intimate contact of the terminal branches of an optic collateral with a distal PA dendrite is carefully illustrated. Small calibered axons are likewise traced from the internal medullary lamina and seen to end by means of end knobs on distal dendrites of both types of neurons. Electron microscopic observations substantiate the Golgi images and reveal 3 different types (I, II, III) of endings in the geniculate neuropil. The large type I endings correspond to the retinal afferents which generally make asymmetric synaptic contact with type II profiles and/or with clusters of microspines on the TSR dendrites. Type II, thought to be the spheroidal dendritic appendages of the PA neurons, form symmetrical synaptic contacts with profiles of its own kind or more commonly with dendrites of the TSR neuron. The type III ending, probably cortical in origin, establishes asymmetrical synaptic contacts with small dendritic profiles. Only profiles of types I and II endings, together with those of other dendritic profiles, form part of the nest like junctions known as the synaptic glomeruli. The significance of the unusual polarization of the geniculate interneurons is discussed.Peer Reviewe