Neurons with presynaptic perikarya and presynaptic dendrites in the rat lateral geniculate nucleus

Abstract

An ultrastructural study of neurons with presynaptic dendrites in the lateral geniculate nucleus (LGN) of albino and hooded rats was carried out. Nearly 30 perikarya of such neurons (presynaptic dendrite or PSD cells) were identified by continuity with presynaptic dendrites and/or by the presence of somato-dendritic synapses. PSD cells are small (6–12 μm diameter) and paucidendritic. Their nuclei are deeply folded, with conspicuous chromatin clumps, and the cytoplasm is rich in free polyribosomes and elements of the Golgi apparatus. Thalamocortical relay cells are larger, never give rise to presynaptic dendrites, and contain unusual membranous organelles and a more extensive granular reticulum. PSD cell mitochondria are considerably smaller than relay cell mitochondria: this may enable the cell types to be distinguished in the absence of other criteria. The surface of the perikarya and proximal dendrites of PSD cells is mostly related to astrocyte processes, and the few synaptic contacts received are from axon terminals with cylindrical vesicles. Synapses made by the perikarya and proximal dendrites are few and focal, with a cluster of discoid vesicles and prominent presynaptic dense projections. The synapses may be made within or outside synaptic glomeruli. The perikarya and main dendrites also give rise to blunt or stalked appendages containing discoid vesicles, that enter and synapse within synaptic glomeruli (gemmules). No axons were traced from PSD cells.