Morphological characteristics of dendrite bundles in the lumbar spinal cord of the rat.

The finding of motoneuron dendrites organized into small compact bundles in cats, monkeys and pigs suggested that a study of this phenomenon in rats should be undertaken. An analysis was performed with electron microscopy, light microscopy and Golgi methods. An extensive dendrite bundle organization was found in the sixth lumbar segment of the spinal cord. Two discrete bundles were localized bilaterally: a lateral bundle in the ventrolateral gray substance, and a medial bundle in the ventral funiculus. The lateral bundle was found to consist of longitudinally oriented dendrites, neurocytons, glial cells and capillaries. As many as 1678 closely packed dendrites were observed in the lateral bundle. The medial bundle contained dendrites directed across the midline and also longitudinally oriented dendrites. Neurocytons in the medial dendrite bundle were found singly or in clusters, and many radiating bundles of dendrites were observed projecting toward the lateral bundle. Golgi analysis confirmed that neurons in the lateral bundle had most of their dendrites oriented longitudinally. It was possible to trace several dendrites into the lateral bundle from dorsally and medially lying neurons. Electron microscopy substantiated the fact that the bundles were composed of dendrites. It also revealed numerous dendrodendritic and dendrosomatic contacts which were desmosomal in type as well as an abundance of small unidentified processes. Various functions which have been attributed to the dendrite bundles are discussed.