Different dendritic arborization patterns of motoneurons in various places of the rat's lumbosacral spinal cord

With the aid of the cobalt labelling technique the distribution and morphology of motoneurons were studied in the 2nd and 3rd lumbar segments of the rat's spinal cord. On the basis of arborization patterns of their dendritic arbors, motoneurons can be classified into six groups. 1. Large stellate-form neurons occupy the centre of the ventral horn. 2. Marginal neurons are found at the ventrolateral border of the ventral horn. 3. Horizontal neurons form a small group in the dorsolateral corner of the ventral horn. 4. Upper stellate neurons situate in the dorsal part of the ventral horn. 5. Triangular neurons constitute a compact group in the medial corner of the ventral horn. 6. Unclassifiable neurons form a scattered group in the ventral part of the ventral horn. The arborization pattern of dendrites of adjacent neurons belonging to the same group frequently resemble one another to such an extent that they are called "twin" neurons and derive probably from a common cell lineage. One member of such twin pairs is often the small counterpart of its much bigger mate. Dendrites originating from the same morphological type of motoneurons and running in the same direction, aggregate into smaller or larger bundles. The majority of dendritic bundles is oriented in, or close to, the transversal plane of the spinal cord; a significant number of longitudinal dendrites are found only in the group of triangular neurons innervating the axial musculature. The possibility is discussed that the differentiation of the different forms is determined both by genetic and epigenetic factors during morphogenesis.