[The topographical localization of spinal motoneurons of the rat and its numerical alternation in regard to development (author's transl)].

The topographical localization of spinal motoneurons innervating the forelimb muscles of the rat and its numerical alternation in regard to development was observed by HRP (horseradish peroxidase) method. HRP method is a histochemical technique to trace the cellular origin of axon terminal net work. When HRP is injected into the muscles, the enzyme is taken up by nerve endings and transported retrogradely by the axoplamic flow to the original cell body, and does not transported hematogenously and does not cross the synapse. The spinal motoneurons were found in spindle-shape longitudinal cell columns according to its innervation. The motoneurons supplying the biceps brachii muscle were located in the dorso-lateral nucleus on the anterior horn of the 4th cervical segment, and extended downward to the ventral nucleus in 8th cervical segment, forming a spindle-shape cell column. The thick spindle-shape cell column consisting of cells which innervated the triceps brachii muscle lied in the ventro-medial nucleus of the 6th, 7th and 8th cervical segment. The neurons innervating the flexor carpi ulnaris muscle were located in 7th and 8th cervical segment, but few in number compared to those of the biceps and triceps. The motoneurons innervating the biceps brachii muscle decreased in number with increasing age according to exponential curve. The total number of HRP-labeled neurons in two week old rat ranged from 2,932 to 2,017 and markedly decreased during first eight weeks, in eight week old rat ranged from 1,637 to 603. After this, they gradually decreased to 312 in 50 weeks on an average, where the number corresponded to about 13% of those of two week old rat. The data indicate that ventral horn has a great deal of excess neurons which has a distinct neuromuscular connection as suggested by Prestige, in the developmental stage of Xenopus laevis. In young rats (ten days to two weeks of age), HRP-labeled neurons were also present in the ventral horn on the non-injected side. These large and multigonal contralateral HRP-labeled neurons constituted a characteristic feature of spinal motoneurons, and distributed through the same segment as those on injected side. The total number of HRP-labeled neurons within non-injected side ranged from 563 to 197 and corresponded from one-fifth to one-tenth of that in the injected side. In five weeks or elder rat, no HRP-labeled neurons could be observed in non-injected side. This indicates that in immature rats the muscles are innervated bilaterally by the spinal nerve, since HRP is transported by retrograde axoplasmic flow and does not cross the synapse, and the bilateral motor innervation has disappeared with maturation. The bilateral motor innervation is therefore, presumed to be an immature mode of innervation limited to the early stage of life. Although bilateral motor innervation following the nerve injury in the limb at the early stage of life has been confirmed by ourselves, the significance of the immature mode of innervation is still hard to explain.