Development of spinal motoneuron innervation of the upper limb muscle in the rat.

Horseradish peroxidase was injected into the biceps brachii muscle of rats at various stages of development, from 10 days to 50 weeks. The retrogradely labeled neurons were found in the ipsilateral ventro-lateral column of the cervical cord, C4--C8, of all stages studied, but the number of labeled neurons decreased according to exponential curve as the age advanced. A striking finding was that the contralateral ventral horn cells were also labeled in the 10- and 14-day-old rats.