Postnatal histogenetic death of rat forelimb motoneurons

Previous workers have reported a five fold decrease in motoneuron numbers occurring from birth to adulthood in the rat. It has also been reported that forelimb muscles receive connections from motoneurons in both ventral horns prior to day 14 of life in the rat and that the contralateral cells subsequently degenerate completely by day 21. In the present study, postnatal changes in motoneuron number and distribution within the ventral horn were studied in the rat using the technique of retrograde horseradish peroxidase (HRP) transport following intramuscular infusion of HRP into biceps brachii (BB). Peripheral nerves other than those to BB were sectioned and ligated to control for HRP diffusion. The spatial organization of BB motoneurons was compared in animals 9-14 days old and adult animals using computer 3-D reconstruction. This allowed observations of the BB motoneurons from a variety of perspectives in relationship to a number of spinal cord landmarks and avoided the necessity for transverse and horizontal sectioning of the cords in alternate animals. A decrease of 50-75% in the number of HRP filled motoneurons number was found from birth to adulthood (adults 98-150, neonates 172-243). The spatial arrangement of the BB motoneurons with regard to the root entry zones and other landmarks such as the dorsolateral convexity of the ventral horn, remains constant from birth to adulthood. No evidence was found to support the existence of inappropriate connections from motoneurons in either the ipsi or contralateral ventral horns of neonate rats. It is suggested that the comparatively small decreases in motoneurons postnatally could be a continuation of the histogenetic processes of cell death begun in utero and is related to the postnatal development of the central and peripheral connections of motoneurons.