A rostrocaudal somatotopic organization in the brachial dorsal root ganglia of neonatal rats.

A rostrocaudal somatotopic organization in the rat lumbar dorsal root ganglia (DRGs) during development has been demonstrated. The present study shows that such a rostrocaudal somatotopic organization also exists in the brachial DRGs of neonatal rats. WGA-HRP or WGA-HRP/HRP injections into the forepaw of neonatal rats gave rise to completely and partially labeled DRGs. Partial labeling always concerned the caudal or rostral halves of the rostral-most or caudal-most labeled DRGs, respectively. Labeling of forelimb nerves (cutaneous branch of the musculocutaneous nerve, median and ulnar nerve) in neonatal rats also resulted in whole labeled DRGs and labeled rostral or caudal halves of DRGs. Accordingly, just as was found for the hind limb nerves, the position of the somata of the sensory neurons of forelimb nerves can be restricted to the rostral or caudal half of a DRG. Moreover, it was observed that the rostrocaudal organization in the brachial DRGs is maintained in the spinal nerves. Application of tracer to the cut end of intercostal nerves in neonatal rats produced labeling of one whole thoracic DRG. It is therefore possible that the rostrocaudal organization in the brachial and lumbar DRGs is a consequence of the formation of the plexuses. In one instance a subdivision in the labeling pattern of a midthoracic DRG was noticed after a WGA-HRP injection into the thoracic wall of a neonatal rat. The whole thoracic DRG was labeled except the dorsorostral corner. This indicates that during development, some kind of somatotopic organization may exist also in the thoracic DRG.