THE ROSTROCAUDAL ORGANIZATION IN THE DORSAL-ROOT GANGLIA OF THE RAT -A CONSEQUENCE OF PLEXUS FORMATION

The dorsal root ganglia (DRGs) of the rat have a rostrocaudal organiza tion. This organization can most easily be demonstrated in fetal and n eonatal rats because the spatial relationships of their DRGs are maint ained better in tissue sections than those of mature rats. This review is concerned with the way in which the rostrocaudal organization of t he DRGs is generated. Wheat germ agglutinin - horseradish peroxidase/h orseradish peroxidase labeling of peripheral nerves of the brachial an d lumbar plexuses shows that the position of the somata of the sensory neurons of the labeled nerves can be restricted to rostral or caudal halves of DRGs. Labeling of the thoracic nerve or its branches always results in labeling throughout the entire thoracic DRG. After applicat ion of the marker to forelimb nerves, it was observed that whenever a DRG is labeled only partially, its spinal nerve is correspondingly lab eled partially as well. These data suggest that the rostrocaudal organ ization in the DRG is related to the formation of the plexuses. During development nerve fibers can be segmentally labeled, using the subdiv ision of the DRGs into a rostral and a caudal half to keep together as they find their way through the plexus. Application of label to forel imb skin, hindlimb skin and even thoracic skin can result in labeling of rostral or caudal halves of a DRG. A possible explanation might be that each dermatome can be divided into a skin area innervated by the rostral half of a DRG and a skin area innervated by the caudal half of the same dorsal root ganglion. In the rat, the segmental sensory inne rvation of muscles during development has not yet been investigated. T he question of whether the segmental unit of innervation of a muscle i s a whole DRG or half a DRG therefore still remains unanswered.