ORGANIZATION OF THE CAUDAL RHOMBENCEPHALIC ALAR PLATE OF THE RIBBED NEWT PLEURODELES WALTL - EVIDENCE FOR THE PRESENCE OF DORSAL COLUMN ANDLATERAL CERVICAL NUCLEI

As part of a recent program on the evolution of somatosensory systems in vertebrates, the cytoarchitecture, chemoarchitecture, and fiber con nections of the caudal rhombencephalic alar plate were studied in the ribbed newt, Pleurodeles waltl. This part of the brain stem includes i ll-defined dorsal column and lateral cervical nuclei. A cytoarchitecto nic analysis revealed that the caudal medullary alar plate consists of an inner and an outer cell layer. The dorsomedial part of the outer c ell layer at the obex level contains the dorsal column nucleus (DCN), whereas its ventrolateral part constitutes the lateral cervical nucleu s (LCN). NADPH-diaphorase histochemistry and calbindin D-28k immunohis tochemistry clearly delineate the main components of the compact inner cell layer, i,e, the nucleus of the solitary tract dorsally and the n ucleus of the descending trigeminal tract ventrally. Neither NADPH-dia phorase-labeled nor calbindin D-28k positive neurons were observed in the DCN and LCN. With anterograde and retrograde tracing, the DCN and LCN were further delineated. Labeling of ascending dorsal root project ions showed that the dorsal column and the DCN are somatotopically arr anged: lumbar primary afferent fibers terminate on medial DCN neurons, whereas cervical primary afferent fibers terminate on lateral DCN neu rons. The LCN is densely innervated by the dorsolateral funiculus. Ret rograde tracing showed extensive, predominantly contralateral projecti ons of both the DCN and LCN to the torus semicircularis and the ventra l thalamus. These data show that even in the poorly segregated caudal rhombencephalic alar plate of urodeles a DCN and LCN can be distinguis hed with afferent and efferent projections comparable to those in anur ans and other terrestrial vertebrates.