Ground plan of the polychaete brain--I. Patterns of nerve development during regeneration in Dorvillea bermudensis (Dorvilleidae).

Abstract

The nervous systems of adult specimens and regenerating fragments of Dorvillea bermudensis ("Polychaeta," Dorvilleidae) were stained with antisera directed against acetylated alpha-tubulin and analyzed by indirect fluorescence and confocal laser scanning microscopy. The anlage of each circumesophageal connective is initially doubled in regenerates and starts with the outgrowth of two nerves from each side of the ventral cord of the amputee. The inner nerves on each side, which become the ventral roots, fuse medially to form the ventral cerebral commissure, whereas the outer nerves become the dorsal roots. As development proceeds, both roots split again, to form the four cerebral commissures. In later stages, the two circumesophageal connectives of each side merge, the point of fusion progressing from the ventral cord toward the brain. In D. bermudensis, this process stops halfway along the connective, thus producing the typical polychaete pattern according to Orrhage ([ 1995] Acta Zool 79:215-234): Each single circumesophageal connective divides near the brain into dorsal and ventral roots, which themselves split into two branches to form the four cerebral commissures. From the results presented here, we conclude that each circumesophageal connective is basically a paired structure but is partially fused in species possessing dorsal roots and completely fused in species lacking them. This may also be true for Clitellata, in which dorsal roots have hitherto not been found. At the posterior end, the outgrowing fibers form five connectives, of which the two outermost of each side fuse in an anteroposterior direction, forming the main connectives. Outgrowing fibers of the stomatogastric nerve stumps soon form the stomatogastric ring, which subsequently is linked with the new brain via stomatogastric connectives.