Nervous system regeneration in Typosyllis antoni (Annelida: Syllidae)

Abstract

Annelids represent a metazoan taxon with remarkable regenerative abilities, used during reproduction as well as after injury. Although many studies of different aspects of annelid regeneration are available, detailed morphological investigations regarding the regeneration of specific organ systems such as the nervous system are scarce. Using immunohistochemical staining techniques combined with subsequent confocal laser scanning microscopy (cLSM) and a 3D-reconstruction based on histological sections of the anterior nervous system in adults for comparison, we investigated the redevelopment of the anterior and posterior nervous system during posttraumatic regeneration in Typosyllis antoni. In both processes, the first signs of a regenerated nervous system are neurites infiltrating the regeneration blastema. During posterior regeneration, the neurites are reorganized in a terminal commissure within the new pygidium. Afterwards, neuronal structures of the ventral nerve cord such as segmental nerves were developed. This occurs in conformity with the development of new segments by the reestablished posterior segment addition zone, comparable to normal growth. During anterior nervous system regeneration, the neurites form loops close to the stump of the remaining nerve cord, that attach its contralateral connectives. Later, these loops become the dorsal and ventral roots of the circumesophageal connectives and the brain reoccurs in an anteriormost position. This kind of anterior regeneration agrees with the general pattern known in annelids.