Nerve development, growth and differentiation during regeneration in Enchytraeus fragmentosus and Stylaria lacustris (Oligochaeta)

Enchytraeus fragmentosus (Enchytraeidae) and Stylaria lacustris (Naididae) are small terrestrial and limnetic oligochaetes that exclusively or seasonally reproduce by fragmentation and regeneration, respectively. We traced the neuronal development and differentiation during regeneration in order to gain information on the basic organization and evolution of the oligochaete nervous system. Subsequent to artificial amputation, the nervous systems have been stained with antibodies directed against acetylated alpha-tubulin. The staining was analyzed by indirect fluorescence in combination with confocal laser scanning microscopy. Both species show unique oligochaete neuronal regeneration patterns: (i) numerous fibers branch off from segmental nerves near the wound site and innervate the blastema; and (ii) the ventral cord is partly reestablished before the circumesophageal connectives develop. In the investigated 'Oligochaeta' the outgrowing fibers of the ventral nerve cord are soon bundled into at least two distinct connective pairs, which prolong into dorsal and ventral roots next to the mouth. Subsequent complete fusion of the doubled roots forms simple connectives. Thus, dorsal roots are not a unique feature for 'Polychaeta'. They occur as a transient structure in 'Oligochaeta' and might be part of the neuronal ground pattern of Annelida. The initially tetra or even pentaneuronal ventral nerve cord also differentiates into an unineuronal one by fusion.