Among invertebrates some annelid species show also remarkable regenerative abilities. Within this group of animals, however, it is not rare to find species that can regenerate posterior regions but not anteriorly. Also, regeneration has not been extensively studied in all taxa. Now, a recent paper from the laboratory of Michael Weidhase and Conrad Helm (http://www.ncbi.nlm.nih.gov/pubmed/25392962) describes for the first time the process of anterior regeneration in the annelid Cirratulus cirratus, focussing mainly in the musculature and central nervous system (CNS). The authors used phalloidin to label the muscle and antibodies against serotonin and FMRFamide to observe the regenerating CNS.
As for the body wall musculature, it is mainly composed of an outer layer of circular fibers and an inner longitudinal muscle layer. The dorsal longitudinal muscles form a plate that covers the whole dorsal body. Ventrally, there is a medial and two ventro-lateral strands of longitudinal muscle. On the other hand, the CNS is a rope-ladder-like system. The ventral nerve cord consists of two main strands and each of them exhibits one ganglion per body segment. From these ganglia there are three major nerves that extend laterally. At the first chaetigerous segment both strands of the ventral nerve cord separate from each other to form the circumesophageal connective surrounding the mouth opening. This connective splits into a dorsal and a ventral root. The brain is located at the anterior end of the circumesophageal connective. Anti-serotonin immunostaining revealed two brain commissures: a stronger stained anterior commissure connected to the ventral root of the circumesophageal connective and a slighter stained posterior commissure connected with the dorsal root.
After amputation, the wound was closed and a blastema was formed within the first week. Then this blastema elongated anteriorly and about 10 days after decapitation the new mouth opening was evident. By the end of the second week the first outgrowing tentacles appeared. The whole regeneration process took place in about 1 month, although the regenerated head showed some differences with the original respect to the number and length of tentacles and branchiae.
By day 6 of regeneration a blastema was formed and contained outgrowths from the pre-existing longitudinal muscle fibers. From that time the longitudinal musculature kept growing and organized into muscular strands. By day 10 the ventral longitudinal strands reached their most anterior position, surrounding the regenerated musculature of the new mouth opening. At this stage the first circular muscle fibers appeared.
Concerning the nervous system, by day 6 after decapitation, a structure represented by three nerve loops was visible inside the blastema, one median loop and two laterals. At this stage the median loop was folded backwards and was connected to the inner bundles of both strands of the ventral nerve cord. On the other hand, the lateral loops were already oriented anteriorly and were connected to the outer bundles of the ventral nerve cord. At day 8 of regeneration the median loop was also oriented anteriorly. This tripartite loop-like structure has not been described in other regenerating annelids. Whereas the authors favour that the median loop will originate the ventral root of the regenerated circumesophageal connective (visible by day 12), they did not obtain clear data about the transition from the lateral loops to the dorsal root. Finally, the brain commissures were visible by day 14 of regeneration. At this stage, the regenerated nervous system elongated together with the blastema; however, distinct ganglia and lateral processes in the new nerve cords were still missing by day 18. Later, the first signs of body segmentation and differentiation of distinct ganglia and lateral nerves were observed.
In summary, this paper describes for the first time anterior regeneration in the annelid Cirratulus cirratus. Further studies are necessary to describe in more detail this process as well as to determine the origin of the regenerative cells that form the blastema and the new muscle fibers and central nervous system, among all the other anterior tissues.