Regeneration of the nervous and muscular system after caudal amputation in the polychaete Alitta virens (Annelida: Nereididae)

Regenerating segments in polychaetes offer a vivid example of epimorphic recovery of the lost organs and tissues. It is also a promising object for studying positional information and the mechanisms maintaining the body integrity. With the aim to develop a convenient standardized model, we described the dynamics of recovery of the major anatomical structures and created a staging system for the caudal regeneration in Alitta virens. In average the normal organization of the posterior body end is restored within 10 days after amputation (dpa). The whole regenerative process was divided into 5 stages: (1) wound healing (0–1 dpa), (2) blastema formation (1–2 dpa), (3) patterning and growth of the blastema (2–3 dpa), (4) differentiation of the first regenerated segment (3–5 dpa), (5) formation and differentiation of the subsequent 5–6 segments (5–10 dpa). The regeneration is carried out mainly by epimorphosis, although the elements of intercalary growth as well as the morphallactic transformation of the stump have been noted. Terminal structures of the pygidium (muscles of the anal sphincter, pygidial cavity, pygidial ring nerve, pygidial cirri) appear at stages 1–3, and then (from stage 3) the formation of new metameres begins in front of the pygidium. Differentiation of the first newborn segment is associated with the tissue remodeling in the last old segment. Formation of the next segments resembles accelerated postlarval growth. The neural elements of the regenerative bud are developing faster than the surrounding muscles. The neurites extending from the CNS and PNS come to the surface of the wound epithelium at stage 1. Later, nerve fibers from the CNS lengthen and thicken along with the growth of the regenerative bud. Ganglion, parapodial nerves, oblique muscles and coeloms of the first segment are detected at stage 4. Longitudinal muscles regenerate in anterior to posterior progression, being constantly in contact with the corresponding fibers of the old tissues. All other muscles differentiate from blastemal cells in isolation from the old musculature of the stump. Our data promote the further using of the posterior body end regeneration in A. virens as an experimental model for resolving crucial problems of developmental biology.