Muscle Regeneration

The most important and conspicuous muscles in the general annelid body plan are those of the body wall (Tzetlin and Filippova 2005; Purschke and Muller 2006). Body wall musculature comprises an outer layer of circular muscles and an inner layer of longitudinal muscle (Figure 10.2C). The circular muscles are located below the basement membrane of the epidermis and usually form finer bands with fewer fibers than longitudinal muscles. They form complete or incomplete fine rings in planes normal to the main anteroposterior body axis and might be completely absent in many species. The longitudinal muscles usually form discrete bundles or bands that run parallel to the main body axis along the complete body length. The number and arrangement of longitudinal bands varies across groups and can go from as few' as four to six separate bands, to several bands forming a continuous layer. In some groups, an additional network of oblique muscle fibers can be seen located between the epidermis and the circular muscle layer (Figure 10.5J-Q).

Traditionally, most descriptions of annelid muscular systems resulted from dissections, histological sectioning, and transmission electron microscopy; more recently, the combination of labeling F-actin with fluorescently labeled phalloidin, laser scanning confocal microscopy, and three-dimensional reconstruction has allowed more sensitive detection of finer muscle fibers and a better understanding of annelid muscular structure (Purschke and Mtiller 2006).

Regeneration of muscles has been studied in several species of annelids and is similar across the phylum (Zattara and Bely 2011; Zattara 2012; Weidhase, Helm, and Bleidorn 2015; Weidhase, Bleidorn, and Helm 2014; Weidhase, Bleidorn, et al. 2016; Weidhase, Beckers, et al. 2016; Kozin, Filippova, and Kostyuchenko 2017). Immediately after amputation, circular muscles adjacent to the w'ound site contract to seal the w'ound. Later, during the initial wound-healing stage of regeneration, these circular muscles are removed, along with the ends of the longitudinal muscle fibers adjacent to the w'ound site. During this stage, dead or injured myocytes are phagocytized, w'hile many surviving myocytes dedifferentiate and move out of the body wall, resulting in a muscle-free distal end of the stump (Figure 10.5J). At the early blastema stage, muscle fibers originating from the old longitudinal muscle bundles extend distally over the growdng blastema (Figure 10.5J-O); in contrast, circular muscle fibers seemingly form de novo, presumably from blastemal cells (Figure 10.5K-Q). In all reported cases, development of circular muscles of the newly forming segments takes place at slightly later stages than development of longitudinal muscles (but see later regarding pygidial muscles). Circular muscle shows an anterior-to-posterior gradient that is very evident during posterior regeneration but can also be seen, albeit more subtly, during anterior regeneration. In contrast, longitudinal muscles develop from anterior to posterior during posterior regeneration but show the opposite gradient during anterior regeneration. Thus, whereas circular muscle develops de novo following the anteroposterior developmental gradient of new' segments (see Section 10.4.7), longitudinal muscle instead develops by extension from fibers from the stump and thus follow's a proximodistal gradient.

During posterior regeneration, development of pygidial tissues precedes differentiation of segmental tissues. As a result, in species with prominent or elaborate pygidial structures (like the nereid Alitta virens or several naidid clitellates), a variably thick ring of circular muscle forms at the posterior end of the blastema, often before the growing fibers of longitudinal muscle reach that tip (Zattara 2012; Kozin, Filippova, and Kostyuchenko 2017) (Figure 10.5N). Additional circular muscle develops over the prospective pygidium into a dense layer that has a sharp anterior boundary where the segment addition zone develops (Figure 10.50 and P). Anterior to that zone, circular muscle develops following the same anterior-to-posterior gradient seen during normal growth (Figure 10.5P and Q).

During anterior regeneration, in contrast, early development of prostomial or peri- stomial circular muscle has not been reported. Muscle fibers extending from the old longitudinal muscle bands reach the distal tip of the blastema, often branching and crisscrossing over it (Figure 10.5J and K). Circular muscle develops soon after, completing the prostomial and peristomial musculature, including the mouth and other anterior structures (Figure 10.5L and M). A transient circular muscle ring forming earlier at the prospective mouth has been reported for the amphinomid Eurythoe cf. complanata (Weidhase, Bleidorn, et al. 2016). This ring might be a feature unique to Eurythoe regeneration; alternatively, a homologous but finer, fainter structure might also be present in other annelids but have gone undetected in previous studies. When labeling F-actin with fluorescently tagged phalloidin, it is common to find a large difference in signal intensity between old longitudinal muscle bundles and newly developing circular muscle fibers. This generates a challenging trade-off when setting dynamic range parameters during imaging, as increasing sensitivity enough to detect the finer fibers often results in signal saturation of the thicker bundles from the old tissues. Thus, future studies should scan for the presence of such fine structures and explicitly report their absence if not found.

The expression of genetic markers of muscle development has so far been only studied during posterior regeneration in the nereid Platynereis dumerilii (Planques et al. 2018). In this species, weak expression of a twist homologue is initially detected after wound healing in scattered cells near the wound site. During early blastema formation, twist becomes strongly expressed in a ring of mesodermal cells located where the pygidial circular muscles will form (see earlier). About a day later, twist becomes also expressed at bilateral patches of mesodermal cells located anterior to the newly formed segment addition zone. This expression precedes formation of circular muscle in the new segments, and it expands as bands as these segments grow and develop. The fact that twist is not expressed in the segment addition zone located between the pygidium and developing segments strongly suggests that cell differentiation does not take place until myocyte precursors leave this zone.

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