What Role Do Annelid Neoblasts Play? A Comparison of the Regeneration Patterns in a Neoblast-Bearing and a Neoblast-Lacking Enchytraeid Oligochaete

Abstract

The term ‘neoblast’ was originally coined for a particular type of cell that had been observed during annelid regeneration, but is now used to describe the pluripotent/totipotent stem cells that are indispensable for planarian regeneration. Despite having the same name, however, planarian and annelid neoblasts are morphologically and functionally distinct, and many annelid species that lack neoblasts can nonetheless substantially regenerate. To further elucidate the functions of the annelid neoblasts, a comparison was made between the regeneration patterns of two enchytraeid oligochaetes, Enchytraeus japonensis and Enchytraeus buchholzi, which possess and lack neoblasts, respectively. In E. japonensis, which can reproduce asexually by fragmentation and subsequent regeneration, neoblasts are present in all segments except for the eight anterior-most segments including the seven head-specific segments, and all body fragments containing neoblasts can regenerate a complete head and a complete tail, irrespective of the region of the body from which they were originally derived. In E. japonensis, therefore, no antero-posterior gradient of regeneration ability exists in the trunk region. However, when amputation was carried out within the head region, where neoblasts are absent, the number of regenerated segments was found to be dependent on the level of amputation along the body axis. In E. buchholzi, which reproduces only sexually and lacks neoblasts in all segments, complete heads were never regenerated and incomplete (hypomeric) heads could be regenerated only from the anterior region of the body. Such an antero-posterior gradient of regeneration ability was observed for both the anterior and posterior regeneration in the whole body of E. buchholzi. These results indicate that the presence of neoblasts correlates with the absence of an antero-posterior gradient of regeneration ability along the body axis, and suggest that the annelid neoblasts are more essential for efficient asexual reproduction than for the regeneration of missing body parts.

Segment formation in Annelids: patterns, processes and evolution

“…Typically, in addition to the prostomium, a fixed number of segments are regenerated ("head segments" in clitellates) but these segments all differentiate more or less at the same time. For example, in Enchytraeus japonicus, where asexual reproduction by fission occurs in addition to anterior and posterior regeneration, a prostomium plus seven head segments are regenerated when the worm is amputated of more than seven anterior segments (Myohara, 2012). By contrast, if the worm is amputated within the "head region", i.e.…”

Investigation into the cellular origins of posterior regeneration in the annelid Capitella teleta

“…the description of neoblasts have been described mostly in clitellates, but also in some polychaetes (Bilello & Potswald, 1974;Cornec, Cresp, Delye, Hoarau, & Reynaud, 1987;Faulkner, 1929Faulkner, , 1932Krecker, 1923;Probst, 1931;Randolph, 1891Randolph, , 1892Stephan-Dubois, 1954;Stolte, 1929;Sugio et al, 2008;Tadokoro, Sugio, Kutsuna, Tochinai, & Takahashi, 2006;Zattara, Turlington, & Bely, 2016). Regeneration abilities are not limited to species with neoblasts; annelids that lack cells with obvious neoblast characteristics can regenerate (Herlant-Meewis, 1964;Krecker, 1923;Myohara, 2012;Stone, 1933). The role of neoblasts in annelid regeneration has not been experimentally tested.…”

On the origin of leeches by evolution of development

“…It has been suggested that a population of ‘set‐aside’ stem cells, or neoblasts , contributes to the blastemal tissue in annelid regeneration (Krecker, ; Randolph, ). It was recently shown that regeneration is incomplete but not totally abolished in the Enchytraeus species lacking neoblast (Myohara, ). Furthermore, knockdown of grimp , a gene expressed in the neoblasts, leads to impaired regeneration in mesoderm, but not in ectoderm and endoderm (Takeo, Yoshida‐Noro, & Tochinai, ).…”

“…The developmental fates of the early and standard m blast cells are the same as the leech, but the supernumerary blast cells may give rise to the stem cells in the PGZ nelid regeneration (Krecker, 1923;Randolph, 1892). It was recently shown that regeneration is incomplete but not totally abolished in the Enchytraeus species lacking neoblast (Myohara, 2012).…”