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Embryonic development and regeneration.
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The fragmenting potworm Enchytraeus japonensis (Oligochaeta, Annelida) reproduces asexually by dividing the body into several fragments that then regenerate to complete individuals in 4–5 days. Such large‐scale regeneration, however, occurs only in some invertebrates. To better our understanding of why regeneration is so limited in many animals, despite their ability to undergo embryonic development from the single cell of a fertilized egg, comparisons were made between regeneration and embryonic development of E. japonensis by using two methods: histochemistry for alkaline phosphatase (ALP) and immunohistochemistry with an antibody against acetylated tubulin that visualizes nervous system development. The analyses revealed that both ALP expression patterns and central nervous system development differ between embryogenesis and the regeneration, suggesting that regeneration is not a simple reiteration of embryogenesis but involves different regulatory mechanisms. The study provides a basis for the elucidation of mechanisms that are unique and crucial to regeneration. Developmental Dynamics 231:349–358, 2004. © 2004 Wiley‐Liss, Inc.
INTRODUCTION
Nuclear transplantation studies over recent years have shown that nuclei of even differentiated mammalian cells contain all the information necessary to produce complete individuals (Wilmut et al., 1997; Kato et al., 1998). This raises an old but as yet unanswered question of why regeneration capacity is so limited in most animals, whereas some can replace lost appendages (amphibians) or fins (fish) or even regenerate complete individuals from small pieces of the body (hydra, planarians). To answer this question, an integrated understanding of the regeneration mechanism of diverse organisms will be required (Sánchez Alvarado, 2000).
Based on a previous study, we have proposed Enchytraeus japonensis, a recently described terrestrial enchytraeid (Nakamura, 1993), as a new material for the study of regeneration (Myohara et al., 1999). Enchytraeids (potworms) are small, whitish oligochaetes that are 2–20 mm long with a body diameter of 0.05–0.5 mm. Among the several hundred species of enchytraeids described worldwide (Didden et al., 1997), eight have been reported to reproduce asexually by fragmentation and subsequent regeneration (Bell, 1959; Christensen, 1959; Nielsen and Christensen, 1963; Bouguenec and Giani, 1987; Nakamura, 1993; Dózsa‐Farkas, 1995). Taking note of its strong potential as a new material for regeneration study, we have carried out a detailed investigation of the regeneration and reproduction of E. japonensis, the only fragmenting enchytraeid reported from Japan (Nakamura, 1993). As a result, we have found that regeneration of a complete individual from a small fragment of the body is accomplished by a combination of epimorphic recovery of the head and tail and the morphallactic transformation of old segments into the appropriate segments to retain the proper body proportions and that both the fragmentation and sexual reproduction can be artificially induced under laboratory conditions (Myohara et al., 1999; Inomata et al., 2000). These properties, together with its simple metameric morphology and ease of culture and handling, make E. japonensis an excellent material for studies of regeneration mechanisms.