A contribution to the problem of the origin of the blastema cells in planarians: A karyological and ultrastructural investigation

Abstract

The basic problem in Planarian regeneration, i.e., the origin of blastema cells, has been tackled by means of a karyological investigation carried out on a triplo-hexaploid biotype of Dugesia lugubris. The main characteristic of this biotype is that male germ cells are karyologically distinguishable from female germ cells and from the somatic cells.Sexually mature specimens were transected at a caudal level, posterior to the testes: from each transected specimen two regenerants were obtained, both of which formed a blastema. Thirty-six hours after the transection most of the metaphase cells of the anterior regenerant blastema (a.r.b.1 had 12 chromosomes. However, cells with 8 chromosomes as well as cells showing signs of chromosome elimination were also present. Three days after the transection only cells with 8 or 12 chromosomes could be detected in the a.r.b.The ultrastructural examination of the 36-hours-old a.r.b. revealed that some blastema cells display features which can be interpreted as indicating a process of dedifferentiation, the others being virtually undifferentiated. In the posterior regenerant blastema (p.r.b.1 of sexually mature specimens only cells with 12 chromosomes were present. The same applies to both the a.r.b. and p.r.b. of sexually immature specimens and to developing embryos. These findings suggest that in the sexually mature planarians studied, the blastema is formed mainly by triploid cells. However, in those regenerants which still possess testes, immature male germ cells can also contribute to the blastema formation. The possible role played by these male germ cells in the reconstitution of the missing tissues in the biotype studied is also discussed.One of the major problems in planarian regeneration concerns the nature and the origin of the cells which form the blastema. As is well known, the process by which the blastema is formed is currently explained according to two main theories. Several authors (cf., Wolff and Dubois, '47; Dubois, '49; Lender, '62; Brsndsted, '69) maintain that neoblasts -pluripotent, embryonic cells dispersed throughout the animal body -are the sole cell type responsible for regeneration. On the other hand, other workers (Steinmann, '08; Lang, '12; Hyman, '51; Woodruff and Burnett, '65; Rose and Shostak, '68; Hay, '68; Coward, '69) believe that, as a result of a regenerative stimulus, many specialized cells can J. EXP. ZOOL., dedifferentiate and migrate to the wounded region where they form the blastema. The missing tissues would then be reconstituted by redifferentiation of the dedifferentiated cells.Evidence obtained from several lines of experiment (for a recent survey on the subject, see Brsndsted, '69; Pedersen, '72; Gremigni, '74; Chandebois, '76) do not yet allow reconciliation of these two conflicting theories. It is possible that the mechanisms adduced by the two quoted theories to explain the formation of the blastema, i.e., the involvement of embryonic reserve cells or the dedifferentiation

Positional information and gonadal differentiation in the planarian Dugesia lugubris (Turbellaria)

“…Coward (1969) in reference to the opinion of other authors admits that the blastema originates from the dedifferentiation of other cells followed by their multiplication migration and redifferentiation in the blastema . One of the more demonstrative cases of such dedifferentiation is that described by Gremigni & Puccinelli (1977) in which spermatogonia reportedly dedifferentiated and then redifferentiated to give rise not only to other male germ cells but also to oocytes and somatic cells . Such dedifferentiation does not however necessarily exclude the possible existence of cells that maintain an embryonic character i .e.…”

The past and present of planarians - An interview with Vittorio Gremigni

“…Thus, I suggested that regeneration in planarians is mainly due to the small undifferentiated totipotent (or pluripotent) cells, the so-called neoblasts, and, to a lesser extent, also to cells (of the germ and perhaps also of the somatic lines) at the very beginning of their differentiation process which, following a planarian transection, can interrupt their pathway towards specialization and go back to the totipotent (pluripotent) state (Gremigni and Puccinelli, 1977;Gremigni et al, 1980 a,b;Gremigni and Miceli 1980;Gremigni, 1981;Gremigni et al, 1982). These findings, obtained at the end of the seventies A B Salò et al, 2009;Abril et al, 2010;Blythe et al, 2010;Hayashi et al, 2006Hayashi et al, , 2010Shibata et al, 2010;Aboobaker, 2011;Adamidi et al, 2011;Fernández-Taboada et al, 2011;Gentile et al, 2011;Qin et al, 2011).…”

Germ Cell Specification and Regeneration in Planarians

“…There are two lines of evidence for some degree of plasticity within the pool of neoblast progeny: One involves the germ cell lineage; the other, the somatic lineage. Experiments conducted by Gremigni and coworkers suggested that the planarian germ cell lineage was capable of contributing to the regeneration blastema and producing somatic cells (Gremigni and Puccinelli 1977;Gremigni and Miceli 1980;Gremigni et al 1980a,b). These experiments used a mosaic strain of Dugesia lugubris (Schmidtea polychroa) in which the somatic cells are triploid.…”