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

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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.) 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.) 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.