RESUMEN
The overall process of arm regeneration in the crinoid Antedon mediterranea is a typical epimorphic process (blastemal regeneration). This can be subdivided into three main phases: a repair phase, an early regenerative phase, and an advanced regenerative phase. The crucial problem of the identification of cell lineages responsible for both repair and regenerative processes has been approached by monitoring cell proliferation during the advanced regenerative phase using light-microscopic and ultrastructural immunocytochemical methods to detect the incorporation of the thymidine analogue bromodeoxyuridine (BrdU) into regenerating tissues. Various treatment protocols and BrdU incubation times have been employed and provided information not only on the sources, sites of proliferation, and recruitment times of the new cells, but also on the cell lineage involved and subsequent fate (differentiation and/or migration) of the labelled cells. Our results are consistent with the following conclusions: (1) The arm regeneration process is due to a massive intervention of active proliferating cells identifiable as migratory, morphologically undifferentiated cells (amoebocytes and coelomocytes). (2) The preferential proliferation sites of these cells are the terminal blastema, the coelomic epithelium, and the brachial nerve of both the regenerating arm and the stump, even far from the amputation. (3) The two main cell components contributing to the regenerate have different origins: the blastemal cells and all the cell lineages derived from the amoebocytes; the coelomic cells from the migratory coelomocytes, in their turn derived from proliferation of the coelomic epithelium. (4) The blastemal regeneration of Antedon is due to a combined recruitment of pluripotent elements, implying the intervention of presumptive stem cells (amoebocytes) and the transdifferentiation/dedifferentiation of differentiated elements of the coelomic epithelium.