PATTERN OF BROMODEOXYURIDINE INCORPORATION IN THE ADVANCED STAGES OF ARM REGENERATION IN THE FEATHER STAR ANTEDON MEDITERRANEA

The overall process of arm regeneration in the crinoid Antedon mediter ranea is a typical epimorphic process (blastemal regeneration). This c an be subdivided into three main phases: a repair phase, an early rege nerative phase, and an advanced regenerative phase. The crucial proble m of the identification of cell lineages responsible for both repair a nd regenerative processes has been approached by monitoring cell proli feration during the advanced regenerative phase using light-microscopi c and ultrastructural immunocytochemical methods to detect the incorpo ration of the thymidine analogue bromodeoxyuridine (BrdU) into regener ating tissues. Various treatment protocols and BrdU incubation times h ave been employed and provided information not only on the sources, si tes 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 m igratory, morphologically undifferentiated cells (amoebocytes and coel omocytes). (2) The preferential proliferation sites of these cells are the terminal blastema, the coelomic epithelium, and the brachial nerv e of both the regenerating arm and the stump, even far from the amputa tion. (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 co elomocytes, in their turn derived from proliferation of the coelomic e pithelium. (4) The blastemal regeneration of Antedon is due to a combi ned recruitment of pluripotent elements, implying the intervention of presumptive stem cells (amoebocytes) and the transdifferentiation/dedi fferentiation of differentiated elements of the coelomic epithelium.