Proliferative response of the stem cell system during regeneration of the rostrum in Macrostomum lignano (Platyhelminthes).

Macrostomum lignano (Platyhelminthes) possesses pluripotent stem cells, also called neoblasts, which power its extraordinary regeneration capacity. We have examined the cellular dynamics of neoblasts during regeneration of the rostrum in M. lignano. First, using live squeeze observations, the growth curve of the rostrum was determined. Second, neoblasts were labelled with 5-bromo-2'-deoxyuridine (BrdU) and an anti-phospho-histone H3 mitosis marker (anti-phos-H3) to analyze their proliferative response to amputation. During the regeneration process, both S- and M-phase cells were present anterior to the eyes, a region that is devoid of proliferating cells during homeostasis. Furthermore, BrdU pulse experiments revealed a biphasic S-phase pattern, different from the pattern known to occur during regeneration of the tail plate in M. lignano. During a first systemic phase, S-phase numbers significantly increased, both in the region adjacent to the wound (the anterior segment) and the region far from the wound (the posterior segment). During the second, spatially restricted phase, S-phase numbers in the anterior segment rose to a peak at 3 to 5 days post-amputation (p-a), while in the posterior segment, S-phase activity approached control values again. A blastema, characterized as a build-up of S- and M-phase cells, was formed 1 day p-a. Altogether, our data present new insights into the cellular response of the neoblast system upon amputation, clearly demonstrating important differences from the situation known to occur during regeneration of the tail plate. Furthermore, the presence of proliferating cells in the region anterior to the eyes shows a clear alteration in stem cell regulation during regeneration.