• apoptosis;
  • BrdU;
  • cell cycle;
  • Macrostomum;
  • platyhelminths;
  • programmed cell death;
  • stem cells;
  • ultrastructure;


In platyhelminths, all cell renewal is accomplished by totipotent stem cells (neoblasts). Tissue maintenance is achieved in a balance between cell proliferation and apoptosis. It is known that in Macrostomum sp. the epidermis undergoes extensive cell renewal. Here we show that parenchymal cells also exhibit a high rate of cell turnover. We demonstrate cell renewal using continuous 5′bromo-2-deoxyuridine (BrdU) exposure. About one-third of all cells are replaced after 14 days. The high level of replacement requires an equivalent removal of cells by apoptosis. Cell death is characterized using a combination of three methods: (1) terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL), (2) specific binding of phosphatidyl-serine to fluorescent-labelled annexin V and (3) identification of apoptotic stages by ultrastructure. The number of cells observed in apoptosis is insufficient to explain the homeostasis of tissues inMacrostomum. Apoptosis-independent mechanisms may play an additional role in tissue dynamics.