Spatial distribution and differentiation potential of stem cells in hatchlings and adults in the marine platyhelminth macrostomum sp.: a bromodeoxyuridine analysis.

Stem cells (neoblasts) in Platyhelminthes are pluripotent, and likely totipotent, undifferentiated cells which retain throughout adult life the capacity to proliferate and from which all somatic cells as well as the germ cells derive. However, basic data on the pool and heterogeneity of neoblasts, their rates of differentiation into sets and subsets of differentiated cells, and their migration to different body regions are still lacking. To fill this gap, S-phase cells in the macrostomid Macrostomum sp. were labeled with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU). S-phase cells were found to be neoblasts and to be distributed in two bands along the lateral sides of the body leaving unlabeled the median axis of the body and the region anterior to the eyes. This distribution is parallel to that of mitotic cells demonstrated using an antibody to phosphorylated histone H3. At different chase times, clusters of BrdU-labeled cells appear, labeled cells migrate to formerly unlabeled areas, and they differentiate into several somatic cell types and into germ cells. Finally, continuous exposure to BrdU shows an extensive renewal of the epithelial cells. Altogether, these results strengthen the idea of platyhelminth neoblasts as an unparalleled stem-cell system within the Animal Kingdom calling for further investigation.