Hidden in the crowd: primordial germ cells and somatic stem cells in the mesodermal posterior growth zone of the polychaete Platynereis dumerillii are two distinct cell populations

In order to verify this hypothesis, embryos and larvae were pulse labeled with the proliferation marker 5-ethynyl-2'-deoxyuridine (EdU) at different stages of development. Subsequently, the PGCs were visualized in 7-day-old young worms using antibodies against the Vasa protein.Surprisingly, the primordial germ cells of Platynereis incorporate EdU only shortly before gastrulation (6-8 hours post fertilization (hpf)), which coincides with the emergence of four small blastomeres from the mesoblast lineage. We conclude that these so-called 'secondary mesoblast cells' constitute the definitive PGCs in Platynereis. In contrast, the cells of the MPGZ incorporate EdU only from the pre-trochophore stage onward (14 hpf).While PGCs and the cells of the MPGZ in Platynereis are indistinguishable in morphology and both express the germline markers vasa, nanos, and piwi, a distinct cluster of PGCs is detectable anterior of the MPGZ following EdU pulse-labeling. Indeed the PGCs form independently from the stem cells of the MPGZ prior to gastrulation. Our data suggest an early PGC formation in the polychaete by preformation rather than by epigenesis.In many species, ranging from sponges, and cnidarians, to flatworms, annelids, tunicates, and sea urchins, both primordial germ cells (PGCs) and somatic stem cells are characterized by the expression of a similar set of genes, namely vasa, nanos, piwi, and PL 10 [1-7]. This fact led to the hypothesis that PGCs and stem cells are not only closely related, but also share a common gene regulatory module, the 'germline multipotency program' [8,9]. Numerous examples show that pluripotent stem cells give rise to both PGCs and somatic derivates, such as choanocytes in sponges, large I-cells in cnidarians, neoblasts in planarians, or epiblast cells in mice [3,7,10,11]. The generation of PGCs by undifferentiated cells occurs after gastrulation and sometimes is referred to as 'epigenesis' [12]. It requires inductive signals by the surrounding tiss