Hemoblasts in colonial tunicates: are they stem cells or tissue-restricted progenitor cells?

Colonial tunicates have hemoblasts, which are undifferentiated coelomic cells that play a key role in tissue renewal during reproduction and regeneration. Some hemoblasts differentiate into somatic lineage cells such as endodermal multipotent epithelial, cardiac and body-wall muscle, and blood cells. There is no well established evidence that somatic hemoblasts are stem cells. Rather, like tissue-restricted progenitor cells, some peripheral hemoblasts give rise to terminally differentiated cells, while other hemoblasts differentiate into germ cells and accessory cells. Unlike somatic lineage cells, germ cells and their precursors express vasa homologues in common. In some colonial tunicates, vasa is indispensable for germ cell development. All vasa-positive hemoblasts appear to differentiate into germ cells, suggesting that most of them are tissue-restricted progenitor cells. When a colony is naturally or experimentally depleted of vasa-expressing cells, vasa and vasa-expressing germ cells can reappear in the colony. We speculate that, in addition to tissue-restricted progenitor cells, highly potent stem cells which regulate the activities of blastogenesis and gametogenesis and eventually cause soma-germ conflict in colonial tunicates may exist in colonial tunicates.