Hemocyte differentiation in the juveniles of the ascidian Halocynthia roretzi

We studied hemocyte differentiation in the juveniles of the ascidian Halocynthia roretzi by light and electron microscopy. In the 2d juveniles post-metamorphosis, the hemocoel opens and hemocytes firstly differentiate in it. Those hemocytes were classified into vacuolated cells, granular cells, giant cells, and undifferentiated mesodermal cells. Around day 5 after metamorphosis the mesodermal cells gradually transform to hemoblasts. Therefore the juveniles have only three categories of differentiated hemocytes and the mesodermal cells or the hemoblasts. The scarcity of the hemocyte types and an abundance of immature hemocytes in the juveniles allowed us to follow their differentiation pathways distinctly. We revealed the process of cell differentiation characteristic to each hemocyte category. The circulating hemoblasts most likely function as hematopoietic stem cells in the early stages of ascidian hematopoiesis.