VEGF-VEGFR Signaling Mechanism Directs the Migration of Newborn Hemocytes from the Hematopoietic Site of Oyster Crassostrea gigas

Hematopoiesis is a complex process of creating new hemocytes and releasing them from hematopoietic tissue. In the present study, the hematopoietic site in oyster <i>Crassostrea gigas</i> was successfully identified in the proximal sector (designated G2-G3) of the gill hinge with a substantial number of newborn cells and a minor presence of stem-like cells. The homologues of VEGF (<i>Cg</i>VEGF) and its receptor <i>Cg</i>VEGFR were characterized, and they interacted with each other. After the oysters received an injection of r<i>Cg</i>VEGF, the number of EdU-positive (EdU⁺) cells increased within the G2-G3 sector and the hemolymph. When the expression of <i>Cg</i>VEGFR was inhibited by RNAi, the percentage of EdU⁺ cells in the hemolymph declined dramatically, but increased significantly in the G2-G3 sector and EdU⁺ cells aggregated in this region. Meanwhile, the phosphorylation levels of <i>Cg</i>Erk and <i>Cg</i>JNK, mRNA transcripts of cell proliferation-related and cell migration-related genes, reduced significantly. These results indicate that the proximal region of the hinge in gill was the site producing hemocytes, and <i>Cg</i>VEGF-VEGFR-MAPK signaling pathway induced the migration of newborn hemocytes from this site to the circulating hemolymph, which provides new clues about hematopoiesis in primary invertebrates.