Composition of thrombi in zebrafish: similarities and distinctions with mammals

Background Blood clots are primarily composed of red blood cells (RBCs), platelets/thrombocytes, and fibrin. Despite the similarities observed between mammals and zebrafish, the composition of fish thrombi is not as well known. Objective To analyze the formation of zebrafish blood clots ex vivo and arterial and venous thrombi in vivo. Methods Transgenic zebrafish lines and laser-mediated endothelial injury were used to determine the relative ratio of RBCs and thrombocytes in clots. Scanning electron (SEM) and confocal microscopy provided high resolution images of the structure of adult and larval clots. Adult and larval thrombocyte spreading on fibrinogen was evaluated ex vivo. Results RBCs were present in arterial and venous thrombi, making up the majority of cells in both circulations. However, bloodless mutant fish demonstrated that fibrin clots can form in vivo in the absence of blood cells. SEM and confocal microscopy showed that larval and adult zebrafish and mammalian thrombi look surprisingly similar externally and internally, even though the former have nucleated RBCs and thrombocytes. Although adult thrombocytes spread on fibrinogen, we found that larval cells do not fully activate without the addition of plasma from adult fish, suggesting a developmental deficiency of a plasma activating factor. Finally, mutants lacking αIIbβ3 demonstrated that this integrin mediates thrombocyte spreading on fibrinogen. Conclusions Our data show strong conservation of arterial and venous and clot/thrombus formation across species, including developmental regulation of thrombocyte function. This conservation supports the possibility that mammals also do not absolutely require circulating cells to form fibrin clots in vivo.