ASK1/p38 signaling axis regulates the tube formation of retinal microvascular endothelial cells. (A) The tube formation was markedly enhanced in the HG group of the HRMECs and was inhibited by blocking of ASK1. Then, the (B) tube meshes, nodes and tube length were counted, and the results revealed that the HRMECs in the HG group formed more meshes and nodes than the control; in addition, the tube total length in the HG group was higher than that of the control group. However, after blocking of ASK1, the tube meshes, nodes and tube length were reduced compared with the HG group. In addition, (C) the tube formation was inhibited by blocking of p38 and the (D) tube meshes, nodes and tube length were counted; the results revealed that the blocking of p38 inhibited the tube formation. *P<0.05 and **P<0.01 compared with the control; #P<0.05 and ##P<0.01 compared with the relative HG-induced HRMEC cell group. ASK1, apoptosis signal-regulating kinase 1; HG, high glucose; HRMECs, human retinal microvascular endothelial cells.