777seem to be prerequisites for the maintenance of cell shape and polarity of cell movement in relation to the substra-31 tum . Endocytotlc vesicles exist, mainly in the cytoplasm luminal part. It is known that the plasminogen activator is associated with vascular endothelium 32 and is released from these cells into the surrounding medium 33. It is now generally accepted that this is the mechanism by which intravascular fibrin is lysed, so it can be thought that re-endothelialization of venous patch graft may be necessary for lysis of deposited fibrin in the vascular graft wall. Recently 34 endothelial cells have been shown to produce a growth factor in vivo which may play a part in the induction of smooth muscle cell movement and differentiation. Our recent work confirmed such a role of endothelium 2, for the formation of a new endothelium is immediatly followed by the appearance of smooth muscle cells in the venous patch wall. Present findings do not indicate that the endothelial cells which cover the venous pateh after the 2nd postoperative week originate from the smooth muscle cells of the patch, since the thrombus formed on the luminal part of the patch prevents cell migration from the medial layer of the patch wall, although a hematogenous re-endothelialization may occur at the level of the suture line 25. Our results indicate that secondary endothelial cells originate mainly in the adjacent arterial endothelium. Future studies using radioactive labelling should clarify this postulate.