Morphologic changes in arterial grafts in rabbit ears.

The morphology of auto- and allografted segments of rabbit central ear arteries was studied at various times after grafting. Autografts showed thrombosis only in the immediate postoperative period. Autografts developed intimal thickenings whose cellular elements at all stages were almost exclusively myointimal in type. Their medial smooth muscle cells were viable at all stages. Allografts frequently thrombosed within 8 weeks of grafting. Intimal thickenings that developed in the first 6 weeks in allografts mainly contained infiltrating hematogenous cells with few myointimal cells. Immune cells infiltrated all of the layers of allograft walls, and the smooth muscle cells of their medias showed increasingly severe degeneration and by 8 weeks had completely disappeared. In long established allografts, the intima was extremely thick and contained myointimal cells and fibrous tissue. Their medias were fibrosed. In long standing allografts, immune cell infiltration was no longer present. The thrombosis of allografted arteries that occurred within 8 weeks of grafting was related to immunologic events observed within the vessel grafts. Differences between myointimal cells and smooth muscle cells with regard to their morphology and orientation were identified. A possible origin of myointimal cells from endothelial cells is suggested.