Ultrastructure of rat aortic grafts.

This study compares the ultrastructure of three syngeneic and three allogeneic grafts of rat abdominal aorta (Lewis to Lewis and BN to Lewis, respectively); the tissue was sampled three months after transplantation (TPL). The endothelial plate was preserved and mononuclear cell adherence was absent. In syngeneic grafts the intima and media remained close to normal with well-preserved smooth muscle cells (SMC). The thickened allograft neointima consisted of elongated spindle cells and rich intercellular matrix. The cells were typical SMC without apparent signs of dystrophy or degeneration. On the other hand, most SMC of the media showed complete disruption and disorganization of membrane and organelles suggestive of accomplished necrosis. However, the framework of elastic lamellae was preserved, without apparent ruptures or lytic changes. Intraintimal migration of medial SMC was not recorded while some cytoplasmic strips were seen to extend across the outer elastic lamella (possible rudimentary outgrowth of SMC?). Lymphocytes and histiomonocytic cells (macrophages) were found in the adventitia but not in the destroyed media. Thus electron microscopy elucidated the histological picture of "anuclear allograft media" and confirmed the predominance of SMC in the thickened neointima. However, signs of the mediointimal SMC invasion were not apparent three months post TPL.