A longitudinal immunohistochemical study of the healing of experimental aneurysms after embolization with platinum coils

BACKGROUND AND PURPOSE: The purpose of this study was to probe the cellular mechanism of healing in aneurysms after platinum coil embolization, by using multiple special stains and immunolabels. METHODS: Elastase-induced aneurysms were created and embolized in 28 rabbits. Aneurysms were excised between 2 and 24 weeks after embolization. Specimens were embedded in paraffin, sectioned, and stained with hematoxylin-eosin, Masson trichrome, and multiple immunostains. RESULTS: At 2 weeks, peripheral sparse spindle-nucleated cells were positive for alpha-smooth muscle actin (SMA), myosin, and vimentin, indicating myofibroblastic differentiation. At 4 weeks, all spindle-nucleated cells in the aneurysm were positive for SMA, myosin, desmin, and vimentin. Ten weeks after embolization, positive immunohistochemical staining in the cells populating the aneurysm significantly decreased. Mean positive SMA cells, per high-powered field were 5 +/- 3, 45 +/- 9, 10 +/- 5, 0 +/- 0, and 0 +/- 0 at 2, 4, 10, 16, and 24 weeks, respectively. Findings of a Kruskal-Wallis test showed these data to be significantly different (P =.0001). Post hoc tests revealed significantly greater amounts of SMA-positive staining in the cells at 4 weeks compared with those at 2, 10, 16, and 24 weeks (P < .05). In addition, the 10-week group had significantly more positive cells than the 16- and 24-week groups (P < .05). There was a 78% decrease in apoptotic cells between 4 (37 +/- 11) and 10 weeks (8 +/- 4) after implantation. Apoptotic cells were completely absent beyond 10 weeks. CONCLUSION: Aneurysm healing, in response to platinum coil embolization, appeared to progress through the stages of thrombus formation, granulated tissue organization, and loose connective tissue formation. Myofibroblasts, the key cellular component involved in healing, appeared within the aneurysm early. They progressively reduced in number with time and finally disappeared through the mechanism of apoptosis.