ULTRASTRUCTURE OF BLOOD-VESSEL REGRESSION IN INVOLUTION OF FOREIGN-BODY GRANULOMA

The diversity of endothelial cell deletion in regressing blood vessels during involution of granulation tissue was investigated in foreign-b ody granulomas induced by a collagen sponge implanted into the dorsum of the rat. Small blood vessel density counts were performed to determ ine the degree of blood Vessel regression in the involution of granula tion tissue in foreign-body granuloma. The density of these small bloo d vessels significantly decreased between 100 and 130 days after spong e implantation. The detachment of endothelial cells from their underly ing basement membrane and the consequent protrusion into and/or out of the lumen in the vascular network of granulation tissue was observed by electron microscopy. Based on the pattern of manifestation in the e ndothelium and the characteristic deformation of nuclei, the detached endothelial cells were classified into two groups: 1) the endothelial cell apoptosis group and 2) the endothelial cell degeneration group. T he essential difference between the two groups was easily distinguisha ble as the nucleus of the former group displayed chromatin condensatio n and margination as the hallmark of early apoptotic changes, while th e nucleus of the latter group displayed a pinch structure and intranuc lear pocket formation. However, the process by which the detached endo thelial cells were shed into the vascular lumen and eventually elimina ted from circulation was the same in both groups. The occurrence of bo th groups increased on the 90th day after sponge implantation and reac hed a maximum on the 110th day, indicating that the appearance of the groups was synchronized. These results suggest that the two major proc esses of apoptosis and degeneration of endothelial cells occur during endothelial cell deletion as a mechanism contributing to blood vessel regression.