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-body granulomas induced by a collagen sponge implanted into the dorsum of the rat. Small blood vessel density counts were performed to determine the degree of blood vessel regression in the involution of granulation tissue in foreign-body granuloma. The density of these small blood vessels significantly decreased between 100 and 130 days after sponge implantation. The detachment of endothelial cells from their underlying 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 endothelium 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. The essential difference between the two groups was easily distinguishable as the nucleus of the former group displayed chromatin condensation and margination as the hallmark of early apoptotic changes, while the nucleus of the latter group displayed a pinch structure and intranuclear pocket formation. However, the process by which the detached endothelial cells were shed into the vascular lumen and eventually eliminated from circulation was the same in both groups. The occurrence of both groups increased on the 90th day after sponge implantation and reached a maximum on the 110th day, indicating that the appearance of the groups was synchronized. These results suggest that the two major processes of apoptosis and degeneration of endothelial cells occur during endothelial cell deletion as a mechanism contributing to blood vessel regression.