Endothelial basement membrane and seamless-type endothelium in the repair process of cerebral infarction in rats.

Ultrastructural analysis of capillary changes during the repair process of experimental cerebral infarction induced in rats was carried out with special reference to the endothelial basement membrane (BM) and seamless-type endothelial cells. Following degeneration of endothelial cells and pericytes, their BMs, without any interruption or fragmentation, were left in the lesion. Newly formed capillaries grew from vessels in the surrounding brain tissues into the reactive zone of infarcts. While the capillaries in cross-section possessed multilayered BMs, these membranes in tangential section comprised an outer BM with extremely wavy profile and an inner one showing a normal trilayered structure, uniformly enveloping the endothelial surface. It is therefore suggested that the sprouting of regenerating capillaries might invade the remaining cavities of BM, resulting from endothelial degeneration. In these new vessels, seamless-type endothelial cells lacking interendothelial contacts were observed frequently. These two different and previously unobserved findings appear to be at the heart of the regeneration mechanism of reactive capillary proliferation.