Ultrastructural aspects of capillary fusion during the development of thyroid hyperplasia

Abstract

When young adult male Fischer rats are fed a low-iodine diet containing 0.25% thiouracil the blood capillaries enlarge and appear to fuse, with the temporary appearance of partial septa between capillary lumina. The mechanism of the fusion was examined by electron microscopy. The partial septum was made up of intact cells with a tight junction near its distal end. A mechanism to form such partial septa must involve the penetration, by endothelial cells, of two basal laminae and a collagen layer between the capillary walls. The penetration may be made by pseudopods on the abluminal surface of many endothelial cells, about half of which penetrated basal laminas. The fusion may involve pairs of pseudopods at cell margins. Multiple small fusion sites between neighboring walls of parallel capillaries observed in serial sections may be an intermediate stage in capillary fusion. Aggregates of blood platelets were observed near collagen exposed when endothelial cells retracted.