ULTRASTRUCTURE AND PERMEABILITY OF ENDOTHELIAL-CELLS IN BRANCHED REGIONS OF RAT ARTERIES

The ultrastructure and the permeability to macromolecules of the endot helia in the branched and unbranched regions of the arteries were comp ared using two different age groups (3 and 12 months) of rats. In the aortic arch, the endothelial cells were longer and thinner and contain ed fewer intracytoplasmic vesicles than those observed in the unbranch ed regions of aorta. Quantitative study revealed that the volume densi ty of intracytoplasmic vesicles in the branched regions of aortic arch in 3-month-old rats was significantly (P < 0.01) lower than the densi ty value in the unbranched regions of aorta. The volume densities of v esicles in both regions of the aorta were lower than those in the caro tid artery. There was an apparent increase in the frequency of the sim ple type of interendothelial contacts and a decrease in the complex ty pe in the branched regions as compared with those in the unbranched re gions of aorta and carotid artery. In addition to the normal interendo thelial contacts, several open junctions with increasing width (25-300 nm) were identified in the branched regions of aortic arch and the bi furcations of carotid artery. For rats at the age of 12 months, local areas of the subendothelial space were expanded. Basal lamina-like and electron-dense materials were accumulated in the subendothelium. The volume densities of vesicles in the aortic endothelia were significant ly (P < 0.01) increased as compared with those in the 3-month-old grou p. The volume density of vesicles in the aortic arch was again signifi cantly (P < 0.01) lower than that in the unbranched regions of aorta. Furthermore, the frequency of the simple type of intercellular contact s was increased, whereas that of the complex type was decreased in bot h regions of aorta. With regard to the junctional complexes, the frequ encies of gap junctions and tight junctions were increased and the jun ctionless intercellular contacts were decreased compared with those of the 3-month-old group.