Electron microscopic studies of cat mesenteric arterioles: A structure-function analysis

Abstract

Arcade arterioles of cat mesentery were fixed in vivo at normal and elevated intravascular pressures and studied by transmission electron microscopy. In general, cells, filaments, and extracellular fibrils on the luminal side of the internal elastic lamina (IEL) were oriented longitudinally while those of the abluminal side were oriented circumferentially. The IEL presented a smooth, cylindrical appearance while the intima, media, and adventitia were quite irregular in cross section and longitudinal section. The vessel lumen contour was also irregular, especially in the vicinity of endothelial cell nuclei. Endothelial cell processes (ridges) projected through longitudinal slits (fenestrae) in the IEL and made contact with vascular smooth muscle (VSM) cells, often causing apparent deformation of the latter. VSM cells typically possessed multiple processes which were oriented circumferentially. Within the VSM cells myofibrils were oriented tangential to the vessel circumference. Dense bodies were preferentially located on the adventitial surface of the VSM but were also found at junctions between myocytes, suggesting a means of force transmission between cells. Vessels which were constricted with venous pressure elevation showed substantial changes in conformation of the wall structures, which may reflect considerable mobility of wall elements. Several areas of the VSM cell membrane appear to be deformed with intravascular pressure elevation, which may be important in the myogenic response to the pressure rise.