Morphology of the constricted arteriolar wall: physiological implications

Microvessels undergo complex shape changes during constriction that could have profound implications for control of resistance. We exploited in vitro cannulation techniques in combination with electron microscopy to assess the effects of physiological degrees of vasoconstriction on the size and form of the lumen of isolated rat mesenteric arterioles. Photomicrographs of vasoconstricted vessels revealed that the luminal surface is folded and thrown into longitudinal ridges several hundred microns long. These ridges begin to form and encroach on the lumen as the vessel is constricted. Ridge height may increase to 5-10 microns, and as many as 50 ridges were observed around the circumference of a 70-microns vessel. Ridges are comprised of endothelial cells, basal elastic lamina, and portions of the smooth muscle cytoplasm including thick filaments. The ridges are major determinants of the relationships among stress on smooth muscle contractile elements, intraluminal pressure, and luminal diameter. The ridges may also limit the precision of measurement of microvessel diameter in situ since it is not known whether the apex or the base of the ridge is measured under typical conditions of in vivo microscopy. Our findings emphasize the need for additional detailed study of wall morphology to fully understand the regulation of microvessel flow resistance by smooth muscle function.