Microvasculature of the canine heart demonstrated by scanning electron microscopy.

In previous studies, a marked diminution in left coronary flow during systole has been demonstrated. This phenomenon has been said to result from capillary compressin by the contraction of myocardium and perhaps also from active changes in the state of the smooth muscle in the wall of the coronary vessels. In order to determine the morphological factors involved in this occurrence, and to gain a three-dimensional concept of the myocardial microvasculature, casts were prepared by perfusion of the coronary arteries with methyl methacrylate. Because of the conditions of the perfusion, these casts were considered to demonstrate a state of muscular contraction. Following maceration, portions of the casts were prepared for scanning electron microscopy (SEM). Arterioles of larger diameter were characterized by the helical configuration of the vascular replicas. Smaller vessels were categorized according to their directional pattern and their diameter. The arterioles gave rise to numerous large-diameter capillaries and also to short vessels that were guarded by sphincters. True capillaries, in some cases guarded by precapillary sphincters, were packed in densely stacked parallel sheets with numerous short anastomotic connections between them. Postcapillary venules were found to enter at regular intervals into larger venules arranged perpendicular to the capillary sheets. Most of these characteristics were found to be consistent throughout the microvasculature. These structural features were considered to be factors in the active regulation of blood flow through the myocardial microvasculature.