The geometry and architecture of ventricular intramural coronary microves sels less than 10 um in diameter, were studed using both scanning and transmission electron microscopy. These vessels were oriented parallel to the longitudinal axes of the adjoining working cardiomyocytes in the differ ent myocardial layers. They were constructed of attenuated endotheliocytes invested by a basal membrane. The endotheliocytes were non-fenestrated and showed a variable number of simple and compound membranous vesicles
which were either attached to the plasmalemma proper of appear free in the cytoplasm. The luminal endothelial surface showed many villous-like flexible outgrowths projecting into the lumen resulting in the appearance of luminal artificial vesicles. The ends of the contiguous endotheliocytes were sealed together by typical desmosomes.
We observed the presence of striking membrane-bounded vesicles of different sizes, containing electron-lucent flocculent material in the minute spaces between myocytes and capillaries. Some of these vesicles appeared to be emanating from the sarcolemmae, and others were detected in the lumina of the adjacent capillaries and post-capillary venules. The present investigation concluded that these vesicles constitute a morphological basis for endocrine activity between the working ventricular cardiomyocytes, and the surrounding capillaries.