MYOCARDIAL BRIDGE MUSCLE ON LEFT ANTERIOR DESCENDING CORONARY-ARTERY DIFFERS FROM SUBEPICARDIAL MYOCARDIUM OF THE LEFT-VENTRICLE IN DOGS

The myocardial bridge (MB) is a muscle band found sporadically above t he coronary artery (CA) in humans and certain animals such as the dog, cat and sheep. The purpose of our study was to compare the structure of the MB muscle with that of tissue from the subepicardial myocardium . The histological studies included toluidine blue staining of 1-mu m- thick sections and Gomori's trichrome staining of canine cardiac sampl es, The MB muscle of the dog heart is characterized by a distinctive s patial arrangement, with individual fibers separated by substantial el ements of intercellular connective tissue in cross-section. Longitudin ally, the long, slender fibers are aligned continuously with intermedi ation of intercalated disks lying perpendicular to the long axis of th e fibers. In other regions of the left ventricular subepicardial myoca rdium, each myocyte is tightly packed in transverse view. There is gre at variation in the thickness (0.11-2.24 mm, average 0.45 mm) of MBs a nd the distance (24-236 mu m, aver age 103 mu m) between MBs and the l eft anterior descending coronary artery (LAD) among the 13 affected do gs examined, with no apparent relationship between the occurrence of M Bs and either age or sex, These results on MB alignment suggest that t he MB muscle generates force along the long axis of the fiber orientat ion as skeletal muscle does, and with minimal constriction of the CA; if so, the function of MB myocytes may differ from that of common card iac myocytes, as does the structure. Then, the long-supposed downward compressive force of MBs on the LAD would be minimal in most cases; ho wever, when the MB produces a systolic narrowing of the LAD known as t he milking effect, the degree of lateral compression and its influence should depend not only on the substantial size of the MB muscle but a lso on the distance between the MB muscle and LAD, The environment sur rounding die LAD may be a crucial factor in determining whether the MB influences the induction of heart disorders or not.