A QUANTITATIVE STUDY OF THE VENTRICULAR MYOARCHITECTURE IN THE STAGE-21-29 CHICK-EMBRYO FOLLOWING DECREASED LOADING

During the early developmental period, ventricular myoarchitecture und ergoes a transition from a smooth-walled cardiac tube, to left and rig ht ventricular chambers filled with a sponge-like network of trabecula r struts. We measured the quantitative changes of ventricular myocardi um properties in normal stage 21-29 chick embryos and after chronic ve rapamil suffusion, which is known to decrease work load and decelerate ventricular growth. The morphologic parametres (compact layer thickne ss, ventricular wall composition, porosity of different layers and tra becular orientation) were determined from scanning electron micrograph s of transversely dissected perfusion-fixed hearts. A vascular bed of stage 21 chick embryos was suffused with 1 ng of verapamil at 1 mu l p er hour up to stages 24, 27 and 29 via a miniosmotic pump. From stage 24, the thickness of the compact myocardium in the left ventricle was greater than that of the right. The increase in thickness was minimal between stages 24 and 27, while the predominantly radially arranged tr abeculae comprised up to 75% of total myocardial mass. The ratio of in tertrabecular spaces to trabeculae (local porosity) decreased from the ventricular center (70%) towards the compact myocardium (0%). In vera pamil-treated embryos, the hearts were smaller and showed delayed deve lopment. The compact myocardium was thinner than normal, and the propo rtion of trabeculae was higher than in controls. The local porosity va lues were similar in control and experimental groups. Decreased load r esulted in delayed growth and morphogenesis, expressed as a persistenc e of trabeculae and a thinner compact myocardium. Embryonic heart pump ing function is largely based on extensively developed trabeculation w ith regionally different properties.