Morphometric study of the total and perfused arteriolar and capillary network of the rabbit left ventricle

A method has been developed to simultaneously and regionally determine several morphometric indices of both the total and perfused arteriolar and capillary beds of the rabbit heart on a quantitative basis. A high molecular weight fluorescein isothiocyanate (FITC) dextran, was injected into the femoral vein of an anaesthetised open chest rabbit. The hearts were removed and frozen in liquid N2 within 14 s after this injection. Sections, 2 micrometers thick, were obtained from the left ventricular wall and the fluorescence photographed to detect perfused arterioles and capillaries. The sections were then stained to visualise the total arteriolar and capillary network. Standard morphometric techniques were employed to study the total and perfused microvasculature. The method was validated to show that it did not alter the microvasculature of the heart, that both the total and perfused microvasculature could be visualised, and that there was little tissue shrinkage. The study found no significant subepicardial vs subendocardial differences in either the total or perfused capillary of arteriolar networks. The total capillary bed had very much larger morphometric indices than the total arteriolar bed. The percentage of the microvasculature perfused ranged from 55 to 64% of the capillary and 51 to 56% of the arteriolar beds for the various indices. Thus at least a part of the control of capillary perfusions is at the level of the arteriole. There was an almost twofold functional reserve in the microvasculature of the heart which could be mobilised by asphyxia.