Lymphatics and pre-lymphatics of the rabbit pericardium and epicardium with special emphasis on particulate absorption and milky spot-like structures.

The lymphatics and pre-lymphatic connective tissue of rabbit pericardium and epicardium were examined by light and electron microscopy under normal conditions and after the injection of India ink and latex particles into the pericardial cavity. A characteristic lattice structure of connective tissue was present between the small mesothelial cells and the submesothelial lymphatic capillaries in the basal region of the pericardium, but not in the epicardium. Milky spot-like structures bulging toward the pericardial cavity were found in the pericardium, similar to those in the omentum and mediastinal pleura. Within 60 minutes after injection, carbon and latex particles were directly absorbed through the intercellular clefts of the adjacent small mesothelial cells into the submesothelial layer particularly at sites of characteristic lattice structure. Carbon particles were already present in the lumens of lymphatic capillaries at this time. Macrophages in the pericardial cavity and submesothelial layers of the pericardium engulfed both carbon and latex particles. Our results suggest two possible routes of drainage of particulate matter from the pericardial cavity into the lymphatics: direct absorption and indirect absorption after phagocytosis by macrophages. Macrophages probably migrate from the milky spot-like structures described in this study. Epicardial lymphatics, in contrast, drain tissue fluid primarily from the myocardium.