Pericardium of the frog, Rana esculenta, is morphologically designed as a lymphatic space

The importance of the pericardium and the pericardial fluid (PF) in the control of cardiac function has emerged over the past few years. Despite the acknowledgment that amphibians are exposed to both dehydration and excessive water accumulation, nothing is known about their pericardial structure and the morphological basis of the PF formation. We have studied the parietal pericardium (PP) morphology in Rana esculenta by electron microscopy. SEM images of the inner surface, which lines the pericardial cavity, revealed the presence of large vesicles and many small circular openings. TEM observations showed that the PP is made up of an inner mesothelial lining, often constituted by two layers …