Surface of the heart pericardium, coloured scanning electron micrograph (SEM). The pericardium is a double-walled, membranous sac surrounding the heart. The pericardium has a tough outer fibrous layer (fibrous pericardium) and a delicate serous layer (serous pericardium). Shown here is the outer surface of the fibrous pericardium that is composed of connective tissue (fibronectin, collagen and laminin). Flattened squamous epithelial cells (from the upper serous layer) can be seen protruding through the fibrous pericardial layer. The inner serous pericardial layer is the wall adjacent to the heart (not seen). It is split into two layers, the epicardium (just above the heart) and the parietal layer. Between the two layers is the pericardial cavity, which contains serous fluid. The serous pericardium lubricates the heart, preventing friction. Magnification: x5,000 when shortest axis printed at 25 millimetres.
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