Histologic and ultrastructural features of normal human parietal pericardium.

Morphologic studies of normal anterior parietal pericardium from seven patients revealed this tissue to be composed of three layers: (1) the serosa, consisting of a surface layer of mesothelial cells and a narrow submesothelial space, (2) the fibrosa, containing variously oriented layers of collagen fibrils and small elastic fibers, and (3) the epipericardial connective tissue layer, containing mainly large coarse bundles of collagen and forming part of the pericardiosternal ligament. Scanning electron microscopic examination is most useful for study of the surface features of pericardial mesothelial cells, which have single cilia and are covered with microvilli. The latter bear friction and increase the surface area for fluid transport. Junctional complexes between adjacent mesothelial cells consist of desmosomes, which reinforce intercellular adhesion and zonulae occludentes, which form permeability barriers. Actin-like filaments (50 A in diameter) are present in microvilli and in immediately subjacent regions of the cells; these filaments mediate changes in cell shape. Intermediate filaments (100 A in diameter) are associated with desmosomes and form bundles in the perinuclear regions; these filaments provide structural support to the cytoplasm.