Cardiac Histology (XI): Atria and Endocardium

Atrial myocytes and endocardium. A. Light micrographs of the lateral wall of a normal adult right atrium. The pectinate muscles are prominent. The atrial wall between the pectinate muscles is composed of a layer of 10 to 20 myocytes located between the connective tissue of the epicardium and the endocardium (H&E 25X). B. Section of atrial tissue adjacent to that shown in A. Note the multiple layers of elastic lamellae arranged in a haphazard fashion throughout the atrial endocardium (Verhoeff 25X). C. View of the endocardium shows three layers. The dense layer directly subjacent to the endothelium collagen and proteoglycan material. The middle layer third shows multiple elastic lamellae mixed with collagen fibers. The inner third of the endocardium shows loose, mostly collagenous connective tissue (Movat pentachrome 100 X). D. Transmission electron micrograph showing the endothelial cell layer and the subjacent collagen bundles. Note the electron-dense basal lamina directly subjacent to the endothelial cell. Embedded within the collagen is a fibroblast that shows a prominent nucleus and very thin cytoplasmic projections. This cell lacks a basal lamina. E. Longitudinal section through the opening of the left superior pulmonary vein into the left atrium. The adventitia of the vein is prominent and abundant. Note the extension of atrial myocytes into the wall of the vein. The arrows point to the transition between atrial myocytes and the wall of the vein. Note the rich elastic network of the wall of the pulmonary vein compared to the thin elastic layer of the atrial endocardium. Also note the thick fibrous tissue layer in the left atrial endocardium. This dense fibrous layer is absent in the right atrial endocardium (see A and B) (Movat pentachrome 5X). F. Ultrasctructure of the atrial myocytes reveals typical electron dense “specific” atrial granules. They are usually round to oval and are limited by single membranes which are slightly ruffled, therefore giving the appearance of being interrupted. These granules store ANP (Atrial Natriuretic Peptide) which plays an important role in the regulation of sodium. (50,000 X)

The next topic is the histology of the pericardium