ABSTRACT
It was well known that atrial myocytes systhesize atrial natriuretic peptide[ANP], and secrete it into the atrial lumen through the atrial endocardium. But the mechanism for regulation of ANP secretion has not been clearly elucidated, because there was little information of the atrial morphology concerning basal lamina. Basal lamina is surmised as one of barriers that control the movement of ANP, a large molecule. This study was attempted to elucidate the morphological characteristics of basal lamina and connective tissue fibers of atrial endocardial layer by scanning electron microscopy. Basal lamina was exposed by removal of the overlying endothelium. This was achieved by using OsO4 maceration, immersion in aqueous boric acid or EDTA treatment. After removal of the endothelial cell, the specimens were exposed to ultrasonic vibration in case of need. The external surface of basal lamina showed a fairly smooth appearance on the whole, although a few irregular folds are often encountered. Fenestrations, 0.1-1 micrometer in diameter, were randomly observed on the basal lamina, and they were circular to oval in shape. Margin of fenestrations was somewhat distinct and some was divided into two parts by linear structures. The structural differences of fenestrations between right and left atria were not found. The fibroreticular lamina under the basal lamina was revealed by removal of the endothelial cells and their basal lamina. This layer was consisted of interwoven fine fibers. These fine fibers were repeatedly divided and fused, forming reticular network. Some fine fibers connected with basal lamina. Some connective tissue fibers below fibroreticular layer were collected into thick bundles running parallel to myocytes. Above results may serve as a basis for the physiological and morphological studies of atrium.