The endocardium of the human heart (400X)

The endocardium is the innermost layer of the heart wall and is composed of a layer of endothelial cells supported by a thin layer of connective tissue. The endothelial cells of the endocardium are continuous with the endothelial lining of the blood vessels that enter and exit the heart, creating a seamless barrier between the heart chambers and the rest of the circulatory system. In this image, the cardiomyocytes are cut across.

Histologically, the endocardium appears as a single layer of squamous or cuboidal cells that form a smooth, continuous surface lining the heart chambers. The cells of the endocardium are bound together by tight junctions, which prevent the leakage of blood and fluid between the chambers. The connective tissue layer that supports the endothelial cells contains scattered fibroblasts and elastic fibers, which provide structural support and help regulate the function of the heart.

While the endocardium may not be the most glamorous part of the heart, it has a certain elegance to it. Its smooth, seamless surface is essential for efficient blood flow and is a testament to the beauty of the human body's design.