Final answer:
The endocardium, the innermost layer of the heart wall, is made up of simple squamous epithelium and lines the cardiac chambers and valves.
Step-by-step explanation:
The layer of squamous cells that lines the cardiac chambers and valves is called the endocardium. This layer is the innermost of the heart wall and is made up of simple squamous epithelium known as endothelium, which is continuous with the endothelial lining of the blood vessels. The endocardium covers the heart chambers and valves, joined to the myocardium by a thin layer of connective tissue. In contrast, the epicardium is the outer layer of the heart wall, and the pericardium is the membranous structure that surrounds and protects the heart.