Endothelium is the simple squamous epithelium that lines the interior surface of blood vessels and lymphatic vessels. It is a specialized type of epithelium that forms an interface between circulating blood or lymph in the lumen and the rest of the vessel wall.

Structure:
• Endothelial cells are thin, flat, and elongated in shape.
• They have a single nucleus and are cylindrical in shape.
• They are tightly packed to form a continuous layer that lines the entire circulatory system.
• The cells are held together by tight junctions and adherens junctions which make the endothelium a selectively permeable barrier.
• They are non-thrombogenic and prevent platelet adhesion and blood clot formation.

Function:
• Endothelium regulates the passage of substances between blood and the surrounding tissues.
• It produces and secretes substances that help to maintain the balance of blood flow and pressure.
• Endothelium plays a role in the immune response and inflammation.
• It regulates the growth and repair of blood vessels.
• Endothelial cells produce nitric oxide, a vasodilator, which helps to regulate blood flow and blood pressure.

Clinical significance:
• Endothelial dysfunction is a common feature of many cardiovascular diseases, such as atherosclerosis, hypertension, and diabetes.
• Damage or dysfunction of the endothelium can lead to impaired blood flow, thrombosis, and inflammation.
• Endothelial dysfunction is also associated with certain conditions such as obesity, smoking, and hypercholesterolemia.