Why the arterial intima is different from other connective tissues - lack of lymph vessels
Earlier we said that the concentration of LDL in the arterial intima is far higher than it is in any other connective tissue in the body. Other connective tissues contain a separate vascular system - the lymph vessels - that limits the build-up of LDL in the tissue spaces. Lymph vessels are thin tubes that reach into almost every nook and cranny in the connective tissues of the body. They act as sump drains, picking up excess proteins and particles that have leaked out across the capillary endothelial cells into the tissue spaces. The lymph vessels work so well that the concentration of LDL in tissue spaces and in lymph fluid is only one-tenth of the concentration in the blood stream.
The arterial intima, however, possesses no lymph vessels. Why not? The answer is not certain, but it probably relates to high pressures within the tissue of the arterial intima. Lymph vessels operate at very low pressure. If lymph vessels attempted to grow into the intima, they would be collapsed and perhaps no lymph would flow. In any case, lymph vessels are found in the adventitial (outside) layer of arteries and partially in the medial (middle) layer, but not in the intima.
LDL are ball-shaped lipid-protein particles, which are tiny compared to cell dimensions and can pass across the arterial endothelial barrier at a slow rate. LDL slowly diffuse throughout the loose structure of the arterial intima. However, when LDL reach the medial layer, they encounter a tightly packed tissue. In the medial layer, spaces between cells and fibers are packed with a carbohydrate-protein (proteoglycan) meshwork of such fine dimensions that LDL-sized particles cannot enter the tiny pores of the meshwork. Therefore, LDL particles stop at the medial layer. No lymph vessels are available to sump the LDL away. The LDL concentration in the arterial intima rises until it matches the LDL concentration in the bloodstream. This is ten times higher than the LDL concentration in other connective tissues throughout the body.
John R. Guyton, MD
Department of Medicine
Endocrinology, Metabolism, and Nutrition-
Duke University