intima

Intima, or tunica intima, is the innermost layer of blood vessels and lymphatic vessels, lining the lumen. It is primarily composed of endothelial cells that rest on a basement membrane and are supported by a thin subendothelial connective tissue layer. In larger arteries, the intima is separated from the tunica media by an internal elastic lamina, a distinct elastic fiber layer.

The structure of the intima varies by vessel type. In arteries, the endothelium is relatively thick and the subendothelial layer is more substantial, with an internal elastic lamina present. In veins, the intima is typically thinner and the internal elastic lamina is usually absent or indistinct. Capillaries consist almost entirely of a simple endothelium with a surrounding basement membrane and little or no subendothelial tissue. The heart has a related lining—the endocardium—but it is a different anatomic structure from the tunica intima of vessels.

Functionally, the intima provides a selective barrier between blood and surrounding tissue, participates in the regulation of vascular tone and hemostasis, and produces antithrombotic and anti-inflammatory factors. Endothelial cells respond to shear stress and contribute to processes such as coagulation, fibrinolysis, and immune cell adhesion.

Clinical relevance arises when the intima thickens or becomes diseased. Intimal hyperplasia and atherogenesis involve the intima and can lead to luminal narrowing and vascular complications. Injury to the intima can trigger neointimal formation after procedures or injury, contributing to restenosis.