The tunica intima is the three-layered inner layer of the lymphatic and blood vessels. In addition to optimal lymphatic and blood flow, this layer provides a barrier to the diffusion of various blood and lymphatic components. Ruptures of the inner tunica intima are a life-threatening phenomenon, especially in the aorta.
What is the tunica intima?
Human blood vessels are composed of different layers. The innermost layer is called the tunica intima. The term comes from Latin and literally means “innermost garment.” The tunica intima not only ensures optimal flow of lymph and blood fluid, but also provides a selectively permeable barrier for dissolved components of the blood. This means that various blood components are allowed entry through the barrier, while other molecules are kept away from the interior of the vessels by the barrier. The tunica intima is itself multilayered, with its individual layers performing different functions. All blood and lymph vessels throughout the body are equipped with a three-layered tunica intima. The individual layers correspond to the endothelium, the underlying subendothelium, and a fiber network with elastic properties. When viewed from the lumen, the tunica intima is directly adjacent to the tunica media, which is the middle layer of the vessels.
Anatomy and structure
The tunica intima has a mean thickness of between seven and 140 µm, depending on the type of vessel. Although the vascular layer is often referred to as the inner layer of the blood and lymph vessels, it itself consists of a total of three layers. Thus, on closer examination, it is actually not “the inner layer” but the “inner layers” of the vascular tissue. The basic layer of the tunica intima is formed by the monocellular and smooth endothelium. This tissue is in direct contact with the contents of the vessels, so especially with blood. Cells of the endothelium are always flattened and have polygonal shape. The nucleus of the endothelial cells lies curved into the lumen of the vessels. In addition to the endothelial layer, the tunica intima carries a subendothelial layer. This layer, as the name suggests, lies beneath the endothelium. Between the endothelium subendothelium lies a thin basement membrane of loose connective tissue. This layer is an elastically fenestrated tissue layer that supports elastic fiber networks, contains fibroblasts, and has smooth muscle fibers.
Function and tasks
A major function of the tunica intima is to establish optimal flow. The inner surface of the endothelial cells is extremely smooth. Because this layer of the tunica intima is in direct contact with the blood, ideal blood flow is ensured. The endothelial surface with lumenal orientation repels solid blood components. Erythrocytes or leukocytes can thus not attach to the vascular tissue. Beyond these tasks, the endothelium assumes the function of a selectively permeable barrier for dissolved blood components. Small-molecule blood components diffuse through the endothelium depending on the concentration gradient, whereas large-molecule components are denied diffusion. In addition, the vascular endothelium is capable of releasing vasoconstrictive as well as vasodilatory substances. In this way, the vascular endothelium controls the state of tension of the vascular smooth muscle and in this way influences blood pressure. The vasomodulatory substances of the endothelium include, for example, nitric oxide. In addition to vascular blood interaction, the endothelium controls interaction and communication with surrounding tissues. This communication and interaction includes the control of vessel wall tone, thrombogenic action, and the control of all processes of metabolic exchange. The endothelium is also relevant for blood coagulation. Drug precursors and active coagulation agents are contained in the endothelium and act on the fibrinolytic coagulation system. For example, heparin sulfate and thrombomodulin inhibit blood clotting and prevent thrombus formation.
Diseases
Erythrocytes accumulate on damaged endothelium. This process characterizes inflammation, for example. The attachment causes the endothelium to produce cell adhesion molecules, which attracts other cells to the scene and induces them to attach to the wall of the blood vessels. Apart from these disease processes, it is the tearing of the tunica intima that has pathological relevance. Tearing of the layer leads to undermining by the blood. Dissection occurs.Aortic dissection is particularly noteworthy in this context. The phenomenon is also called aneurysm dissecans aortae and is associated with a splitting of the wall layers within the aorta. The causative tear of the tunica intima entails hemorrhage between the layers of the blood vessels and causes sudden and severe pain. In the case of aortic dissection, there is acute danger to life. As a consequence of the phenomenon, the aorta may burst, for example, or at least cause circulatory disorders of individual organs. Apart from these disease processes, even a minor injury to the tunica intima can have serious consequences. For example, all injuries to the inner vascular layer can cause intimal hyperplasia. In this case, the tissue layer of the inner vessel wall increases due to exaggerated processes of cell division. In most cases, mainly the smooth muscle cells within the tunica media are involved in the cell proliferation. The processes are subject to stimulation by platelet derived growth factor. Platelets attach to the vascular injury. Extracellular matrix such as collagen and elastin is produced in excess by the influences and may result in narrowing of the lumen or even complete vessel occlusion. Furthermore, as a disease related to the tunica intima, atherosclerosis is relevant, in which fat, thrombi, connective tissue, and calcium attach to the inner endothelium of the vessels. Arteriosclerosis promotes thrombus formation, strokes, and myocardial infarctions.
