Tunica Media of Vessel

Structure

The tunica media is the middle layer of the artery wall and comprises mainly of smooth muscle. This layer is generally the largest layer within arteries.

The composition of the tunica media contributes directly to the mechanical properties of the vessel. The tunica media is formed by layers of smooth muscle cells, which play a role in vasoconstriction and vasodilation. Furthermore, a scarce elastic lamella is present throughout the tunica media enabling it to return to its original shape after being stretched. Small collagen bundles also provide structural support, and proteoglycans embedded throughout the tunica media allow the vessel to withstand compressional forces.

Anatomical Relations

The tunica media is found between the tunica intima and the tunica externa within the artery wall. The external elastic lamina separates the tunica media from the tunica externa. The internal elastic lamina divides the tunica media from the tunica intima.

The smooth muscle in this layer is predominantly arranged in a helical manner, this means that the smooth muscle is coiled in a spiral fashion. Located towards the outer portion of the tunica media are layers of longitudinal vascular smooth muscle cells that synthesize and secrete elastin, collagen, and other extracellular components to supplement the structural integrity of the tunica media (Pawlina, 2016).

Function

The mechanical strength of the tunica media comes from structural collagen and smooth muscle. The smooth muscle coiled around the vessel can contract or relax, altering the diameter of the vessel's lumen, which causes the lumen to decrease or increase. This is known as vasoconstriction and vasodilation, respectively.

List of Clinical Correlates

—Hypertension

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Pawlina, W. 2016. Histology: A text and atlas with correlated cell and molecular biology. 7th ed. Philadelphia: Wolters Kluwer.