Tunica

Tunica is a Latin term meaning cloak or covering. In biology and medicine, it denotes a covering or layer that envelops an organ, tissue, or structure. The concept is most often used to describe the walls of tubular organs, particularly blood vessels, but it extends to other coverings as well.

In blood vessels, the wall is typically described as consisting of three tunicae: tunica intima, tunica media, and tunica externa (also called adventitia). The tunica intima is the innermost layer, made up of an endothelial lining and a subendothelial layer. The tunica media contains smooth muscle cells and elastic fibers that regulate vessel caliber and blood flow. The tunica externa is a connective tissue sheath that anchors the vessel and may house small vessels (vasa vasorum). Arteries generally have a thicker tunica media, while veins have a relatively thick tunica externa and a thinner media. Capillaries are simplified to a single tunica intima.

Other commonly named tunicae include the tunica albuginea, a dense connective tissue capsule surrounding certain organs, notably the testes and the corpora cavernosa of the penis. The tunica vaginalis is a serous covering of the testes with visceral and parietal layers that provides lubrication and allows movement.

In plant biology, the term tunica also appears in the histology of the shoot apical meristem, describing an outer layer (the tunica) around an inner mass of initials (the corpus) that influences growth patterns.

Etymology and usewise, tunica consistently marks a distinct envelope or layer that serves protective, mechanical, or functional roles in diverse organisms.