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A membrane or layer of tissue that covers or envelops an organ or other anatomical structure.



(1) In animals, the peripheral layer of skin coverings of ascidians, salpids, and other animals of the subphylum Tunicata, phylum Chordata. The tunica is a thickened cuticle of cutaneous epithelium that is colonized by cells, a unique phenomenon in the animal kingdom. It serves to protect the internal organs.

The tunica is composed of tunicin, a carbohydrate closely related to cellulose. This substance has a gelatinous consistency, which makes possible the implantation in the tunica of cells that migrate from the cutaneous epithelium. In histological structure, the tunica resembles connective tissue with a gelatinous basal substance. Blood vessels may grow into the tunica. In appendicu-larians, the tunica separates from the body as the result of a process that resembles molting, forming a transparent gelatinous or chitinoid membrane, which serves as an outer covering of the animal or which is appended to the animal externally. This structure is adapted to filtering food particles out of seawater.

(2) In plants, one or more external layers of cells of formative tissue (meristem) covering the corpus of the growing point (apex) in the form of a vault. Dicotyledons have as many as five tunicae, while monocotyledons have as many as three.

Cell division in the tunica predominantly occurs anticlinally, or perpendicular to the surface of the organ; hence the surface of the tunica enlarges without change in the number of cell layers. The epidermis is usually formed from the outer layer of the tunica, which corresponds to the dermatogen according to the histogen theory of the German botanist J. von Hanstein; the primary cortex or part of it is formed from the inner layer of the tunica. The tunica-corpus theory was proposed by the German scientist A. Schmidt in 1924. Sometimes only the layers of cells located higher than the rudimentary leaves are called the tunica. The number of layers of the tunica sometimes increases gradually, but always remains a characteristic taxonomical feature of the plant.

References in periodicals archive ?
Percentage thickness of tunica media with respect to full wall thickness was calculated.
Moreso, sections of ductus arteriosus with lumen, tunica intima, prominent tunica media and thin tunica adventitia were equally observed in all foetal age groups (Fig.
According to the review of interstitial cells of blood vessels, [sup][24] a new cell type termed interstitial cell was indicated in the tunica media of both veins and arteries.
2) The dissection of the coronary artery wall in ECM has been postulated to be due to the disruption of the vasa vasorum, which also explains the presence of hemorrhage between tunica adventitia and tunica media that displaces the inner artery wall toward the lumen.
2) Moreover, the initiation and maintenance of atherosclerotic lesions in parrots is believed to occur mainly in the tunica media.
In some samples, the collagen deposits were extensive, extending from the tunica adventitia into the tunica media, and were typically seen in association with small blood vessels (Fig.
During this length, the vessel has a well-developed adventitia and an underlying thick elastic lamina, with a broad tunica media consisting of smooth muscle, collagen and elastic fibres.
In addition he also showed that as aging occurs smooth muscle cells (SMCs) progressively migrate from the tunica media and accumulate into the tunica intima9.
Those lesions that do develop are believed to arise from the tunica media or from pluripotential undifferentiated mesenchymal cells.
There is variation in the structure of all its coats namely tunica intima, media and adventitia specially tunica media which is the thickest of the three coats.
In all the slides, thickness of tunica intima was measured from luminal border upto internal elastic lamina; tunica media was measured between internal elastic lamina and external elastic lamina whereas thickness of tunica adventitia was measured from external elastic lamina up to its outermost boundary which could be easily identified.