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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 ?
The modified procedure with dartos fascial flapI was similar except that in the second stage after tubing of the neourethral plate, circumcision incision was made subcoronally.
Snodgrass hypospadias repair with vascularized dartos flap: the perfect repair for virgin cases of hypospadias J Urol 2002:168;1723-6.
The testis was placed in the scrotum fixed to dartos fascia with an absorbable suture through tunica albugenia and skin was approximated with su cuticular or closed with absorbable sutures (Fig- 2 and3).
This neourethra was then covered with a vascularised dartos flap harvested from subcutaneous tissue of dorsal prepucial skin.
Although secondary calcification of epidermal cysts, dystrophic calcification of the dartos muscle and calcification of eccrine cysts seem to be considered as the causes of scrotal calcinosis, it is still assesed as an idiopatic disorder.
The resection margin can be divided in 3 important areas that need to be analyzed: the skin of the shaft with underlying dartos and penile fascia; corpora cavernosa with albuginea; and urethra with periurethral cylinder that includes lamina propria, corpus spongiosum, albuginea, and penile fascia (Figure 1).
The tunica dartos envelops each testis and extends between them to form a septum that isolates the testes in separate compartments.
2006), persistiendo como una estructura anatomicamente evidente solo a nivel del musculo dartos (Besana-Ciani & Greenall, 2005).
DartOS, which is master-less, load sharing and redundant, is made up of layers of software that execute in parallel on multiple CPUs and micro-controllers.
But many still continue to use the adjacent genital tissue like dartos as a covering layer.
The structural anomalies of dartos are accused for its development but Wan et al also included skin tethering, corpora cavernosa disproportion to fibrotic dartos and Buck fascia as factors causing ventral penile curvature.