1. A projection from the surface, especially of a mucous membrane. If the projection is minute, as from a cell surface, it is termed a microvillus. 2. An elongated dermal papilla projecting into an intraepidermal vesicle or cleft. See festooning. [L. shaggy hair (of beasts)]
- anchoring v. a chorionic v. that is attached to the decidua basalis.
- arachnoid villi tufted prolongations of pia-arachnoid that protrude through the meningeal layer of the dura mater and have a thin limiting membrane; collections of arachnoid v. form arachnoid granulations that lie in venous lacunae at the margin of the superior sagittal sinus; the spongy tissue of the a. v. contains tubules that serve as one-way valves for transfer of cerebrospinal fluid from the subarachnoid space to the venous system. Both a. v. and the granulations formed from them are major sites of fluid transfer. SEE ALSO: arachnoid granulations, under granulation.
- chorionic villi vascular processes of the chorion of the embryo entering into the formation of the placenta.
- floating v. SYN: free v..
- free v. a chorionic v. that is not attached to the decidua basalis, but is “free” in the maternal blood of the intervillous spaces. SYN: floating v..
- intestinal villi [TA] projections (0.5–1.5 mm in length) of the mucous membrane of the small intestine; they are leaf-shaped in the duodenum and become shorter, more finger-shaped, and sparser in the ileum. SYN: villi intestinales [TA].
- villi intestinales [TA] SYN: intestinal villi.
- villi pericardiaci [TA] SYN: pericardial villi.
- pericardial villi [TA] minute filiform projections (synovial villi) from the surface of the serous pericardium. SYN: villi pericardiaci [TA].
- peritoneal villi [TA] synovial villi on the surface of the peritoneum. SYN: villi peritoneales [TA].
- villi peritoneales [TA] SYN: peritoneal villi.
- pleural villi [TA] shaggy appendages (synovial villi) on the pleura in the neighborhood of the costomediastinal sinus. SYN: villi pleurales [TA].
- villi pleurales [TA] SYN: pleural villi.
- primary v. the first stage of chorionic v. development, with columns of cytotrophoblastic cells covered by syncytiotrophoblast.
- secondary v. an intermediate stage of chorionic v. development following invasion by a connective tissue core.
- synovial villi [TA] small vascular processes given off from a synovial membrane. SYN: villi synoviales [TA], synovial fringe, synovial tufts.
- villi synoviales [TA] SYN: synovial villi.
- tertiary v. the definitive chorionic v. with a vascular core separated from maternal blood by connective tissue, cytotrophoblast, and syncytiotrophoblast.

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vil·lus 'vil-əs n, pl vil·li -.ī a small slender vascular process: as
a) one of the minute fingerlike processes which more or less thickly cover and give a velvety appearance to the surface of the mucous membrane of the small intestine and serve in the absorption of nutriment and of which each has a central blindly ending lacteal surrounded by blood capillaries and covered with epithelium
b) one of the branching processes of the surface of the chorion of the developing embryo of most mammals that are restricted to particular areas or diffusely arranged and over parts of the surface become vascular and help to form the placenta

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n. (pl. villi)
one of many short finger-like processes that project from some membranous surfaces. Numerous intestinal villi line the small intestine. Each contains a network of blood capillaries and a lacteal. Their function is to absorb the products of digestion and they greatly increase the surface area over which this can take place. Chorionic villi are folds of the chorion (the outer membrane surrounding a fetus) from which the fetal part of the placenta is formed. They provide an extensive area for the exchange of oxygen, carbon dioxide, nutrients, and waste products between maternal and fetal blood. See also arachnoid villus, chorionic villus sampling.

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vil·lus (vilґəs) pl. vilґli [L. “tuft of hair”] 1. a small protrusion resembling a tuft of hair. 2. a small vascular process or protrusion, especially one on the free surface of a membrane.

Medical dictionary. 2011.

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