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A harmless variant or derivative of a pathogen that stimulates a host's immune system to mount defenses against the pathogen; Substance designed to trigger formation of antibodies without inducing disease.
A preparation of dead or weakened pathogen, or of derived antigenic determinants, that is used to induce formation of antibodies or immunity against the pathogen. Vaccines are generally composed of killed or weakened cells, or of proteins.


A membrane-enclosed sac taking up most of the interior of a mature plant cell and containing a variety of substances important in plant reproduction, growth, and development.
A large, fluid-filled compartment in the cytoplasm of a plant cell or a smaller vesicle (a compartment that can involve cellular secretion, storage or nutrient uptake) in the cytoplasm of any eukaryotic cell. Usually food vacuoles (associated with the digestion of food) or contractile vacuoles (association with the excretion of fluid). Small vacuoles may be called vesicles.


Part of the female reproductive system between the uterus and the outside opening
The birth canal in mammals; also accommodates the male's penis and receives sperm during copulation.

Valence shell

The outermost energy shell of an atom, containing the valence electrons involved in the chemical reactions of that atom


1. Value (in psychology)
2. A measure of the number of chemical bonds formed by the atoms of a given element (in chemistry).
A measure of the reactivity of an element.


Preciseness and exactitude of statistical results.

Valvular disease

Any disease affecting any of the four valves of the heart that ensure that blood flows in the correct direction.
A defective valve can either fail to close completely and leak (regurgitation), or cannot open completely so that blood pumps through a smaller opening (stenosis).

Van der Waals dispersion force or Van der Waals interactions

Weak attractions between molecules or parts of molecules that are brought about by localized charge fluctuations; the weakest of the imtermolecular forces.
Present on all particles and increasing strength with increasing size.  Results from the fact that a preponderance of electrons can end up on one side of an atom. The dispersion force which in fact is an induced dipole - induced dipole interaction depends on the polarisability of the interacting molecules and is inversely proportional to the sixth power of separation. In the case of e.g. two CH4 molecules at a separation of 3Å, the dispersion interaction energy is of the order of -1.1 Kcal/mole.


Artificial vanilla flavouring frequently used as an ingredient in chocolate.


The change from a liquid to a gas; evaporation.


A symbol used to represent a value.

Variable-number-of-tandem-repeats (VNTR) locus

Locus that is hypervariable because of tandemly repeated DNA sequences.
Presumably variability is generated by unequal crossing over or slippage during replication. A chromosomal locus at which a particular repetitive sequence is present in different numbers in different individuals of a population or in the two different chromosome homologues in one diploid individual.

Variable penetrance

The proportion of individuals with a specific genotype and environmental conditions who manifest that genotype at the phenotype level.
Usually has with it adjectives like "incomplete", "complete", or "variable".

Variable region

A region in an immunoglobin molecule that shows many sequence differences between antibodies of different specificities; the part of the antibody that binds to the antigen.

Variable surface glycoprotein (VSG)

One of a battery of antigenic determinants expressed by a microorganism to elude immune detection.


A statistical measure of the dispersion of a set of values about its mean; a measure of the variability of the data.
It is determined by squaring the deviations of each value about the mean and dividing by one less than the number of individuals in the pool. The variance is the square of the standard deviation.

Variance Model

The variance model is one of four least squares linear regression types. The four types of linear regression are the linear, exponential, logarithmic, and the power line fits.

Variance Expression

The variance expression of the response, computed as a function of the mean response of a variance line, is either the variance or the coefficient of variation (CV) of the responses at a single point.

Variance Line

A variance line is an equation which describes the average variance of a response point as a function of the mean of that response. The response variance is computed from the means and variances of the reference assay responses. The heteroscedasticity of the test method, the random error, and the systematic error are the principle elements contributing to the variance line.


Diversity among the members of a population.Variation among individuals can exist at many levels, including genetic, physiologic and behavioral.


Dilated and twisted veins of the testis . Manifests itself as a swelling on the side of the scrotum which may look and feel like a "bag of worms". May be surgically corrected if causing discomfort. This condition may also cause reduced sperm count and male sterility due to sluggish blood flow elevating testicular temperature.


The occurrence within a tissue of sectors or clones with differing phenotypes. Patchiness; a type of position effect that results when particular loci are contiguous with heterochromatin.


Containing or concerning vessels that conduct fluid.


Membrane-bound fluid-filled space within a cell. In most plant cells, there is a single large vacuole filling most of the cell's volume.
Some bacterial cells contain gas vacuoles.

Vas deferens (Plural = vas deferentia)

The tube in the male reproductive system in which sperm travel from the epididymis to the urethra.

Vector (in mathematics & physics)

A quantity that is determined by its magnitude and direction: forces and fields.
See Scalar.

Vector (in molecular biology)

In recombinant DNA, a small, self-replicating DNA molecule, or a portion thereof, into which a DNA segment can be spliced and introduced into a cell; generally a plasmid or a virus. An agent used as a vehicle to introduce recombinant DNA into a host cell; The DNA "vehicle" used to carry experimental DNA and to clone it.
The vector provides all sequences essential for replicating the test DNA. Examples of vectors are phagesplasmids, cosmids and YACs (yeast artifical chromosome), all of which are self-replicating DNA molecules that can be joined with DNA fragments to form recombinant DNA molecules that can be introduced into a host.
Plasmids currently are the biological vectors of choice; though viruses and other biological vectors such as Agrobacterium tumefaciens bacteria or BACs are increasingly being used for this purpose.
Nonbiological vectors include the metal microparticles (coated with genes) which are "shot" into cells by the Biolistic R gene gun.

Vector transmission

Transmission of microorganisms via an intermediate host (flea, mosquito, tick, etc.)

Vegetative cell

In contrast to the dormant spore, which has no metabolic activity until activated when food is cooked, the vegetative cell is in an active metabolic state in which the bacteria are growing and multiplying at a rate depending on the food temperature, acidity, water, additives, etc.

Vegetative state

Active state of a bacterium where the cell takes in nourishment  grows


The solution in which a drug is administered. The solvent used to dissolve drugs  often water or isotonic (0.9%) saline solution. May itself be administered to some animals in a control experiment. See also placebo.

Vehicle transmission

Transmission of microorganisms by an inanimate object, such as equipment used in treating the patient.


A vessel that returns blood to the heart.


The rate of change of distance with respect to time. The first derivative of the position function.

Vena cava

A large vein that brings blood from the tissues to the right atrium of the four-chambered mammalian heart. The superior vena cava collects blood from the forelimbs, head, and anterior or upper trunk; the inferior vena cava collects blood from the posterior body region.


Of the blood vessels that drain tissues and return blood to the heart.


A small hole drilled through an earmold to allow the passage of air and to modify sound reaching the eardrum.


Any method of increasing contact between the respiratory medium and the respiratory surface.


Pertaining to the undersurface of an animal that holds its body in a horizontal position; to the front surface of an animal that holds its body erect..

Ventral horn

In the spinal cord the ventral region of the central butterfly-shaped region that appears gray in histology. The ventral horn contains the motor nerve cell bodies (whose axons run out of the spinal cord to control various target muscle structures).


1. Fluid filled spaces in the brain.
2. A muscular chamber of the heart that receives blood from an atrium and pumps blood out of the heart, either to the lungs or to the body tissues.
The fluid acts to cushion and protect the brain, like shock absorber fluid.


A very small vein.

Verbal apraxia

Impaired control of proper sequencing of muscles used in speech (tongue, lips, jaw muscles, vocal cords). The muscles are not weak but their control is defective. Speech is labored and characterized by sound reversals, additions and word approximations.

Verbal dyspraxia

A common speech disorder in which a person is unable to produce the sequential, rapid, and precise movements required for speech.  Nothing is wrong with the vocal apparatus, but the brain cannot give correct instructions for the motor movements involved in speech. This disorder is characterized by many omissions. Some verbally dyspraxia children speak only in vowels, making their speech nearly intelligible, and have very slow, halting speech with many false starts before the right sounds are produced. Their speech errors may be similar to those of children with phonological impairment.


The excessive use of speech (wordiness) in which individuals use words that have little meaning to them.


The preferred plural of vertebra . (The alternate plural is vertebras.) See also: Cervical vertebrae ; Coccygeal vertebrae ; Lumbar vertebrae ; Sacral vertebrae ; and Thoracic vertebrae .


The point on an angle where the two sides intersect.


Intracellular membrane-bound sac.

Vertebral column

The backbone. In nearly all vertebrates, it forms the supporting axis of the body and protects the spinal cord.


A chordate animal with a backbone: the mammals, birds, reptiles, amphibians, and various classes of fishes.


Generic term for any repair or reconstruction of a vertebral body. Titanium vertebral cages or bone allografts are common ways of replacing damaged vertebral bodies.


A small, intracellular membrane-bound sac.


A tube in the body that carries fluids: blood vessels or lymph vessels.

Vestibular system

Pertaining to the vestibular system in the middle ear (consisting of three semi-circular canals and two otolith organs) and the brain which senses movements of the head. Disorders of the vestibular system can lead to dizziness, poor regulation of postural muscle tone and inability to detect quick movements of the head.

Vestigial organ

A type of homologous structure that is rudimentary and of marginal or no use to the organism


The ability to survive to adulthood.


Speciation which occurs as a result of the separation and subsequent isolation of portions of an orginal population.

Videoswallow study (Also may be referred to as a modified barium swallow study or oral-pharyngeal motility

A videofluoroscopic study from which a detailed analysis of the structures and function of the oral and pharyngeal mechanisms during the feeding and swallowing processes can be obtained.
Information obtained not only identifies whether aspiration occurs, but also helps to determine the factors causing the aspiration so that appropriate decisions regarding treatment and methods for nutritional intake can be made.

Villus (Plural = villi)

In vertebrates, one of the minute, fingerlike projections lining the small intestine that serve to increase the absorptive surface area of the intestine.

Vineland Social Maturity Scale

A standardized assessment procedure for evaluating adaptive behavior.

Viral oncogene

A viral gene that contributes to malignancies in vertebrate hosts.


A virus particle.


A plant pathogen composed of molecules of naked RNA only several hundred nucleotides long; Infective forms of nucleic acid without a protective coat of protein; Unencapsulated small single-stranded self-splicing RNA molecules.
Naked RNA, possibly of degenerated virus, that infects plants. Accompanies several plant viruses, including tobacco ringspot virus.


Pathogenic or poisonous potential of bacteria, fungi, or other agents.
VirulentCapable of overcoming a host's defense mechanisms and causing a disease sometimes of rapid onset and severe symptoms.

Virulent phage

A phage that cannot become a prophage; infection by such a phage always leads to lysis of the host cell.


A submicroscopic, noncellular particle composed of a nucleic acid core and a protein coat (capsid); parasitic; reproduces only within a host cell.
Infectious microorganisms without a cell wall, that can reproduce only within a host cell after inserting itself into the host cell and hijacking the reproduction mechanism for its own ends.
A virus is a particle consisting of a nucleic acid (RNA or DNAgenome surrounded by a protein coat (capsid) and some animal viruses are also covered by a membrane. Smallest of all organisms and often not considered alive because they strictly depend on a cellular host organism (bacteria, plant, animal) to reproduce.  Viruses have no metabolism of their own and depend on passive carriers to transport them around. A virus particle may exist free of its host cell but is incapable of replicating on its own. Viruses can replicate only after infecting a host cell. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus. Viruses are infectious particles with a DNA or RNA based small genome that can control the cellular mechanism of infected cells prompting the host cell to synthesize new viruses. Infections often cause mild to severe symptoms, yet some viruses do not cause any harm or not in all host organisms (called carriers). Because they have no metabolic activity they are not susceptible to antibiotics and most drugs, unless a drug can interfere during the infection and viral replication stages. Outside cells, viruses are passive and are easily destroyed by chemical intervention.
They cause diseases such as mumps and Hepatitis A and can be transmitted through food.


The collective term for the internal organs of an animal.

Visceral arches (also known as branchial arches, gill arches, or pharyngeal arches)

Columns of mesenchyme found in the neck of the developing vertebrate embryo derived from cranial neural crest.
In lower vertebrates, blood vessels formed here become part of the gills; in higher vertebrates derivatives include portions of the jaw and middle ear.

Visceral muscle

Smooth muscle found in the walls of the digestive tract, bladder, arteries, and other internal organs.


An instrument for rapid and accurate determination of the viscosity of a fluid.


The internal friction of a fluid; Having a relatively high resistance to flow.
Measure for the flow properties of a substance (expressed in mPa.s). Thick fluids have a high viscosity and thin fluids low.

Visible light

That portion of the electromagnetic spectrum detected as various colors by the human eye, ranging in wavelength from about 400 nm to about 700 nm.

Visual acuity

The clarity or sharpness of vision.
We would be very familiar with tests of visual acuity such as the Snellen Eye Chart we would have encountered when going for a driving test, or at the optomoetrists.

Visual closure

The ability to identify an object from an incomplete image.

Visual figure ground

Differentiating an object from its surrounding background and hold the image while scanning the total pattern.

Visual tracking

Visually following an object as it moves through space.

Vital capacity

The maximum volume of air that can be exhaled after a maximal inspiration.


The belief that natural phenomena are governed by a life force outside the realm of physical and chemical laws.


Name that is given to 13 organic substances which are essential in the diet because they cannot be manufactured by the body.
Vitamins serve primarily as coenzymes or parts of coenzymes.

Vitamin D

A fat soluble vitamin that can be found in some foods but is mostly made by the skin when exposed to ultraviolet rays from the Sun.  The two major forms are D2 and D3. Active vitamin D functions as a hormone because it sends messages to the intestines to increase the absorption of calcium and phosphorus. Vitamin D deficiency is known to cause several bone diseases, including rickets and osteoporosis.

Vitreous Humor

transparent, semigelatinous fluid in the chamber filling the cavity behind the crystalline lens of the eye


Referring to a type of development in which the young are born alive after having been nourished in the uterus by blood from the placenta.

V-J joining

The joining of a variable (V) gene segment and a joining (J) gene segment in the first step of the formation of a functioning immunoglobulin gene.

Vocal nodules

A thickening at a specific locale on a true vocal cord.
These are caused by abuse of the true vocal cords. When a node develops on one cord, there is often an irritation at the corresponding site on the other fold, and then bilateral nodes can be the result. As the nodes thicken, they lower the fundamental pitch of the cords with their increased mass. Other presenting symptoms include a breathy or hoarse sound, this being due to the air leakage caused by poor approximation of the folds with the presence of a mass. Most vocal nodules will disappear with proper voice therapy to reduce vocal abuse, but in some causes, surgical reduction is necessary. The presence of vocal nodules is considered to be a voice disorder.

Voice disorders

Speech impairments characterized by either an inappropriate pitch (too high, too low, never changing or interrupted by breaks); loudness
(too loud or not loud enough); or quality (harsh, hoarse, breathy, or nasal) of the speech itself.

Voiding cystourethrogram

An imaging technique (X-ray) displaying the urethra and bladder while urinating.


Differences in potential (or electric state) related to the electrical forces that 'push' charges through a conductor. 
Can be thought of as the pressure which pushes electricity through a wire.

Voltage-gated ion channels

Membrane ion channels that open and close in response to changes in membrane potential (voltage)
The sodium and potassium channels of neurons are examples


Measurement of space.

Volumetric heating

Heating by internal energy generation throughout the volume of a material


An abbreviation for "volume element" or "volume cell."
A voxel is the 3D counterpart of the 2D pixel. Each voxel is a quantum unit of volume and has a numeric value (or values) associated with it that represents some measurable properties or independent variables of the real objects or phenomena.  The term voxel is routinely used in brain imaging studies to describe a volume of activity.