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Studies that are in vitro (Latin: in glass; often not italicized in English) are performed with cells or biological molecules studied outside their normal biological context; for example proteins are examined in solution, or cells in artificial culture medium. Colloquially called "test tube experiments", these studies in biology and its sub-disciplines are traditionally done in test-tubes, flasks, petri dishes etc., but which now involve the full range of techniques used in molecular biology and larger commercial applications.
Examples of in vitro studies include: the isolation, growth and identification of microorganisms; cells derived from multicellular organisms (cell culture or tissue culture); subcellular components (e.g. mitochondria or ribosomes); cellular or subcellular extracts (e.g. wheat germ or reticulocyte extracts); purified molecules (often proteins, DNA, or RNA, either individually or in combination); and the commercial production of antibiotics and other pharmaceutical products. Studies that are conducted using components of an organism that have been isolated from their usual biological surroundings permit a more detailed or more convenient analysis than can be done with whole organisms. In contrast, in vivo studies are those conducted in animals including humans, and whole plants. These may involve investigations into the pathogenesis of infections using live bacteria or their toxins, or the effects of antibiotics or drugs etc.
Viruses, which only replicate in living cells, are studied in the laboratory in cell or tissue culture, but many animal virologists refer to such work as being in vitro to distinguish it from in vivo work on whole animals.
Living organisms are extremely complex functional systems that are made up of, at a minimum, many tens of thousands of genes, protein molecules, RNA molecules, small organic compounds, inorganic ions and complexes in an environment that is spatially organized by membranes and, in the case of multicellular organisms, organ systems. For a biological organism to survive, these myriad components must interact with each other and with their environment in a way that processes food, removes waste, moves components to the correct location, and is responsive to signalling molecules, other organisms, light, sound, heat, taste, touch, and balance.
This extraordinary complexity of living organisms is a great barrier to the identification of individual components and the exploration of their basic biological functions. The primary advantage of in vitro work is that it permits an enormous level of simplification of the system under study, so that the investigator can focus on a small number of components. For example, the identity of proteins of the immune system (e.g. antibodies), and the mechanism by which they recognize and bind to foreign antigens would remain very obscure if not for the extensive use of in vitro work to isolate the proteins, identify the cells and genes that produce them, study the physical properties of their interaction with antigens, and identify how those interactions lead to cellular signals that activate other components of the immune system.
Cellular responses are often species-specific, making cross-species analysis problematic. Newer methods of same-species-targeted, multi-organ studies are available to bypass live, cross-species testing.
The primary disadvantage of in vitro experimental studies is that it can sometimes be very challenging to extrapolate from the results of in vitro work back to the biology of the intact organism. Investigators doing in vitro work must be careful to avoid over-interpretation of their results, which can sometimes lead to erroneous conclusions about organismal and systems biology.
For example, scientists developing a new viral drug to treat an infection with a pathogenic virus (e.g. HIV-1) may find that a candidate drug functions to prevent viral replication in an in vitro setting (typically cell culture). However, before this drug is used in the clinic, it must progress through a series of in vivo trials to determine if it is safe and effective in intact organisms (typically small animals, primates and humans in succession). Typically, most candidate drugs that are effective in vitro prove to be ineffective in vivo because of issues associated with delivery of the drug to the affected tissues, toxicity towards essential parts of the organism that were not represented in the initial in vitro studies, or other issues.
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