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Sometimes eukaryotic cells exhibit unusual and alternative growth characteristics both in culture and in vivo. Such cells are usually referred to as transformed or neoplastic cells.
The transformation of some cells may be induced using viruses.
For example, when cultures of mammalian cells are exposed to Polyoma virus or Simian virus 40 (SV 40), some of the cells in the culture are transformed into a cell line that manifests a number of unusual characteristics, including:
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(1) The cells continue to grow and divide even after confluence is attained (with the result that they grow on top of one another instead of maintaining a monolayer),
(2) The cells are no longer anchorage-dependent (thereby permitting growth in multilayers),
(3) The cells grow to an indefinite population size,
(4) Growth continues for many more generations than in the corresponding un- transformed cells (i.e., the cells appear to ignore or to have lost the built-in counting mechanism keeping track of the number of cell doublings), and
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(5) The cells grow irregularly in size and in shape and often reveal an abnormal chromosome content.
One of the most famous (and useful) examples is the “HeLa” line of transformed cells originally derived in 1951 from a cancerous cervical tumor and that has been continuously sub-cultured since that time. (HeLa is an acronym for Henrietta Lacks, the woman from whose uterus the cervical cells were removed.)
The relationship that exists between cellular transformation and cancer should be apparent; indeed, when transformed cells are injected into laboratory animals, the cells form tumors or neoplasms, whereas normal cells do not.
In tissue culture, transformed cells do not form conventional junctional complexes with neighboring cells. Hence, it is easy to understand why, when cellular transformation occurs in vivo, the transformed cells metastasize, that is, spread to other body tissues where they form new tumors, growing and crowding out the tissue’s normal cells.
