At any time during the culture of a cell strain, cells in the culture may become transformed, meaning that they are no longer subject to crisis and senescence but can be passage indefinitely i.e. they will have an infinite lifespan. Immortal cell cultures are called continuous cell lines, to distinguish them from primary cultures and cell strains that have a definite lifespan.
Senescence:Normal cells usually divide only a limited number of times before losing their ability to proliferate, which is a genetically determined event.
Immortalization can occur spontaneously during the passage of a cell strain, or it can be induced by
- treatment with chemical mutagens,
- infection with tumorigenic viruses, or
- transfection with oncogenes.
In addition, cells cultured from tumor tissue frequently establish immortal cell lines in culture. Also known as heteroploid cultures, continuous cell lines are termed so as they can be serially cultivated indefinitely. The standard continuous cell lines have been derived from human cancer cells such as HeLa (derived from cervical cancer of a lady, HeLa by name), Hep2, and KB cells.
Continuous cell lines are maintained either by the serial subculture or by storing in deep freeze at -70°C.
Some of the distinguishing properties of continuous cell lines are:
- Cells of a single type capable of infinite growth In Vitro.
- Grow faster and their chromosomes are haploid
- Derived from immortalized cell lines (cancer cells) often of epithelial origin
- Are not used for the preparation of viral vaccines as vaccines prepared in cancer cells are considered unsafe for human use.
Uses of Cell lines
Most of the cell cultures used in the diagnostic laboratories are continuous cell lines. The specimen is introduced into the cell culture medium and the presence of viruses is detected by observing the cells for a cytopathic effect (CPE). Cell culture techniques are specialized and labor-intensive and some viruses either cause no cytopathic effects or cause a CPE that takes a week or more to evolve.
| Name of the virus | Cell lines used |
| Influenza virus | Primary monkey kidney (PMK) cell culture Madin-Darby canine kidney (MDCK) cell line |
| Herpes simplex virus (HSV) | Primary human embryonic cell or human diploid cell lines eg. MRC-5 |
| Measles virus | Primary human fetal kidney cell |
| Rabies virus | Murine lymphoblastoma cell |
| Dengue virus | Intrathoracic or intracerebral inoculation of Toxorhynchites mosquito. Mosquito cell lines eg. Ap-61, TRA 248, etc. |
| HIV | Phytohemagglutinin stimulated peripheral blood mononuclear cell |
Difference between Primary and Continuous Cell Culture
| Properties | Primary Cell Culture | Continuous Cell Culture |
| Source | Mixture of cells freshly derived from the tissue of origin e.g., laboratory animal, human biopsy sample, etc. | Mixture of cells derived from human and animal tumors. |
| Life span | Cell line derived from primary culture has a limited lifespan. They usually divide only a limited number of times before losing their ability to proliferate ( a genetically determined event known as senescence) | Can be sub-cultured indefinitely. |
| Morphological change | All the cells retain their normal karyotype | Cells will not display the same karyotype. |
