Primary cells are cells obtained directly from freshly killed animals; and they can only be passaged or subcultured once or twice. They support the cultivation of many viruses. The cells used here can also be obtained from humans. Primary cells are heterogeneous at the stage of collection. But they are homogenized with trypsin, a proteolytic enzyme prior to their usage. Chelating agents like ethylene diamine tetra-acetic acid (EDTA) can also be used for the dispersal of cells in tissues prior to cell culture. The homogenization of these tissues helps in the release of single cells and small aggregates of cells capable of initiating optimal growth of the cultivated virus(s). Kidney cells and lung tissues from animals are typical examples of sources of cells for primary cell culture. Primary cell culture though best, is expensive; and it is usually difficult to obtain a reliable supply of normal cells from freshly killed animals to undertake primary cell culture during viral cultivation. Primary cell lines are generally used for viral isolation and for vaccine preparation.
- Continuous cell cultures: Continuous cells are cell lines that are capable of more prolonged and indefinite growth. They are immortalized cell lines that can be passaged or subcultured several times unlike the primary cell lines that can only be passage once or twice. HeLa cells (human carcinoma of cervix cell line) are typical examples of continuous cell lines capable of indefinite growth. Continuous cell lines are usually derived from tumour or cancerous cells. These cells are not used for vaccine preparation since they are derived from malignant tumours or cancer cells. Continuous cell culture can only handle a limited number of viruses unlike the primary cell culture that handles a wide variety of viruses even though they are the easiest type of cell culture technique to undertake for viral cultivation.
- Semi-continuous cell culture: Semi-continuous cell lines which can otherwise be known as diploid cell lines are cell lines that contain the same number of chromosomes as the parent cells from which they are derived. Though they can be passaged several times, semi-continuous cell lines cannot be subcultured indefinitely like the continuous cell lines used in continuous cell culture. However, semi-continuous cell lines can be passaged up to 50 times unlike the primary cell lines that can only be passaged once or twice. Examples of semi-continuous cell lines include cells from the rhesus monkeys and human embryonic lungs. They can be used for vaccine preparation and for the isolation of fastidious viruses.
One of the disadvantages of using cell culture techniques in carrying out viral cultivation is that cell culture requires specialized and trained personnel with experience to do it. It is not a routine practice in some hospital laboratories – since they rarely isolate and identify pathogenic viruses from clinical samples. Most clinical samples collected for viral examinations are sent to reference laboratories where trained personnel’s and equipments are available for their processing. Cell/tissue culture technique does not support the cultivation of some animal viruses. Nevertheless, cell culture technique has a high sensitivity is identifying a virus and the technique is relatively cheap and easy to perform.
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