Cell culture procedures are conducted with two types of cells:
Primary Cells – Cells isolated directly from human or animal tissue using enzymatic or mechanical methods. Once isolated, they are placed in an artificial environment in plastic or glass containers supported with specialized medium containing essential nutrients and growth factors to support proliferation. Primary cells could be of two types – adherent or suspension. Adherent cells require attachment for growth and are said to be anchorage-dependent cells. The adherent cells are usually derived from tissues of organs. Suspension cells do not require attachment for growth and are said to be anchorage-independent cells. Most suspension cells are isolated from blood system. Some tissue-derived suspension cells are available such as hepatocytes or intestinal cells which are suitable for ADMET applications. Although primary cells usually have a limited lifespan, they offer a huge number of advantages compared to cell lines. When performing primary cell culture, researchers are acquiring an opportunity to study donors and not just cells. Several factors such as age, medical history, race, and sex can be considered when building an experimental model. With a growing trend towards personalized medicine, such donor variability and tissue complexity can only be achieved with use of primary cells and are difficult to replicate with cell lines that are very systematic and uniform in nature and do not capture the true diversity of a living tissue.
Cell Lines – Cells that have been continually passaged over a long period of time and have acquired homogenous genotypic and phenotypic characteristics. Cell lines can be finite or continuous. An immortalized or continuous cell line has acquired the ability to proliferate indefinitely, either through genetic mutations or artificial modifications. A finite cell line has been sub-cultured for 20-80 passages after which they senesce. Cell lines are preferably used for convenience as they are easy to handle and widely published. However, they are less preferred as a biologically relevant option, since they have lost the true characteristics of the original tissue from which they were isolated. Serial passaging is known to cause genotypic and phenotypic variation in cell lines. Variation can often be so far from that of the original tissue to where they do not mimic the in vivo environment very closely. Cells that do not represent the original tissue could result in false negative or false positive findings.