When studying tissues or diseases, scientists often want to understand the underlying molecular mechanisms. If we only focus on in vivo models, we often run into experimental constraints. Hence, why we use in vitro cell models to understand cellular mechanisms of interest. There are different types of cell models, most of which can fall under primary cells, immortalized cell lines, or stem cell culture.
Primary cells: getting cells straight from the source
Primary cells are the closest you can get to studying the in vivo system in an in vitro cell experiment. Primary cell culture involves isolating cells from the tissue of interest. These types of samples are invaluable. For instance, if we want to dissect signaling pathways in a disease, isolating and culturing cells from patients (or animals, depending on your research) is a powerful tool. These cells can now be manipulated in ways that are not feasible with in vivo systems. We can challenge these cells with new conditions or treat them with small molecules and better understand how they work.
Unfortunately, these models do have their share of problems. First, establishing primary cell culture models depends on access to tissues of interest and good protocols for isolation. You need to isolate enough cells for your experiment, purify cell populations if necessary, and minimize any damage to the cells along the way. Secondly, removing these cells from their natural habitat and placing them in often plastic plates can be jarring for cells. It can alter their behavior, reduce their lifespan, and can make them difficult to grow. Along with that is the variability that occurs with cells from different patients/animals, making reproducibility difficult.
Bring in the immortalized cell lines!
Unlike primary cells, immortalized cell lines can be continuously passaged over time. These cells either acquire mutation(s) or are modified to maintain their ability to proliferate. This means we can more readily freeze, thaw and distribute these cells. Perhaps the most famous example are HeLa cells. Companies, such as ATCC, stock and sell these cells, providing a level of uniformity not possible with primary culture.
Immortalized cell lines are a staple in research, though they have limitations. For starters, these cells are not “normal.” There is a reason these cells stop dividing and die over time in actual tissues. The longer these cells are cultured, the more chances they have to acquire additional mutations and chromosome abnormalities. This then brings them further away from the original in vivo cell type, so scientific inferences about tissues or diseases must be made with that in mind.
Where do stem cells fit?
Stem cells (in vivo or in vitro) by definition, are cells that can continuously proliferate and maintain a level of potency, meaning they can give rise (differentiate) into other cell types. As such, stem cell lines are distinct from both primary cells and immortalized cell lines. Since they can proliferate and be frozen down for future use, they provide a consistency missing in primary cell culture. Plus, since we are taking advantage of their natural ability to proliferate, we generally do not need to modify or mutate them to keep them going. Utilizing their pluripotency (embryonic stem cells) or multipotency (adult stem cells) gives the added benefit of being able to model other cell types under the appropriate conditions. In biomedical research, a huge advancement was the development of induced pluripotent stem cells (iPSC). To develop an iPSC line, isolated adult/somatic cells are reprogrammed back into pluripotent stem cells. By isolating cells from patients, scientists can develop disease-in-a-dish models.
Of course, like the other models, stem cells have their faults. There are established mouse and human embryonic stem cells, providing the consistency of immortalized cell lines. However, once again, they are cells adapted to grow on tissue culture plates—something normal stem cells don’t do. And just like immortalized cell lines, the longer they are passaged, the more likely they will run the risk of gaining mutations. Additionally, these cells can easily differentiate and, if you are not careful, become cell types other than those you are interested in.
It all depends on your research
The key is to place your scientific findings in the context of the questions you are asking. Cell lines are instrumental in scientific research, but at the end of the day, they are models. It is important to validate your work with additional experiments using in vivo, in vitro, or even clinical data, if appropriate. However, there is no doubt that cell lines are key to many monumental discoveries. As science continues to progress and advancements like 3D cell culturing emerge, these models will only continue to improve and become more reliable and indispensable.
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