What is cell culture?

Cell culture aims at growing cells under artificial conditions in the lab. The type of cells used range from mammalian cells (including human), other animal cells, plant cells, yeast, bacteria, insect cells… You name it. Let’s assume for now you’re using animal cells. The source of these cells can be primary or cell lines. Primary cells are those obtained directly from the animal tissue, such as blood or skin cells. They are quite hard to grow as they require conditions as close as possible to those found in the body. Cell lines can be bought from life science suppliers; they are generally easier to grow as they have standard cell culture protocols with defined growth medium and feeding schedule. They originate from primary cells genetically modified to keep growing, or sometimes from primary cancer cells that acquire mutations which produce the same effect.

What is the purpose of cell culture?

Cell culture serves two main purposes: researching the behavior of cells under specific conditions, and producing cells or cellular components for medical or industrial use. Examples of applications in the area of research include testing potential drugs, identifying drug targets and understanding the development of diseases. In the area of medical production of cells or cellular components, cell culture is being used for growing artificial organs, producing blood components or IV fertilization, while in the industry the main target is the production of drugs (think antibiotics for example), materials (some bacteria produce polymers with excellent properties) and fuel (some types of bacteria produce alcohol).

Cell types

There are two types of cells according to cell attachment:

Adherent cells:

These cells come from solid tissues, so they are used to attaching to surfaces in order to grow. They need specific flasks that meet their surface requirements. Cell culture medium is added to cover the cell layer at the bottom of the flask.

Non-adherent cells:

These other cells come from liquid tissues (i.e. blood), or have been modified to think they don’t need to attach to grow. These cells only need to be suspended in medium (the flask itself doesn’t matter that much). Growth medium is added in the right proportions so that it’s not too overcrowded with cells.

Cell culture types

Cell culture can also be classified according to the environment where cells grow:

2D cultures

Cell culture in 2D is done in flasks or wells, with a specific supply of medium which needs to be manually replaced regularly. These cultures are easy to handle and give reproducible results. However, they restrict the surface and space available for cells to settle, and generally require the addition of growth factors.

3D cultures

Culturing cells in 3D involves using a matrix, sponge, gel or other 3D structure to give cells additional support and mechanical signals more similar to the tissue where they originated. Because the environment is closer to the one in the body, cell growth happens more similarly. This can be important for research related to cell proliferation, cancer drugs etc, but also for the research around cells producing valuable chemicals. However, 3D cultures are a lot harder to maintain and cells are not always recoverable.

Bioreactors

Bioreactors represent a quantum leap for bioscientists. Even though 2D cultures are very easy to set up, maintaining them is cumbersome and is more prone to contamination, spillages and generally mishandling. While this can still happen in bioreactors, once they start running most of the tasks are performed automatically. Pumps feed medium delivery and recover waste (if any), temperature is maintained at constant levels by a heating system (or in the incubator), gas concentrations are monitored… Bioreactors are currently being used for cells that produce valuable substances, such as antibiotics.

Looking after cell cultures

To have a healthy culture, there is an optimal range of cell densities (i.e. ratios of cells to growth medium volume) that you need to maintain. If your cell density is too high, your cells will grow more slowly (or not at all) due to:

• contact inhibition: being closer to other cells sends signals for the cells to stop growing
• deprivation of nutrients & excess of metabolites: cells eat and produce waste like you and I. Similarly, they are happier when they are fed regularly and their environment is clean (these metabolites can be toxic at higher concentrations!).
• lack of access to oxygen: your cells need to breathe. Air diffusion is critical in cell culture.

In addition, you need to maintain your cells under conditions similar to the ones in the body (37°C, 5% CO₂) by keeping them in an incubator.

You can tell if you have too many cells by simply looking at your flask under the microscope. If the bottom looks completely covered (adherent cells) or the suspension looks very dense (non-adherent), then you have reached confluence, which is a fancy word for “cell overcrowding” (visually) or growth break (graphically – if you plot your cell numbers over time). If your cells are not getting the right amount of oxygen, you will also see a growth break or slowdown.

If your medium has too many metabolites, its color will change due to a decrease in the pH (metabolites are acid). It is a sign that your cells are starving because they have been growing really well. BUT always check your cultures under the microscope for contamination, because other organisms also eat and release acid substances that change the color of the medium (check here).

Different cell types have different needs, but there’s always a manufacturer’s protocol where they state the cell density, frequency of media change and environmental conditions required.

Let’s get hands on with the job!