Category Archives: Cell Culture

Passaging/subculturing cells

  • Cell culture is not static. Cells acquire changes when maintained for a long time in culture. Stressful condition accelerates such changes, which results in inconsistency in the experimental outcome. Therefore, it is important to maintain culture under specified culture condition.
  • Cells in culture grow and divide in presence of nutrients and proper physiological condition. As they grow, cell density in culture increases and culture becomes confluent. With the increase in cell density, cell-cell contact becomes more prominent, which affect the cell physiology in various ways, leading to transient (inhibition of cell division by contact inhibition in untransformed cells) or sometimes even permanent changes in cell characteristics (e.g., may induce differentiation in cells).
  • Moreover, highly confluent culture consumes nutrients from the medium quickly, causing nutrient deprived state. Nutrient deprived state results in poor proliferation and cell death, leading to accumulation of toxic products in the medium. Accumulation of toxic products further enhances cell death.
  • In order to avoid complications and attain reproducibility in cell culture-based experiments, cell culture must be maintained under a condition, which allows exponential growth. Therefore, it is necessary that culture must not reach 100% confluency (adherent cell line) or gets overcrowded (suspension culture). To reduce the confluency/cell density, cells are regularly diluted by transferring a small amount of cells from the existing culture to another culture dish containing fresh culture medium, where cells further grow. The process of transferring a small proportion of cell to another fresh tissue culture dish is called passaging or subculturing.
  • The procedure of passaging is dependent on growth mode of cells. Adherent cells need to be detached from the substratum for passaging. Many methods of detaching cells from the substratum have been developed. Enzymatic treatment (Trypsin, Collagenase) of cell detachment is the most common methods used in most laboratories for routine passaging of adherent cells. However, other methods, like mechanical means (using rubber policeman or vigorous shaking for semi-adherent cells) or treatment with chelators can also be used, which depend on the cell type or experimental requirement.
  • Enzymatic methods often aim to make single cell suspension of cells which require both disrupting the cell-substratum as well as cell-cell contacts. However, use of only mechanical means often results in small cell clumps.
  • Detached cells are further diluted with the fresh culture medium and placed into new culture dishes.
  • Non-adherent cell culture (suspension cell culture) is simply diluted with culture medium and placed in new culture dishes. However, sometimes cells are treated with the enzymatic solution to make single cells suspension for the experiment.

Subculturing adherent cells using trypsin-EDTA

  • Subculturing/passaging can be defined as preparation of fresh culture by transferring cells from an existing culture.
  • Subculturing is done by transferring a small amount of cells (usually 1/3 to 1/10 cells of the existing semi confluent culture) from an existing culture dish to a new culture dish containing fresh growth medium.
  • Trypsin-EDTA method, also referred to as  trypsinization, is a most commonly used method for passaging adherent cells.
  • Trypsin-EDTA method of subculturing of a cell culture involves following steps.
    • Washing of cells with Ca2+- free  and Mg2+ – free PBS
    • Trypsin – EDTA treatment
    • Inactivation of trypsin
    • Preparation of fresh culture dish from the cell suspension
Washing of cells with Ca2+– free  and Mg2+ – free PBS
  • This step involves removing old culture medium from the culture dish, followed by washing with PBS which is free of Ca2+- free  and Mg2+ ions.
  • This step is intended to remove divalent cations and serum-containing medium from the cell culture. Serum in culture medium has trypsin inactivating activity (trypsin inhibitors) and divalent cations strengthen the cell-cell and cell-substratum interaction by stabilizing them.
Trypsin – EDTA treatment
  • This step involves brief incubation (few minutes, varies from 1 – 5 min for most cell lines) of adherent cell culture with Trypsin EDTA solution at 37°C.
  • This step is intended to disrupt both cell-cell and cell-substratum interactions. These interactions are mediated by various proteins (cadherins, integrins, extracellular matrix proteins like fibronectin, vitronectin) and their interactions are strengthen by divalent cations (e.g., Fibronectin-integrin interactions is promoted by Ca2+).
  • Trypsin, a serine protease, cleaves the polypeptide at C-terminal of lysine or arginine amino acid, except when either is followed by proline. Trypsin shows optimal activity at 37°C and pH 8.0.
  • EDTA, a chelating agents, sequesters divalent cations (e.g., Ca2+, Mg2+).
  • Trypsin disrupts cell-cell and cell substratum interactions by digesting proteins and EDTA weakened these interaction by chelating divalent cations.
Inactivation of trypsin
  • Since trypsin digests proteins, excessive trypsin treatment can cause high cell death by disrupting the plasma membrane. Therefore, inactivation of trypsin is an essential step in this method.
  • Usually trypsin is inactivated by adding serum-containing growth medium. In specific conditions where serum can not be added, other trypsin inhibitors, e.g., soybean trypsin inhibitor, are used.
Preparation of fresh culture dish from the cell suspension
  • This step aimed to distribute cell suspension into fresh culture dishes. Usually when the purpose is to maintain a culture in a healthy state, a rough estimation of cells called split ratio is used to distribute cells to fresh culture dishes. Split ratio suggest that how many culture dishes can be prepared from the existing culture dish. For example you can prepare 4 – 6 culture dishes if a recommended split ratio is 1:4 to 1:6  for a specific cell line. Alternatively calls can be counted and a specified number of cells can be transferred to another fresh flask containing  medium.

Cryopreservation of cell culture

  • Cell culture is not static. Cells in culture acquire changes which can be either genetically programmed (e.g., senescence in primary culture) or due to accumulation of genetic abnormalities (mutations, gain or loss of whole chromosomes or part of chromosomes). In addition to this, changes in gene expression pattern and epigenetic modifications due to several reasons including fluctuations in culture condition, contamination, mishandling and stressful condition to culture, can also lead to permanent changes in cell behavior (e.g., stem cell culture can differentiate, or lose its ability to differentiate). Therefore, we need a method to preserve cell culture which stop or slow down these processes.
  • Cryopreservation is an efficient way to preserve cells at ultra-low temperature (below -135°C) which stop all physiological processes and biological aging. It is a routinely used technique in all cell culture laboratories.
  • During preservation at ultra-low temperature, cells die due to many reason including lysis due to ice crystal formation, pH change, dehydration, and alterations in the concentration of electrolytes. Four distinct phases of cell preservation and revival process can cause to damage to cells…………
    • when temperature reduced to above freezing point (hypothermia)
    • when temperature reduced to below freezing point
    • during frozen state
    • during revival
  • Cryopreservation methods ensure that cells are alive at ultra-low temperature and maintain their features when revived after long term frozen state.
  • Most cryopreservation methods rely on
    • cryoprotectants
    • slow cooling
    • rapid revival
  • To cryopreserve cells, cells are suspended in freezing medium, followed by slow cooling and subsequently storage in liquid nitrogen.
  • Freezing medium is nothing but growth medium supplemented with cryoprotectant. Serum containing growth medium contains high amount of serum (upto 90%).
  • Cryoprotectants, the most important component of freezing medium, function by preventing the formation of ice crystals, thus protect cells from lysis.
  • Polyalcohols (e.g., glycerol, ethylene glycol, 2,3 butanediol) and DMSO can be used as cryoprotectants, often a concentration varies from 5 – 20%. Most cryoprotectants have ability to penetrate the cell membrane and function by replacing part of the water in the cell.
  • DMSO is most frequently used cryoprotectant. However, some cells lines are sensitive to DMSO. In such situation, glycerol can be  used. Glycerol is less toxic than DMSO, however, osmotic problem associated with glycerol at the time of thawing restrict its uses.
  • High concentration of serum can also be added in freezing medium. High serum concentration correlate with better survival upon thawing.
  • Serum-free chemically defined freezing medium are also available which are prepared by adding cryoprotectant to serum-free chemically defined medium growth.
  • Serum-containing freezing mediums are used for cell lines growing in serum-supplemented growth medium whereas serum-free freezing medium is used for those cell lines which are maintained in serum-free chemically defined medium.

Cell culture

  • Cells are the basic structural and functional unit of life. Cells from all organisms including multicellular organisms can perform all physiological functions independently. Therefore, cells can be maintained under artificial environment if the right condition is provided.
  • Cell culture can be defined as a process of maintaining cells under the artificially controlled environment in a culture dish, outside their natural environment. This definition can be applied to any organism including prokaryotes, as well as unicellular and multicellular eukaryotes. However, in practice, the term cell culture is used for cells from multicellular organisms, especially multicellular animals. Specific terminology, like bacterial culture (maintaining bacteria in the controlled laboratory environment), yeast culture (maintaining yeast in the controlled laboratory environment), plant culture are used frequently to denote other types of culture.
  • Cells in culture behave as an independent unit like unicellular organisms and perform all necessary functions including cell division and metabolism in the culture dish.
  • Classically, the term ‘Tissue culture’ was used to grow plant and animal explant in a controlled artificial environment in the laboratory. The term ‘Animal tissue culture’ refers to cell culture derived from multicellular animals whereas ‘Plant tissue culture’ refers to the culture of plant cells/tissues.
  • Culture condition must maintain cell’s characteristics as it possesses in its natural environment. Practically, it is very difficult, in part, due to limited knowledge of physiological requirements of specific cell type. However, many different cell types have been maintained in culture and are in use in both basic and applied research. One such example is a successful maintenance of stem cells (embryonic and adult stem cells) in culture.
  • Cell culture can be classified on the basis of cell’s characteristics including growth mode, lifespan, morphology and cell types.
  • Based on growth mode of cells, culture can be broadly classified into two types – suspension culture and adherent culture. Semi-adherent culture, which contains loosely adherent cells to the dish surface, also exist.
  • Based on cell morphology in the culture dish, cell culture can be broadly classified into three types – Fibroblast-like, Epithelial-like, and Lymphoid-like.
  • Cell culture is not static. Cells in culture acquire changes which can be genetically programmed (e.g., senescence in primary culture) or due to the accumulation of genetic abnormalities (mutations, gain or loss of whole chromosomes or part of chromosomes). Furthermore, in response to fluctuations in culture condition, cells in culture can show altered behavior due to changes in gene expression pattern which sometimes lead to permanent changes in cell behavior (e.g., stem cells can differentiate).
  • Cell culture technology has found wide application both in basic research and applied research including industry (pharmaceuticals, medical sciences, cancer research, diagnostics, drug and product development, manufacturing of biological compounds, etc.)

Bacterial contamination in cell culture

  • Bacterial contamination is one of the most common cell culture contamination.
  • Poor aseptic culture condition, including handling, incubator and laminar flow hood, or culture media, can be common source of bacterial contamination.
  • Contaminated culture often becomes turbid and the medium turns yellow (phenol red containing medium).
  • Microscopic inspection of such culture is often sufficient to confirm the presence of bacteria. Motile bacteria and bacterial clumps are often observed in contaminated culture, which can easily be distinguished from cell debris. However, low level of contamination may go undetectable in the presence of antibiotics in culture.
  • In such cases, suspected contaminated culture can be grown in absence of antibiotics, which will allow the bacteria to grow, thus contamination can be detected easily.
  • Usually a contaminated culture once confirmed is discarded immediately. In case of precious culture, one can try to rescue the culture by treating the culture in presence of high concentration of antibiotics. Frequently replacing the culture medium containing high concentration bactericidal antibiotics may help to eliminate the bacterial contamination.