Cell culture is the process of growing living cells outside their original organism, in a controlled laboratory environment. Cells are removed from animal or plant tissue, placed in dishes or flasks containing nutrient-rich liquid (called culture medium), and maintained under carefully regulated temperature, humidity, and gas conditions. The practice underpins virtually every corner of modern biomedical research, from drug testing and vaccine production to cancer biology and regenerative medicine. What makes cell culture both powerful and tricky is that living cells are demanding tenants: they need the right food, the right atmosphere, and absolute protection from microbial invaders, and even small lapses in any of these can ruin an experiment or, worse, produce misleading results.
How Cell Culture Got Started
The roots of cell culture stretch back to the 1880s, when Wilhelm Roux kept embryonic chick cells alive in a warm salt solution for several days. Ross Granville Harrison built on that work in the early 1900s with experiments that are generally credited as the first true tissue culture, growing frog nerve fibers in clotted lymph. Alexis Carrel then pushed the field further, famously claiming he had kept chicken heart cells alive indefinitely, an assertion that turned out to be flawed but that nonetheless spurred enormous interest in the idea that cells could be maintained outside the body for extended periods.1PubMed Central. A Brief History of Cell Culture: From Harrison to Organs-on-a-Chip Since those early efforts, the field has expanded from simple tissue fragments sitting in glass dishes to complex microfluidic chips that recreate organ-level physiology.
Primary Cells and Immortalized Lines
There are two broad categories of cultured cells, and the choice between them shapes every downstream result. Primary cells come straight from living tissue. They retain the shape, signaling behavior, and functional characteristics of the tissue they were taken from, making them the closest thing to studying real human (or animal) biology in a dish.2PubMed. Primary cell cultures versus immortalized cell lines: critical comparative insights into biological fidelity and translational applications The trade-off is that primary cells have a limited lifespan. After a certain number of divisions they slow down, change character, or stop dividing altogether.
Immortalized cell lines solve the lifespan problem. These are cells that have been genetically altered, either naturally or through laboratory techniques like introducing the enzyme telomerase or viral genes such as SV40 large T antigen, so that they divide indefinitely.3PubMed Central. Stem cells immortalized by hTERT perform differently from those immortalized by SV40LT in proliferation, differentiation, and reconstruction of matrix microenvironment HeLa cells, derived from a cervical cancer patient in 1951, are the most famous example. Immortalized lines are convenient and widely shared between labs, but they frequently accumulate genetic and functional changes over time that make them less faithful representatives of the tissue they originally came from.2PubMed. Primary cell cultures versus immortalized cell lines: critical comparative insights into biological fidelity and translational applications Researchers have to weigh that trade-off for every experiment: biological realism versus practical convenience.
Aseptic Technique
The single most important skill in any cell culture lab is keeping things sterile. Bacteria, fungi, and mycoplasmas thrive in the same warm, nutrient-rich conditions that cells need, and a single stray microorganism can overrun a culture in hours. The umbrella term for all the practices used to prevent this is aseptic technique, which covers everything from how you sterilize your equipment and media to how you move your hands inside a sterile workspace.4PubMed. Aseptic technique for cell culture
Most cell culture work takes place inside a biosafety cabinet or laminar-flow hood, a box-like enclosure that pushes filtered air across the work surface to sweep away airborne particles. Before anything enters the cabinet, it gets sprayed with ethanol or passed through an autoclave. Media bottles are opened only briefly, pipettes are used once and discarded, and gloves are changed frequently. These precautions sound simple, but maintaining them consistently across months of daily work is where many labs stumble. The culture and storage of mammalian cells must be carried out under conditions that rigorously exclude microorganisms, and the equipment and protocols used in modern labs are all designed to enforce that requirement.5Encyclopedia of Industrial Biotechnology. Aseptic Techniques in Cell Culture
What Cells Eat and Breathe
Cells in culture live in liquid medium, a cocktail of salts, sugars, amino acids, and vitamins dissolved in water. Common formulations like DMEM and RPMI 1640 serve as the base, but on their own they lack the growth factors and attachment proteins that cells need. That gap is traditionally filled by adding fetal bovine serum, a blood product collected from unborn calves. Serum provides a broad mix of hormones, growth factors, carrier proteins, and trace elements that support cell survival and division.6PubMed. Alternatives to the use of fetal bovine serum: serum-free cell culture
Fetal bovine serum works, but it has real drawbacks. Its composition varies from batch to batch, which can make experiments hard to reproduce. It raises animal welfare concerns. And for clinical manufacturing, using an animal-derived product introduces contamination risks. These issues have pushed researchers to develop serum-free, chemically defined media in which every ingredient is known and controlled. The advantage is consistency and the absence of animal products; the disadvantage is that chemically defined media tend to be expensive and cell-type specific, so a universal serum-free medium that works for all cells remains out of reach.7Heliyon. Alternative to FBS in animal cell culture – An overview and future perspective – Section: Efforts made to reduce or replace FBS One promising middle ground is human platelet lysate, which recent studies have validated as a viable alternative to fetal bovine serum for certain cancer explant models, producing comparable marker expression over a 10-day culture period.8PubMed. Xeno-free alternatives to the use of fetal bovine serum in head and neck cancer explant culture
Beyond nutrients, cells need the right pH and gas environment. Mammalian cells prefer a pH around 7.4, and maintaining that in culture relies on a buffering system that mimics the body’s own chemistry. Incubators pump in a carbon dioxide-enriched atmosphere, typically around 5% COâ‚‚, and the culture medium contains bicarbonate salt. The COâ‚‚ and bicarbonate together form a buffer that holds pH steady, the same basic system your blood uses.9PubMed Central. Evidence-based guidelines for controlling pH in mammalian live-cell culture systems If the incubator’s COâ‚‚ level drifts, the medium’s pH drifts with it, and cells can quickly become stressed or die. Most mammalian cell incubators also hold temperature at 37 °C and humidity near saturation to prevent the medium from evaporating.
Passaging, Detachment, and Counting
Cells growing on the bottom of a flask eventually fill the available surface, a state called confluency. At that point they need to be split, or “passaged,” into fresh flasks with more room. For adherent cells, those that stick to the flask surface, splitting first requires detaching them. The classic tool is trypsin, an enzyme that chews through the proteins anchoring cells to the plastic. A brief exposure, around five minutes at low concentrations, is usually enough to release cells while keeping them healthy. Longer or harsher trypsin treatment can strip important surface proteins from cells and reduce their ability to reattach afterward.10PubMed Central. The use of mild trypsinization conditions in the detachment of endothelial cells to promote subsequent endothelialization on synthetic surfaces
Alternatives to trypsin exist. Synthetic enzyme substitutes like TrypLE detach cells about as quickly but tend to be gentler on surface markers. Collagenase and non-enzymatic dissociation reagents also work, though they generally take much longer, around 60 minutes to reach peak cell yields compared to five minutes for trypsin-based reagents.11PubMed Central. Effects of Different Cell-Detaching Methods on the Viability and Cell Surface Antigen Expression of Synovial Mesenchymal Stem Cells The choice of detachment method depends on what you plan to do with the cells afterward; if downstream analysis depends on intact surface proteins, gentler methods are worth the extra time.
Once cells are detached and suspended in liquid, you need to know how many are alive. The most widely used quick check is the trypan blue exclusion test. A small sample of cell suspension is mixed with trypan blue dye, and the mixture is examined under a microscope. Living cells have intact membranes that keep the dye out, so they appear clear. Dead cells absorb the dye and turn blue.12PubMed Central. Trypan Blue Exclusion Test of Cell Viability It is a rough-and-ready method, not perfect, but it gives a fast snapshot of how healthy a culture is before you seed new flasks or set up an experiment.
Freezing Cells for the Long Term
Maintaining a living culture indefinitely is impractical and risky. Cells accumulate changes with every passage, and a contamination event can wipe out months of work. Cryopreservation, freezing cells for long-term storage, solves both problems. The standard approach is to suspend cells in medium containing about 10% dimethyl sulfoxide (DMSO), a cryoprotective agent that prevents the formation of lethal ice crystals inside cells. The mixture is cooled slowly, roughly one degree Celsius per minute, and then stored in liquid nitrogen at around minus 196 °C.13PubMed Central. Cryopreservation: An Overview of Principles and Cell-Specific Considerations
Getting the cooling rate right matters. Too fast, and ice forms inside the cells. Too slow, and the cells dehydrate fatally before they freeze. For sensitive cell types like induced pluripotent stem cells, even the state of the cells, whether they are single or clumped into small aggregates, changes the outcome. Aggregates are more sensitive to supercooling and need careful control of the starting temperature to avoid internal ice formation.14PubMed Central. Freezing Responses in DMSO-Based Cryopreservation of Human iPS Cells: Aggregates Versus Single Cells When done properly, cells can sit in liquid nitrogen for years and be thawed back to life with high viability.
The Contamination and Misidentification Problem
Even experienced labs get bitten by contamination. Bacterial and fungal infections are usually obvious because the culture medium turns cloudy or changes color. Mycoplasma contamination is far more insidious. Mycoplasmas are tiny bacteria that lack a cell wall, making them invisible under a standard microscope and resistant to most antibiotics used in cell culture. They can alter virtually every aspect of a cell’s behavior, from growth rate to gene expression, without the researcher noticing anything wrong.15PubMed Central. Mycoplasma contamination of cell cultures: Incidence, sources, effects, detection, elimination, prevention Routine testing with PCR-based detection kits is the only reliable way to catch it.
Cross-contamination between cell lines is an equally serious and underappreciated problem. When one fast-growing line, often HeLa cells, sneaks into another culture through shared reagents or careless technique, it can silently take over. A comprehensive survey of cell lines used in China found that about a quarter of samples tested were cross-contaminated, and over 93% of the contaminants in lines established in Chinese labs turned out to be HeLa cells.16PubMed. Genetic profiling reveals an alarming rate of cross-contamination among human cell lines used in China Another study examining 482 human tumor cell lines by genetic profiling and species-identification testing found that more than 20% were incorrectly identified, with contamination spanning both same-species and cross-species mix-ups.17Scientific Reports. A Combination of Species Identification and STR Profiling Identifies Cross-contaminated Cells from 482 Human Tumor Cell Lines The consequences for published research are sobering: experiments attributed to liver cancer cells might actually have been performed on cervical cancer cells, invalidating conclusions.
Short tandem repeat profiling, a DNA-fingerprinting technique, is now the standard method for authenticating human cell lines. Many journals and funding agencies require authentication before publication. Yet compliance remains patchy, and well-known misidentified lines continue to circulate in labs worldwide.18PubMed Central. Guidelines for the use of cell lines in biomedical research
Why Passage Number Matters
Every time cells are split into new flasks, the passage number ticks up by one. It might seem like an administrative detail, but passage number has real biological consequences. Cells gradually change their gene expression patterns with serial passaging. A recent transcriptomic study showed that cells progressively diverged in their gene expression profiles with increasing passages, following a nonlinear pattern with significant intermediate fluctuations rather than a steady drift.19Scientific Reports. Cell passage number drives transcriptomic drift as an overlooked factor in experimental reproducibility In practical terms, at higher passages cell health begins to decline and experimental data can become variable.20PubMed Central. The effect of cell passage number on osteogenic and adipogenic characteristics of D1 cells
This is why most protocols specify a passage range. If your experiment uses cells between passage 5 and passage 15, you can be reasonably confident the cells are behaving similarly across replicates. Drift beyond that window can introduce artifacts that look like real biological effects but are really just the cells aging in culture. Keeping frozen stocks at low passage numbers and thawing fresh vials regularly is one of the simplest ways to guard against this.
From Flat Dishes to Three-Dimensional Culture
Traditional cell culture grows cells in a single layer on a flat plastic surface. This two-dimensional setup is convenient and well understood, but it forces cells into an unnatural geometry. In the body, most cells exist within three-dimensional tissues where they interact with neighbors on all sides and with an extracellular scaffold that influences their shape, movement, and gene activity. Two-dimensional cultures disturb those cell-to-environment interactions, alter cell shape and polarity, and can change how cells divide.21PubMed Central. 2D and 3D cell cultures – a comparison of different types of cancer cell cultures
Three-dimensional culture methods aim to close that gap. One common approach is the spheroid model, where cells are placed in conditions that prevent them from attaching to a flat surface, encouraging them to clump together into small ball-like structures. These spheroids develop their own extracellular matrix and can be produced cheaply without specialized additives.22Scientific Reports. Comparative analysis between 2D and 3D colorectal cancer culture models for insights into cellular morphological and transcriptomic variations Other approaches use scaffold materials, hydrogels, or hanging-drop techniques to provide three-dimensional architecture. The resulting cultures tend to better mimic drug responses and tissue behavior, which is especially valuable in cancer research where flat-dish results have historically translated poorly to real patients.
Organs-on-Chips
The most ambitious extension of three-dimensional culture is the organ-on-a-chip. These are microfluidic devices, usually about the size of a USB stick, containing tiny channels lined with living cells arranged to mimic the architecture of a specific organ. By recreating the interfaces between different tissue types, the physical microenvironment, and the continuous flow of fluid that cells experience in the body, these chips produce levels of tissue function that neither flat-dish nor simple three-dimensional cultures can match.23PubMed Central. Microfluidic organs-on-chips
Fluid flow turns out to be particularly important. In the body, cells lining blood vessels and intestinal walls constantly experience shear stress from passing fluid. Exposing cultured endothelial cells to similar shear forces improves their differentiation and function, something that conventional static models cannot provide.24PubMed Central. Microfluidic organ-on-chip technology for blood-brain barrier research Studies on gut-lining cells have shown that even low levels of microfluidic shear stress can induce significant changes in cell structure and function compared to cells grown without flow.25PubMed. A systematic investigation of the effect of the fluid shear stress on Caco-2 cells towards the optimization of epithelial organ-on-chip models Lung-on-a-chip, liver-on-a-chip, and blood-brain-barrier-on-a-chip models are all active areas of development, with the long-term hope that interconnected multi-organ chips could someday reduce or replace some animal testing in drug development.
Stem Cell Culture and Feeder-Free Systems
Human pluripotent stem cells, including embryonic stem cells and induced pluripotent stem cells, are among the most demanding cells to grow. They need very specific conditions to remain in their undifferentiated, “blank slate” state. Traditionally, stem cells were cultured on top of a layer of mouse embryonic fibroblasts, feeder cells that secrete growth factors keeping the stem cells from specializing prematurely.26PubMed Central. Concise review: The evolution of human pluripotent stem cell culture: from feeder cells to synthetic coatings The problem with feeder cells is that they introduce animal-derived material, making the cultures unsuitable for clinical therapies where you need to avoid foreign biological contaminants.
Feeder-free and animal-free culture systems have been developed to address this. By coating culture surfaces with defined proteins like recombinant laminin fragments, researchers can maintain stem cells without any animal products. One system using a laminin-511 fragment allowed efficient colony formation of human stem cells in completely feeder-free and animal-free conditions.27Scientific Reports. A novel efficient feeder-free culture system for the derivation of human induced pluripotent stem cells These advances are critical for moving stem cell therapies from the bench toward clinical use.
Plant Cell Culture
Cell culture is not restricted to animal or human cells. Plant cell culture has carved out its own niche, particularly for producing complex proteins and high-value biomolecules. Plant cells can properly fold, assemble, and add sugar chains to complex proteins without the contamination risks associated with mammalian or microbial production systems. Growing plant cells in bioreactors rather than in open fields also offers more controllable conditions, keeps genetically modified material contained, and can reduce the cost of purifying secreted proteins from the growth medium.28PubMed Central. Recent advances towards development and commercialization of plant cell culture processes for the synthesis of biomolecules Some pharmaceutical products, including certain vaccines and therapeutic enzymes, are already manufactured using plant cell systems, and the field continues to grow as the technology matures.
Scaling Up
Most research-scale cell culture happens in flasks that hold a few milliliters of medium. Manufacturing applications, whether for biopharmaceuticals, cell therapies, or cultured-meat prototypes, require vastly larger volumes. Scaling up introduces new challenges: cells need uniform mixing without being sheared apart, oxygen has to reach every cell in a large vessel, and the risk of contamination multiplies with every additional connection and transfer step. Wave bioreactors, which gently rock a flexible bag of cell suspension back and forth, have proven effective for scaling insect and mammalian cell cultures up to 50 liters and beyond for recombinant protein production.29Biochemical Engineering Journal. Comprehensive study on Wave bioreactor system to scale up the cultivation of and recombinant protein expression in baculovirus-infected insect cells Stirred-tank bioreactors, hollow-fiber systems, and packed-bed reactors each offer different trade-offs in cell density, shear stress, and process control. Choosing the right platform depends on the cell type, the product being made, and how sensitive the cells are to mechanical disturbance.