Safe medical treatments are crucial tools in the fight against human diseases. Ensuring that medical treatments are safe inevitably requires investigating their effects on our most basic building blocks: cells. While using animals for research is useful and often necessary, it can be expensive and time-consuming (Doke & Dhawale, 2015). Fortunately, a variety of techniques allow cells to be isolated from animals and humans and grown in the lab for study. These techniques for culturing cells offer a viable alternative for experimental use.
While cell culturing is a modern essential tool for scientists, humans have been culturing cells for millennia, primarily for the purpose of brewing alcoholic beverages. A current hypothesis is that during the development of agriculture, yeast cells floating in the air landed in open containers with grains that had been exposed to water (Meussdoerffer 2009). This mixture of raw ingredients combined with sufficient time created proto-beers, leading to the accidental creation of the first brewed beverages.
An unexpected beneficial side-effect of this accidental brewing was the conversion of non-potable water into a drinkable form, a huge and often necessary benefit to early civilizations. As the technique to produce beers was harnessed and improved over generations, brewers began to pass their stock cultures from one to another and from generation to generation. This system of maintaining living cells in a liquid medium became the first way to culture cells (Bostwick 2014).
Modern cell culture techniques focus on different types of cells: tissue cells found within both animal and human bodies. Tissue cell culturing techniques were developed in the 1900s as a way to research medical questions (Taylor 2015). Ross Harrison and Leo Loeb devised ways to culture cells from frog tissues in a liquid medium. A solution of blood clots, agar, and salt provided cultured cells with nutrients that allowed them to survive and grow, which led to it being called growth medium. Harrison then created the “hanging-drop” method in which a droplet of blood plasma is suspended by gravity (imagine a droplet of water hanging from your fingertip) and held stable by surface tension (Navis 2007), rather than spreading across a culture plate.
Following this, Alexis Carrel refined those early techniques, improving these growth media, and was able to establish cultured cells from the tissues of chicks and tumors. In order to prevent culture contamination by bacteria and fungi which also thrive in media, he introduced antibacterial and antifungal compounds to the media (Taylor 2015). For his contributions to the field, he became the first American to win the Nobel Prize in Medicine in 1912.
Cell lines and their current uses
Despite advances in cell culture media, growing tissue and cells for research was expensive because the cells would die. This factor, unfortunately, slowed down research, as the cells needed to be continuously replenished with fresh tissue, which needed to be identical to the cells being replaced (Cotter & Al-Rubeai, 1995). Scientists required a solution to these problems that would be cheaper and easier to manage for cell culture to really take off.
A significant leap forward was made in 1951 by cell biologist George Otto Gey when he successfully cultured the first confirmed, immortal human cell line from cervical cancer cells: HeLa. An immortal cell line is a population of cells which, due to mutations, can evade the normal aging process. These “immortal” cell lines can be grown in the lab indefinitely, which means experiments could be repeated easily by growing them back from stock. In time, the ease of culturing them encouraged the sharing of cells between scientists and made them easily accessible. Finally, since the cells would remain identical over time, this would allow for standardization between labs and replicates. These advantages made immortal cell lines much cheaper and desirable.
Within a single year after being isolated, HeLa cells were used to develop the polio vaccine (Cox 1953). As demand grew for HeLa cells, they were used for physiology studies and research in genetics and medicine. In fact, HeLa cells are so persistent and ubiquitous, concerns have arisen that HeLa cells may be contaminating other cell lines, and efforts continue today to ensure the purity of other cell lines (Gartler 1968).
More immortal cell lines emerged following HeLa, including the Vero line, which was isolated from kidney cells extracted from an African green monkey (Chlorocebus sp.). This lineage can be maintained in culture indefinitely, without showing signs of aging. The main application for this line is the development of vaccines against diseases as detrimental as polio and rabies (Montagnon 1989).
In addition to the numerous benefits of animal cell culture lines, plant cells can also be cultured. Some plant cell lines are remarkable because single cells can grow into complete plants, unlike animal cell lines. For instance, carrot cells isolated from unfertilized eggs were made to double their DNA and then developed into an entire new carrot (Kiełkowska et al., 2014). Another widely-used plant cell line is the tobacco BY-2 cell line, which has several advantages including a high proliferation rate and the ability to be genetically altered through the direct incorporation of foreign DNA (M. Srba et al., 2015), a useful process to produce pharmaceuticals (Baeshen et al., 2014).
New techniques/technologies
Cell culture technology has continued to advance as our needs change. Embryonic stem cells (ESCs) are the foundation for every cell, tissue, and organ in the human body, and were first derived in 1981 from a mouse embryo for later use in medical treatments, such as cardiovascular diseases (Martin 1981). Currently, embryonic stem cells are cultivated and used in numerous therapies in the field of biomedicine, for example in degenerative diseases that affect elderly people (He et al., 2003).
Traditional cell lines are grown on flat media and exist as a single cell layer. While this has led to tremendous advancements, essential cellular functions, such as protein synthesis and hormone responses that are present in tissues are missed by traditional cell culturing techniques (Ouyang et al., 2015). Similarly, tissues, like skin, are made of several layers of different cell types that are arranged in a specific fashion in 3D space to provide their function.
Moving forward, new techniques which could mimic the functions of living tissues require transitioning from these so-called monolayers to cultures with many layers in 3D space. The development of 3D cultures can more closely resemble real tissues and will likely have a strong impact on drug development and many other fields, with the potential to reduce testing on laboratory animals and cut down the total cost of medical research. These 3D cultures would allow researchers to better understand how different cell types communicate and work together. However, establishing 3D cultures as a mainstream approach requires the development of standard protocols, new cell lines, and advanced analysis methods such as 3D imaging.
Conclusion
Cell culturing continues to have numerous applications in research and medicine because of its flexibility in experimental applications. Tissue cell culturing allows researchers to conduct experiments using the basic building blocks of all organisms. After years of developing and refining cell culturing techniques, immortal cell lines isolated from living tissue can now be maintained in the lab with basic equipment and low cost. Although modern tissue cell culture techniques have marked some of the most monumental advancements in science and medicine, it has been deeply rooted in our history because of the cultural significance of one of its earliest uses that still lives on: brewing beverages.