Established cell lines are a foundational technology in modern biomedical research, serving as consistent and reproducible models for biological processes. These cell populations are cultivated outside of an organism, typically in a flask with a nutrient-rich medium, and possess the unique ability to grow indefinitely. This characteristic is the primary distinction that separates an established line from a primary cell culture, which can only divide a finite number of times before ceasing growth. Established lines offer an inexhaustible supply of uniform cells, making them an indispensable tool for investigating disease mechanisms and developing new medical treatments.
Creating Immortal Cells
Normal human cells are subject to the Hayflick Limit, undergoing only about 40 to 60 divisions before entering irreversible growth arrest called senescence. This limit relates to the progressive shortening of telomeres, the protective caps on the ends of chromosomes, which lose DNA with each replication. Established cell lines, sometimes called continuous cell lines, bypass this natural limit through genetic modifications that grant them immortality.
The most common mechanism for achieving indefinite proliferation is through activating the enzyme telomerase, which rebuilds telomeres and prevents their shortening. In the laboratory, cells can be deliberately immortalized by introducing genes, such as the human telomerase reverse transcriptase (hTERT) component, or by using viral oncogenes like the Simian Virus 40 T antigen (SV40Tag). These genetic changes disable the cell cycle checkpoints that would normally halt division, allowing the cell to proliferate continually. Cell lines derived from tumors, such as the initial HeLa line, often possess this immortal characteristic naturally because they have already acquired the necessary genetic mutations to evade senescence.
Essential Tools for Research and Medicine
The unlimited supply and homogeneity of established cell lines make them versatile platforms for scientific investigation and medical development. They are routinely used in drug discovery and toxicology testing as a rapid, high-throughput method to assess the effects of thousands of potential therapeutic compounds. Specific lines, such as the Caco-2 line derived from human colon cancer, are used to model the intestinal barrier to predict how well a drug will be absorbed into the body.
Established lines are fundamental to vaccinology, serving as biological factories needed to grow viruses for vaccine production. The Vero cell line, derived from African green monkey kidney cells, is a common substrate used to propagate viruses for vaccines against diseases like polio and rotavirus. Other lines, including HEK 293 and MRC-5, are used to manufacture vaccines, providing a consistent and scalable alternative to traditional egg-based methods.
For cancer research, established tumor cell lines model the behavior of human malignancies. This allows scientists to study tumor growth, metastasis, and the effectiveness of new chemotherapy agents in a controlled environment.
The Ethics and Legacy of Cell Lines
The history of established cell lines is inseparable from the story of the first and most widely used line, HeLa, derived from Henrietta Lacks in 1951. Lacks, an African American woman, had her cervical cancer cells taken during a biopsy at Johns Hopkins Hospital without her knowledge or consent. The cells proved uniquely robust and proliferative, and the resulting HeLa line was distributed globally, becoming the foundation for countless medical breakthroughs.
This historical event highlighted ethical failures regarding patient autonomy and informed consent. While collecting and using patient tissue without permission was common practice in the 1950s, the commercial success and scientific impact of HeLa spurred a movement for change. Following the Lacks case, international guidelines like the Declaration of Helsinki and the establishment of Institutional Review Boards (IRBs) became standard practice to ensure strict consent processes for the use of human biological materials. The legacy continues today, with agreements acknowledging the family’s contribution and seeking to address the historical injustices surrounding the commercial use of her cells.
Ensuring Quality and Authenticity
Despite their utility, established cell lines are prone to issues that can compromise research validity, making quality control a necessity. One challenge is cell line misidentification, which occurs when a faster-growing line, such as HeLa, accidentally contaminates and overgrows a slower-growing culture. This cross-contamination can lead to erroneous research conclusions, as experiments are conducted on the wrong biological model.
To combat this, authentication testing is routinely performed, with Short Tandem Repeat (STR) profiling serving as the industry standard for human cell lines. STR profiling creates a unique genetic fingerprint by analyzing repeating DNA sequences, which are compared against reference databases. Researchers must also periodically test for microbial contamination, particularly by Mycoplasma, a small bacterium that can alter cell behavior without causing visible turbidity.