A cell line is a population of cells grown in vitro that can be subcultured over several generations. Normal cells cease to divide after a certain number of cell divisions, a process known as replicative senescence. This number, called the Hayflick limit, was conceptualized by Leonard Hayflick in 1961 when he observed that fetal cells grown in culture could only divide 40-60 times. This limit is due to the shortening of the telomeres during each round of cell division, preventing cell division beyond insufficient telomere length. Overexpression of the enzyme telomerase prevents telomere shortening, and is one of the methods to produce immortal cell lines.

Types of Cell Lines

Primary cell lines obtained directly from animal tissue retain the approximate genotypic and phenotypic characteristics of their cells of origin. For example, human lung cell line BEAS2B and retinal cell line RPE1 have close to the normal number of 46 chromosomes. In contrast, cell lines obtained from cancer cells can proliferate indefinitely and are called transformed cell lines. These cell lines show additional attributes like anchorage independence and lack of contact inhibition. Transformed cell lines also commonly have an altered number of chromosomes. For example, the SW480 and A549 cell lines can have up to 56 and 66 chromosomes, respectively.

Validating Cell Lines

Cell lines are prone to genomic instability and cross-contamination in the lab. Therefore, it is essential to validate them routinely. Techniques such as spectral karyotyping help identify numerical and structural chromosomal aberrations and detect cross-contamination. Cell lines can also be validated at the molecular level by STR profiling, a method used to analyze the number of short tandem repeats (STR) in DNA which are unique to each cell line.