By the time Henrietta Lacks died in 1951 at the age of 31, she had already achieved a sort of immortality.

Without her knowledge, her doctor had harvested cells from a tumor on her cervix, where her cancer proliferated, and was attempting to keep them alive outside her body. Normally, human cells do not survive long once they have been severed from the organism they belong to: they will divide no more than about 50 times, then die through a process called apoptosis. Lacks’s doctor at Johns Hopkins Hospital, George Gey, figured he could cure cancer if only he had a line of cells that could reproduce indefinitely, but all of his cultures failed, until he met Henrietta. Her cells, later labeled HeLa, just kept dividing.

What was special about this woman, who was recently featured in an HBO movie, The Immortal Life of Henrietta Lacks?

The answer has to do with particular mutations in her cells caused by the human papillomavirus that had infected them. HPV inserts its own DNA into that of the host, resulting in a genetic hybrid. Not all HPV infections lead to cancer, and not all cancer has the potential to be an immortal cell line, but Lacks’s specific mutations had at least two characteristics that made her cervical cells special.

Scanning electron micrograph of just-divided HeLa cells.Zeiss Merlin HR-SEM.
These two HeLa cells have just divided -- a process that will go on for as long as the cells are nurtured.

For one, HeLa cells are prolific dividers. Gey was surprised at just how quickly his cultures doubled in number. Even among cancers, these cells were reproductive superstars.

Secondly, they have an enzyme called telomerase that is activated during cell division. Normally, it is the gradual depletion of telomeres — a repetitive strand of DNA on the ends of the chromosomes — that stops cells from dividing indefinitely. But active telomerase rebuilds telomeres cut during division, allowing for indefinite proliferation.

HeLa cells are not the only immortal cell line from human cells, but they were the first. Today new immortal cell lines can either be discovered by chance, as Lacks’s were, or produced through genetic engineering.

Multiphoton fluorescence image of HeLa cells stained with the actin binding toxin phalloidin (red), microtubules (cyan) and cell nuclei (blue).Nikon RTS2000MP custom laser scanning microscope.
HeLa cells are stained with fluorescent compounds and photographed under a microscope.

The cells have been a boon to biomedical science, playing a role in the development of the polio vaccine and thousands of other patented discoveries. HeLa gives researchers a way to conduct repeatable experiments on human cells without testing directly on humans, although the cells are arguably no longer human at all.

Genetically, HeLa cells contain parts of Henrietta Lacks’s own DNA, mutations introduced by the strain or strains of HPV that infected her, as well as uncounted numbers of new mutations introduced organically through cellular division after the original cells were harvested from her body. A normal human cell has 46 chromosomes — a HeLa cells tends to have between 70 and 90.

Title: HeLa-IV Description: Scanning electron micrograph of an apoptotic HeLa cell. Zeiss Merlin HR-SEM. Categories: Research in NIH Labs and Clinics Type: Color, Photo Source: National Institutes of Health (NIH) Date Created: 2013 Date Added: 8/9/2013
The HeLa cell is in the process of apoptosis -- a programmed cellular death.

According to some scientists, the HeLa cell line should properly be considered its own species. It is a sort of single-cellular organism that reproduces asexually through division and evolves through mutations that compound over time. It is a domesticated species, dependent on humans for food and shelter — the cow of microbial life, perhaps.

The story of the woman who non-consensually gifted HeLa to the world was largely unknown for decades after her death, even as the cells themselves contributed to significant advances in medicine. That changed thanks to The Immortal Life of Henrietta Lacks, a book that chronicles the efforts of author Rebecca Skloot to find Lacks’s family and tell her story. The HBO movie, starring Oprah Winfrey, is based on that account.

Multiphoton fluorescence image of HeLa cells Multiphoton fluorescence image of HeLa cells with cytoskeletal microtubules (magenta) and DNA (cyan).Nikon RTS2000MP custom laser scanning microscope.
 In this multiphoton fluorescence image, the microtubules are stained purple and the HeLa DNA is cyan.
Photos via National Institutes of Health / Flickr, Tom Deerinck / Flickr (1, 2, 3), National Institutes of Health / Wikimedia