Why are HeLa cells used for many different types of research?

Answer 1

HeLa cells were the first type of cancer cell to survive continuous culture passage for experiments. They were immortal cells that remained viable over many passages during an experiment unlike fresh tissue samples. They made it possible to use the same cell base to compare later experiments. This made HeLa cells a common controlled factor.

At first they were simply the most reliable, rapidly growing, immortal cell line. Later it was found they had developed subtypes or strains with genetic differences.
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Answer 2

Henrietta Lack stumbled into a doctor's office some decades ago and keeled over dead. Autopsy showed a 40 pound tumor and upon taking a sample they noticed that the cells lived forever on an augar culture. They were unique and no one knows why but her cells have been used ever since for a variety of reasons where cells are needed. They never show any signs of ageing!