Knowing what a workhorse line does and does not model: why a hepg2 cell line and hepg2 cells retain some hepatic function and lose much of the drug metabolising capacity people assume, where primary hepatocytes, human primary hepatocytes and primary mouse hepatocytes, or plain mouse hepatocytes as a catalogue lists them, answer what the line cannot, what u2os cells, caco2 monolayers, hct116 and hct116 cells, k562 cells with the k562 cell line and k562 cell lines listings behind them, thp-1 cells and bv2 cells are each conventionally used for and what each is known to drift on, how human ipsc derived material, human ipsc cell lines and ipsc cell lines generally change the question and the cost, what bl21 de3 is doing in the same conversation as an expression host rather than a model, and why authentication and passage discipline matter more for the lines everybody uses than for the rare ones

Using a workhorse line honestly

1. State what the line models. Write down which property of the biology the line is standing in for. If that property is one the line is known to have lost, the experiment needs a different system or an explicit caveat.
2. Authenticate and screen on arrival. Short tandem repeat profiling against a reference and a contamination screen, before the line joins the culture area. The most used lines are also the most misidentified, precisely because they are shared so widely.
3. Fix a passage window. These lines drift with passage, and the drift is faster in some than others. Bank early at low passage, work from the bank, and record passage number with every experiment.
4. Control the differentiation state. Several of these lines behave very differently depending on confluence, medium and whether they have been differentiated. Fix the protocol and describe the state, because the same line can be two systems.
5. Pair with a primary system where the claim needs it. Where a conclusion depends on physiology the line has lost, a primary or stem cell derived system is what supports it. The line is then the screening tool and the primary system the confirmation.

Convenience has a cost

These lines are used because they are robust, cheap and familiar, and every one of those properties comes from adaptations that took them away from the tissue. That is a reasonable trade when the question suits it.

The failure is not using them; it is using them and describing the result as though it came from the tissue.

Provenance travels badly

A line handed between laboratories for years accumulates undocumented passages, medium changes and occasionally an identity swap. The stock in your freezer may differ substantially from the one in the original publication.

Buy from a repository where possible, authenticate on arrival, and record where the stock came from. It costs little and it is the first question a careful reviewer asks.

Common questions

Is a hepatic line a model of liver?

Partly. It retains some hepatic functions and has lost much of the drug metabolising capacity of primary hepatocytes, so it models some questions well and metabolism poorly. Say which you are relying on.

Why authenticate a line everybody uses?

Because widely shared lines are the most likely to have been swapped somewhere in the chain, and morphology will not reveal it. Profiling is inexpensive and settles it permanently for that stock.

Do these lines need a passage limit?

Yes. They drift, and a study spanning a wide passage range has an uncontrolled variable. Bank at low passage and return to the bank rather than extending a culture indefinitely.

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