There is another area of toxicity testing where scientists have been equally reluctant to use human cells. Sometimes chemicals only become hazardous when metabolized, or broken down, in the liver, so researchers often include liver cells in their in vitro tests to mimic the body's main metabolic reactions. The Ames bacterial test, which is designed to identify mutagens and cancer-causing chemicals, routinely includes cells from the rat's liver. Yet common sense would demand the use of human liver cells since differences in metabolism between rats and people are known to be the rule rather than the exception.(45) Differences even occur between different rodent species as discovered during an investigation of 74 chemicals by the Edinburgh-based contract laboratory Inveresk Research International. Each substance was assessed for mutagenicity by in vitro bacterial tests incorporating liver cells from either rats, mice or hamsters. "Marked differences" in metabolism between the species were found,(46) suggesting that reliance on animal tissues might fail to identify some mutagens.
As long ago as 1982, Britain's Guidelines for the Testing of Chemicals for Toxicity stated that,.
“.. in the assessment of risk to man there are obvious theoretical advantages in the use of [a liver cell mixture] prepared from human tissues, which may differ from tissues prepared from rats . . . in their ability to activate or detoxify chemicals.”
Nevertheless, a survey of papers published by the scientific journal Mutagenesis during 1989 showed that in nearly every in vitro test where liver cells had been added to metabolize test chemicals, the tissue had originated from rats or hamsters.
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