Ch. 8— Alternative to Animal Use in Testing • 177 
USE OF LIVING SYSTEMS IN TESTING 
As detailed in chapter 6 ; two kinds of living sys- 
tems can reduce whole-animal use— in vitro sys- 
tems based on animal or human components (cell, 
tissue, and organ cultures) and systems based on 
organisms not considered animals for purposes 
of this report (micro-organisms and invertebrates). 
(Some people consider both of these in vitro 
systems.) 
In Vitro Systems 
Cells, tissues, and organs can be kept alive out- 
side a living organism and used for testing. Al- 
though animals are still required as a source for 
these in vitro systems, the animal would experi- 
ence distress for a much shorter time, and per- 
haps less distress overall, than occurs with whole- 
animal testing because it would be killed before 
any experimental manipulations were carried out. 
Occasionally, different cells, tissues, or organs from 
the same animals can be used for different inves- 
tigations. In addition, many fewer animals would 
be required for a given test, in part because varia- 
bility in the toxic response is smaller than it is with 
whole-animal tests and in part because one ani- 
mal can be used for multiple data points, further 
reducing variability. The fact that human tissues 
sometimes can be used confers an additional ad- 
vantage because the need for extrapolation from 
animal data is obviated. 
These isolated components also have disadvan- 
tages. They are usually unable to produce the com- 
plete physiologic responses of a whole organism. 
The components often become undifferentiated 
and lose their ability to perform their special func- 
tions when isolated from the organism, particu- 
larly when the sample is broken up into its con- 
stituent cells, and even more so when the cells 
replicate. Another disadvantage is that the effect 
of the route of exposure, a variable that can have 
profound effects on test results, is often impossi- 
ble to determine. 
There are many measures of damage to differen- 
tiated or undifferentiated cells— the rate of repro- 
duction, the rate of synthesis of certain substances, 
Microscopic View of Cell Culture From Rabbit Corneal Epithelium 
Photo credit: Kwan Y. Chan, University of Washington 
