Animals in Research Facilities / 93rodents or other species, including humans and even jellyfi sh, can
be inserted into mice to merge with other genes.^22 Because large
numbers of rodents kept in close quarters present ideal conditions
for disease outbreaks, breeders developed “specifi c pathogen free”
mice, which have none of the bacterial or viral pathogens carried
by “conventional” mice. “Gnotobiotic” mice are delivered by C-sec-
tion; the mother dies in the process. The sterile uterus is placed in
a sterile environment. Scientists can infect the fetuses with just
a single microbe, allowing for the study of the effects of only the
one organism. Scientists engineered “transgenic” mice by altering
embryos with one or more genes that produce a trait of interest,
such as a predisposition to develop human breast cancer. Selec-
tive breeding results in generations of mice characterized by the
genetic alteration. “Chimeric” mice are produced by altering the
DNA sequences of embryonic mouse stem cells and adding them to
mouse embryos. Some of the offspring of chimeric mice will have
the altered DNA sequences.
The production of rodents, particularly mice, for laboratories
has become a huge industry. Researchers can consult extensive cat-
alogues and choose from over thirty-fi ve hundred strains.^23 Mice
became the species of choice for most laboratory research because
of the convergence of three trends concerning how scientifi c work
takes place. Two of these were apparent in Little’s efforts to develop
inbred mice. The fi rst is the need for standardized equipment, mate-
rials, and procedures. Although taken for granted today, scientists
did not seek standardization until the mid-to-late-nineteenth cen-
tury. Previously, researchers had used a variety of species in their
experiments.^24 As other aspects of the lab, such as machinery for
measurement, became standardized, the next logical step was to
standardize the animals used in research. Scientists moved away
from using a variety of species, and the purpose-bred mouse or rat
became the basic “laboratory animal.” The commercial production
of rodents, who reproduce quickly and respond well to selective
breeding, met the new demand for standardization. In the transi-
tion from diversity to generality, rodents used in research lost their
identity as individual animals and became “tools of the trade, part