Source: Kay Stewart, RVT, RLATG, CMAR; Valerie A. Schroeder, RVT, RLATG. University of Notre Dame, IN

Millions of mice and rats are bred for use in biomedical research each year. Worldwide, there are several large commercial breeding facilities that supply mice to research laboratories, but many facilities choose to also breed mice and rats in-house to reduce costs and increase research options. When breeding in the animal facility, researchers are able to manipulate the genetics of the animals, time the pregnancies to meet the needs of the research, and work with embryos and neonates as required.

Mice and rats can be bred in a variety of schemes and methods. Technical procedures, such as the use of vaginal cytology, visualization of the vaginal area, and observation of copulatory plugs, have been developed to assist with the synchronization of breeding to correspond to research requirements. This manuscript is an overview of the basic fundamentals of mouse and rat breeding and technical procedures used. More detailed descriptions of the complex breeding schemes, and the full description of the methods for vaginal cytology, are available in the list of references.

1. Information needed when pairing animals includes strain/stock of the animal utilizing proper nomenclature, dates of birth for the breeder male and female, and the setup date. Accurate recordkeeping is imperative with breeding colonies.

2. Sex determination of mice and rats is done by comparing the anogenital distances. In females, the distance between the anus and the external genitalia is shorter than it is for males. The presence of a scrotal sac in male animals is another sex indicator.<

In-house breeding colonies offer flexibility to research, especially with projects that utilize embryos or neonates. Using techniques such as timed mating with copulatory plugs and vaginal cytology, the date of conception and gestation can be more accurately predicted. This allows researchers to more carefully plan their experiments. Controlling environmental factors such as light cycles, temperature, humidity, and vibrations will optimize the breeding outcomes.

1. Breeding Strategies for Maintaining Colonies of Laboratory Mice. A Jackson Laboratory Resource Manual. http://ko.cwru.edu/info/breeding_strategies_manual.pdf.
2. Byers, S. L., Wiles, M. V., Dunn, S. L., and Taft, R. A. 2012. Mouse estrous cycle identification tool and images. PloS One. 7, 1-5.
3. McLean, A. C., Valenzuela, N., Fai, S., Bennett, S. A. L. 2012. Performing vaginal lavage, crystal violet staining, and vaginal cytological evaluation for mouse estrous cycle staging identification. J. Vis. Exp. 67, e4389. doi:10.3791/4389.
4. Mamrot, J., Pangestu, M., Walker, D., Gardner, D. K., and Dickerson, H. 2015. Confirmed dioestrus in pseudopregnant mice using vaginal exfoliative cytology improves embryo transfer implantation rate. Reproductive BioMedicine Online. 31. 538-543.
5. Institute for the Laboratory Animal Research. 2011. Guide for the care and use of laboratory animals, 8^th ed. Washington (DC): National Academies Press.