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LABORATORY ANIMAL MEDICINE AND SCIENCE - SERIES II ...

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18 <strong>LABORATORY</strong> <strong>ANIMAL</strong> <strong>MEDICINE</strong> <strong>AND</strong> <strong>SCIENCE</strong> - <strong>SERIES</strong> <strong>II</strong>53. Creating strains To decrease variation from animal to animal within a species(maximizing isogenesity), animal breeders produce inbred strains ofrats and mice. A strain is designated or identified only after >20consecutive generations of brother x sister matings or youngest parentx offspring matings. The Committees on Standard Nomenclature forInbred Strains of Rats and Mice ensure that existing and newlydeveloped strains are uniformly identified. Researchers should becautioned that strains from different vendors may have slightdifferences, and strains from the same vendor should be usedthroughout an experiment to ensure consistency and validity of results.54. Maintain stocks Rats and mice can be outbred in an attempt to maintain diversity(heterogenesity) between animals of the same species yielding anoutbred stock of animals. Stringent procedures are employed tominimize inbreeding. The International Nomenclature System forOutbred Stocks of Laboratory Animals ensures uniform identification ofoutbred stocks of rats and mice.55. Sexual maturity The age at puberty varies between the species, strain, or stock of ratsand mice. In general, rats reach puberty around 50 to 60 days of age,while mice reach puberty around 28 to 49 days of age. Althoughfemale mice can have their first estrus as early as 39 days, their firstfertile mating generally occurs around 7 to 10 weeks of age. The usefulbreeding period for inbred female mice lasts through about 10 to 12months of age, although breeding could continue for several monthslonger. Rats are usually bred after 9 weeks of age and have a usefulbreeding period through about 45 weeks of age.56. Estrous cycleThis graph shows the regular estrous cycle of rats andmice. The cycle lasts approximately 5 days in the ratand 4 to 5 days in the mouse. Estrus or “heat,” theperiod of sexual receptivity of the female, usuallyoccurs on day 3 of the cycle (based on vaginalcytology) and lasts for 12 to 14 hours, typically atnight. Ovulation occurs within 8 to 12 hours after theonset of estrus. Rat and mouse females arepolyestrus, breeding throughout the year.57. Monitoring the estrous cycleThe stages of the estrous cycle can be determined byperforming daily vaginal flushing or lavage usingsterile saline (shown) and microscopically examiningthe cell types during the cycle (shown are methyleneblue stained cornified epithelial cells indicating estrus).A sterile eyedropper can be used for the flushing andcollection of the fluid. This procedure facilitatesbreeding of animals on the day of ovulation, producingtimed-pregnant animals of known gestational age.

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