LABORATORY ANIMAL MEDICINE AND SCIENCE - SERIES II ...

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18 LABORATORY ANIMAL MEDICINE AND SCIENCE - SERIES II53. 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.
V-9042 RATS AND MICE: Care and Management 1958. Mating For mating, females may be caged with males, but if housedseparately, female(s) should be placed in the male’s cage for breedingpurposes. Mating or copulation usually occurs at night, when thesenocturnal animals are most active. Receptive females will exhibitlordosis, a rigid posture with the hindquarters elevated when downwardpressure is applied to the pelvis. Females will breed during apostpartum estrus that occurs between 14 and 28 hours afterparturition.59. Pheromonal influences The following phenomena are pheromone mediated:_ Whitten Effect⎯Synchronization of estrous cycles may be inducedby the introduction of any male mouse into a room or cage offemale mice._ Bruce Effect⎯A similar introduction of an unfamiliar male into acolony of females 4 days after they have been bred will oftenresult in the termination of any pregnancies._ Lee-Boot Effect⎯Another influence on the estrous cycle in whichthe absence of males causes irregular estrous cycles andsuppresses estrus.60. Detection of matingMating in rats and mice is detected normally by thepresence of a vaginal plug in the females or byvaginal lavage and examination for the presence ofsperm. The plug, as shown in these pictures, isformed from the mixing of vesicular and coagulatinggland secretions at the time of ejaculation. Vaginalplugs may be retained in mice for 16 to 48 hours, butoften fall out of rats within hours after breeding. Directobservation of the vaginal plug in the vagina orpresence of the plug in the waste pan beneathsuspended wire caging is a good indication thatmating took place.61. GestationThe length of the gestation period may vary with thestrain or stock of animal. The range in rats is from 21to 23 days (usually averaging 21 days) and 18 to 21days in mice (19 days average). Pups are usuallydelivered at night, with all pups delivered within 1 to 4hours after the onset of parturition.62. Parturition Periodic stretching and extension of the hindlegs signals impendingparturition; and for delivery, females stand in a semi-crouched position.Fetuses are delivered with either head or breech presentation. Thefemale usually eats the placenta, then turns her attention to the pups.Delivery of all pups may take between 1 to 4 hours.
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