MOUSE AND RAT BREEDING

General

RAR can provide breeding services for investigators. This includes mating, weaning and separation of sibs. RAR will maintain breeding records as requested by the investigator and will follow whatever breeding scheme that is requested. Investigators are billed for the technician's time. Requests should be made to the area veterinarian.

Breeding rodents in the research setting creates some unique problems some of which are addressed in the information provided in the next two sections. There are a number of scientific and animal welfare issues that are important. Managing a breeding colony is time consuming and expensive (see Maintaining a Colony of Wildtype Mice). In general, it is not desirable to breed animals if there is an adequate commercial supply available from vendors. It is important to carefully monitor animal breeding closely to ensure that breeding cages do not become overcrowded, that reproductive problems are not occurring, and that the genetic integrity of the strain is being maintained. There is a tendency to overproduce offspring in breeding programs resulting in unused animals and unnecessary expense. Finally, when breeding transgenic or mutant mice, particular attention must be paid to discomfort the animals may experience as a result of the phenotype and/or the genetic monitoring methods. Investigators breeding rats or mice should refer to the following information:

Biology

The laboratory mouse (Mus musculus) is a prolific breeder. Its naturally high fecundity can be utilized successfully, but it can also be a source of frustration when controlled reproduction is desired. The mouse reaches sexual maturity by six weeks of age and litter mates must be separated by this time to prevent inadvertent inbreeding. The laboratory rat (Rattus norvegicus) has a reproductive biology very similar to that of the mouse and all information herein is applicable to either species unless otherwise indicated.

The female is polyestrus, with the estrus lasting 4-5 days. The length of estrus can vary considerably depending on a number of factors. Males are able to breed at any time after puberty. Estrus can be detected by performing a vaginal swab or lavage and examining cells microscopically. There is an increasing population of cornified squamous epithelial cells with pyknotic nuclei, as the animal approaches estrus. Leukocytes gradually infiltrate during diestrus. Anestrous is characterized by non-cornified epithelial cells. Red blood cells are not prominent during proestrus as in the canine. Estrus can be induced in mice with progesterone and estrogen or PMSG and HCG, though it is more practical to just wait another 4-5 days for the next estrus. Estrus can be supressed by the presence of other females or the presence of a strange male. Estrus can be stimulated by introduction of a male.

Female mice are only receptive to male mice while they are in estrus. Mating generally occurs within 24 hours of introducing a male and an 'in estrus' female mouse, though the female is most receptive during the first few hours. There may be frequent, short copulations before adequate intromission and ejaculation occur. Mice copulate by having the male mount the female from behind while she is in a normal standing position. The secretions of the accessory sex glands in the male will form a waxy congealed plug in the vagina of the female. This plug may fall out after 24 hours. Visualization of this plug in the vagina or in the cage can be used as an indication that mating has occurred. The actual mating may not be observed as it often occurs at night.

Ovulation occurs 8-12 hours after the onset of estrus. Therefore, depending on when mating occurs, fertilization of the egg may occur at almost any time within the 24 hours of estrus. Fertilization occurs in the proximal oviduct ampulla. The embryo moves down the oviduct 18-22 hours later and it is 68-72 hours after fertilization before it enters the uterus. Implantation may not occur until day 5. Implantation can be delayed even longer if the dam is lactating heavily. Delayed implantation does not occur in the rat. Physical stimulation of the cervix and vagina is necessary for luteal development and maintenance of pregnancy. This is important if artificial insemination is performed.

If fertilization does not occur after mating, a corpus luteum can still be formed and cause a pseudopregnancy for a period of 10-13 days. During this time no estrus will occur. If fertilization has occurred, it can be expected that fetuses will be palpable abdominally by day 14 and nipple enlargement should occur. The female may start nest building approximately 5 days pre-partum. Gestation in the mouse normally lasts 19-21 days.

Prostaglandins secreted by the placenta cause luteolysis, inducing parturition. A clear discharge from the vagina may be seen 1.5-4 hours pre-partum. Parturition may last 1-3 hours. Parturition is infrequently observed, because it often occurs at night. The mother should clean the amniotic sac from the pups and may ingest all fetal membranes and any dead pups. She may also ingest live pups.

Nursing may not begin until all of the pups are delivered. Rejection of the litter is rare, but should be monitored. A foster or surrogate dam may be necessary. The female experiences estrus within 24 hours of parturition and is sexually receptive. Lactation can suppress further estrus, however, this is dependent on a number of factors and is not absolute. Lactation normally lasts 3 weeks. It is a good idea to wean the litter and sex segregate them at this time. By continually supplying surrogate litters, a female's lactation can be extended up to 70 days.

Breeding Systems

There are several standard methods of mating mice. Monogamous mating means introduction of an individual male and individual female mouse to the same cage for mating. They may be kept together long term. Polygamous mating utilizes one male with up to four female mice. Females are removed as they become pregnant. Harem or colony mating is similar to polygamous mating, except the female are not removed. Harem mating can generate the greatest number of offspring per male mouse. The disadvantage is that it is difficult to keep track of the age and genetics of each litter. Polygamous mating allows better monitoring of the offspring, but because the males and females are separated before parturition, the post-partum estrus can not be utilized and total offspring generated over a period of time will decrease. Monogamous mating allows for excellent monitoring of the offspring and allows mice to mate during the post-partum estrous. The disadvantage is that more male mice and more cages are required.

The mating system that is currently used in RAR is the monogamous system. Depending on the number of offspring desired, males and females may be separated before or after parturition. Normally the mice are kept together for a minimum of 5 days to insure that breeding has occurred. If timed pregnancies are needed, the females are monitored daily for the presence of a vaginal plug. An alternative method is to separate the male and female in a divided cage for 2-3 days and then introduce them. Mating should occur within the first day of introduction.

Mice are observed for abdominal enlargement and can be palpated abdominally to determine if they are gravid. It should be emphasized that these are extremely imprecise methods of pregnancy diagnosis. Animals are monitored daily for parturition.

If a mouse (male or female) has been previously observed to cannibalize its litter, and is valuable enough to continue breeding, it is recommended that they be observed regularly during gestation and the litter removed immediately post-partum and placed on a surrogate mother. Alternately the litter may be derived surgically and farmed out. Mice tend to adapt to being surrogate dams quite easily. A new litter is placed with a female who has just had its litter removed. This may be done at any time during lactation, but is best done within the first 24 hours of parturition by the surrogate. Observe the surrogate and pups to make sure the surrogate is not aggressive to her new litter. It may be necessary to briefly anesthetize the surrogate before placing the pups in her cage to allow the new litter to acquire the odor of the cage.

RAR will perform routine random cross breeding for investigators for the purpose of building colony size. We do not perform complex inbreeding, backcrossing etc. nor do we maintain pedigree charts. This is the responsibility of the investigator. While we can achieve timed pregnancies it should be recognized that because of the difficulty of diagnosing pregnancy, the stage may be somewhat approximate. Investigators requiring more exactly staged pregnancies can opt to purchase timed-pregnant animals from a specialty vendor, or breed and monitor their own animals.

Because of the potential complexity of breeding mice and rats, it is important to have as much information beforehand as to what is desired by the proposed mating.

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The information contained in this site is intended as a reference for University of Minnesota investigators, and animal husbandry and veterinary staff. Drug information and dosages are derived from a variety of sources and do not necessarily guarantee safety or efficacy. Information obtained through this site should not be relied upon as professional veterinary advice. Any medications administered or procedures performed on animals should only be performed by or under order of a qualified, licensed veterinarian.