Mice are an extremely powerful research tool, but they do take extra time and effort. While you can purchase mice from vendors like Jackson Labs or outsource your genetic engineering to a number of companies, it’s often necessary, and more economical, to start a breeding colony. Even if you get your mice commercially, you’ll probably need to do some breeding in-house from time-to-time.
Plan In Advance
To establish your breeding colony, you’ll need to do a little forecasting. It will take roughly 12 weeks to generate a new litter – three weeks gestation, three weeks weaning and six weeks to reach maturity – so plan accordingly. A single, giant cohort can be unmanageable if your testing takes several days or more. On the other hand, you don’t want to be sitting around waiting for mice to reach maturity.
To figure out what you need, ask yourself the following questions:
- How many mice do you need?
- What is the average litter size for your strain?
- Do you need to generate homozygous mutants?
- Are you only using female mice for your experiments?
- How frequently do you need the mice?
Set Up the Breeding
Now it’s time to select a breeding pair. Breeders need to be at least six to eight weeks old but not older than eight months. To start, you’ll need at least one male and one female mouse (duh!). If you need a large number of mice quickly, you may want to consider trio or harem breeding. Instead of a breeding pair, a single male mouse can be placed with two or more female mice.
Breeders often get a high-fat diet and special nesting materials, but other than that, there isn’t much you need to do to get mice to breed. Place a male and female mouse in a cage and they’re often busy before you leave the room. However, you can increase the likelihood of conception by selecting females in estrus. To see if a mouse is in estrus, pick up the mouse by the tail, let them grip the cage bar, then tilt the tail up. Mice in estrus will have a swollen vulva.
The day after setting up your breeders, check for a copulatory plug in the vagina. The plug is hardened semen and seminal fluid and should be visible 12 hours after mating. Remember that the plug is not a guarantee of conception, but it is a good sign.
Gestation is about 20 days, so mark your calendars!
Check the cage starting on day 19 and record any births. When you find pups, schedule ear punching or tattooing and tail biopsies with your lab animal staff (or make time in your calendar to do it yourself).
The mother will care of the pups for three to four weeks, leaving you relatively little to do. But sometimes, like humans, mice do not know what to do as first time parents. Unlike humans, this can lead to cannibalization of the first litter, so you may not want to plan on the first litter for experiments. Even if the pups aren’t cannibalized, some labs choose not to use the first litter to avoid confounding variables that may arise during this crucial period of development.
Some strains can’t nurse or are just not good at raising pups, so you may need a foster mother. Choose a mouse that has recently given birth, and replace some of her pups with the fosterlings. A good way to introduce the fosterlings to the new mother is to mix her litter with the fosterlings and some of their soiled bedding. After half an hour, check to make sure the foster mother has all of the pups piled up next to her and remove your fosterlings if she rejects them.
Prior to weaning, take tail snips around two weeks of age. At this time, snips can be done without causing major pain. After three weeks of age, tailbones mineralize and nerve endings mature, so you’ll need to anesthetize the mouse before performing the tail biopsy.
Pups stay with their mother until three weeks, and then it is time to wean. To wean, split up male and female mice into separate cages. Make sure to put softened food pellets in the bottom of the cage, and use a water bottle even if you have an automatic watering system. This is a good time to do ear punches and tag the mice. However, if you need to identify your mice before weaning, you’ll need to arrange for tattoos on the tail or ear.
Maintaining Your Breeding Colony
Once your line is started, make sure you set aside mice to maintain the breeding colony. Don’t use them all for experiments! Keep careful records of your breeders and make sure to regularly retire females after six litters or when they reach eight months of age.
If you have an important strain or transgenic line, consider cryopreserving these mice in case of an emergency or if you have trouble breeding. Cryopreservation can also be used to introduce a strain to a new facility instead of obtaining a breeding pair. If you are getting mice from a collaborator or company, you won’t have to worry (as much) about the pathogens present at other facilities. Recovering a strain after cryopreservation, like everything in science, takes time and money, so you may only want to do this for infrequently used strains.
And remember, as with all animal experiments, follow the protocols in your country regarding laboratory animal care and use and always get institutional approval before starting experiments.
Stay tuned for my next post in which I’ll discuss the do’s and don’ts of maintaining your line.