The generation of transgenic rats is a more recent development. As such, less research has been done on the efficiency of the technique using different rat strains. In our laboratory, we have used only the Sprague-Dawley (SD) strain of rats, but Mullins et al. (11) have successfully used fertilized eggs derived from mating SD females and WKR males.
Fertilized eggs are obtained from immature (4-5 wk old) superovulated females mated with mature stud males. These females will yield 100-200 injectable eggs. To supply 20 immature females/week, 30 breeding pairs should be more than sufficient. At least 20 stud males caged individually are required for mating with the immature females and should be replaced every 8 mo to a year. An individual male should be presented with a female only every alternate day.
Estrous exbreeder females (females that have successfully pupped at least one litter) of between 8 and 16 wk are mated with vasectomized males the night before they are implanted with the microinjected eggs. Although virgins tend to produce larger litters than exbreeders, they often eat the resulting pups.
Vasectomized males are needed to engender pseudopregnancy in the exbreeder females. Experienced males are preferred, and once vasectomized, have to be replaced every 6-8 mo as they grow quickly to a large size.
18.104.22.168. Foster Mothers
Foster mothers (with pups of about the same age) are occasionally required for the rare litter that is delivered by Caesarean section.
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