Mature male rats that are sexually active are vasectomized and used to produce pseudopregnancy in sexually mature female rats. These pseudopregnant female rats are used as surrogate mothers. Microinjected fertilized one-cell eggs are implanted into the oviduct of such recipients. As no sperm is emitted from the vasectomized rat during mating and as vasectomized rats, unlike vasectomized mice, do not leave a copulatory plug after mating, there is no direct indication of a successful sterile mating. Hence, a proestrous female is usually chosen to mate with a vasectomized male rat because such a receptive female at this stage of its estrous cycle is more likely to copulate with the male rat.
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