For maintaining stocks of normal and transgenic strains, mice and rats are allowed to mate and breed with minimal intervention. This is simply a matter of regulating the transfer of males and females between cages, the provision of nesting material to pregnant mothers, and weaning off the pups.
Natural matings between mature female mice (6-7 wk) and stud males (over 7-8 wk) can be used to supply the eggs for microinjection, but yield around 10 F2 hybrid eggs/mouse. Administering gonadotrophins to immature females to induce an increased release of ova (superovulation) and mating these with male studs can yield 30-40 eggs/mouse depending on the strain used. In superovulation, the timing of ovulation, copulation, and fertilization is controlled to optimize egg yield. Eighty to 100% of treated mice, and 60-100% of similarly treated rats are successfully impregnated.
It is essential both for natural and timed matings that the animals are maintained on a constant light-dark cycle. Simple on/off regimens are adequate (e.g., 1700-0600 dark; 0600-1700 light), but sophisticated regimens that simulate natural conditions are preferable (e.g., 1830-0630 0% light; 0630-0830 25% light; 0830-1130 75% light; 1130-1300 100% light; 1300-1600 75% light; 1600-1830 25% light).
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