The use of in vitro studies allows a reduction in the number of test animals needed to perform a study. In 24-hr post-implantation embryo culture (from day 10 to day 11 post-insemination) the 5-8 somite stage is considered to be the best starting point. A time-controlled mating procedure has been developed using a positive lordosis reaction and a short period for mating to synchronize pregnancy in rats. With this procedure, 243 hr post-mating 97% of embryos are in a stage of development ranging between 5 and 8 somites. The pregnancy rate (96%) and embryo viability rate (93.4%) are excellent. In these conditions only two or three females are needed to obtain 20 embryos at the required somitic stage. Compared with commonly used mating procedures, this technique reduces considerably the number of females needed for each experiment and workload involved. Furthermore, it allows better planning of whole embryo culture studies and can be easily adapted to obtain embryos at any somitic stage. © 1990.
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