Optimal Number of Embryos for Transplantation in Obtaining Genetic-Modified Mice and Goats

The technology of creating genetically modified animals (placental mammals) by microinjection into the pronucleus of a fertilized egg suggests, as one of the key stages, the transplantation of early embryos into female recipients. However, there is a wide range of opinions among researchers about the optimal number of embryos to be transferred to the female recipient. Thus, data on transplantation of 20–60 mouse embryos and from 2 to 6 goat embryos to one recipient are given in the methodological literature and experimental articles devoted to the method of creating genetically modified animals. Thus, the standard recommendation is the transfer of a much larger number of embryos than that which develops in animals of both species in physiological pregnancy. At the same time, technology of transplantation of bovine embryos (cattle) involves the transfer of one embryo, which is the physiological norm for this species of animals. Clinical protocols of assisted reproductive technologies for the transplantation of human embryos also recommend the transfer of one embryo, because transferring the number of embryos greater than in physiological pregnancy leads to increased risks. In our work, we analyze the results of experiments on obtaining genetically modified mice and goats and provide data indicating the need to revise the standard recommendations on the number of transferred embryos downward. We believe that the number of transferred embryos should not exceed the number of embryos characteristic for physiological pregnancy. Excess of the number of transplanted embryos leads to a pathological course of pregnancy and a significant decrease in overall performance.