Studies on the Transfer of Fertilized Mouse Eggs to Uterine Foster-Mothers
Authors: McLaren Anne , Michie Donald 
Published: April 2021

ABSTRACT
The origin and potential uses of the method of egg transfer in mammals are briefly surveyed. An experiment is described in which genetically labelled fertilized mouse eggs were transferred to the left uterine horns of recipient female mice. Eggs were obtained both by induced ovulation of sexually immature donors and by spontaneous ovulation of adult donors. Both pregnant and pseudo-pregnant recipients were used. The post-coital stages of donors and recipients were independently varied. At or days post-coitum the recipients were killed and their uterine contents recorded. The operation had no effect upon the recipients’ chances of becoming pregnant, nor did it have substantial effects upon the implantation and subsequent survival of eggs in the umnjected horn of the uterus. In the injected horn, the implantation rate was reduced by about one-third in recipients both at and days post-coitum. Post-implantational mortality in the injected horn was increased in day recipients, but not in day recipients, except when the operation was accompanied by gross surgical trauma. The yield of live embryos from eggs transferred to recipients which had themselves been mated to fertile males was highest in the days combination, lowest in the days combination, and intermediate in the two synchronous combinations. These differences may in part be attributable to competition between native and transferred eggs. Such competition was shown mainly to occur before, during or shortly after implantation, and to be a property of non-synchronous rather than synchronous donor-recipient combinations. But the differences were in part independent of the presence of competing native eggs, as shown by transfers to recipients mated to sterile males; the yield from the combination was still very much greater than that obtained from the combination. 6; Transfers of eggs artificially ovulated from sexually immature donors gave results in all respects similar to those obtained with eggs spontaneously ovulated by sexually mature donors. The distribution of alien embryos among the recipients suggested that apart from the random loss of parts of inocula through escape or death of individual eggs, there was another and distinct process at work causing the loss of whole inocula as units. Over the range tested (0-18 eggs) the number of alien embryos and the number of implantations of all sorts in the injected horn rose linearly with increasing numbers of eggs injected. The number of native embryos in the injected horn declined with increasing numbers of eggs injected. When the number of implantations, with increasing numbers of eggs injected, began to exceed the normal quota for one horn, the number of live embryos in the injected horn (alien + native) increased less steeply and the proportion of dead and resorbing embryos began to rise. The day series gave some evidence that when the number of implantations in the injected horn was raised above the normal level, successful implantation in the uninjected horn was reduced, so that the total number in the two horns combined never exceeded an average of about . The experimental results are discussed in the light of previous work and of future application. We conclude that with reasonable control of natural and technical hazards a yield of about 50% of fertilized mouse eggs recovered as live young should be attainable.