[Factors influencing implantation of mouse embryos fertilized in vitro]

To examine the process and the rate of implantation of mouse eggs fertilized in vitro, we transferred the eggs to the opposite uterine horn of the donor mouse and assessed the effect of culture conditions on the early-stage embryos. In vitro fertilization of mouse ova Crj: CD-1 (ICR) was performed according to the technique of Toyoda et al. Embryo transfer (2 cell, 4 cell, morula and blastocyst stage) was performed by a modified McLaren's technique. Sister chromatid exchanges (SCE) were examined as indicators of cytogenetical factors, and a comparison was made of three different environmental conditions: I. in vitro fertilization-in vitro culture, II. in vitro fertilization-in vivo development, III. in vivo fertilization-in vitro culture, IV. in vivo fertilization-in vivo development. The results were as follows. 1) The implantation rate of blastocysts transferred within donor mice was 20% and the growth rate to fetus was 14.5%, but embryos from the 2 cell to morula stage could not be implanted on the endometrium of the contralateral uterine horn. 2) Where donor and recipient were different mice, there was a decrease in the implantation rate, when the recipient and donor were at a synchronized stage of the cycle. An improvement in the rate was secured when the recipient's stage was delayed 24 hours. 3) The SCE score increased significantly in groups I and III (p less than 0.001, Student's t-test) as compared with that of group IV. The decrease in the IVF embryo implantation rate vs. natural conception is due to the non-physiological environment and growth retardation of the IVF embryo causes asynchronization with endometrial age.