Embryo-dependent induction of embryo receptivity in the mouse endometrium

The effect of intraoviductal embryos on endometrial receptivity was studied by intraendometrial and intrauterine embryo transfer. Five-week-old female ICR mice were mated after superovulation; a vaginal plug confirmed day 1 o f pregnancy. On day 4 (90h after hCG injection), blastocrysts were collecte d and transferred to pseudopregnant female mice and to recipient mice in wh ich the uterotubal junction had been ligated bilaterally on day 1 of pregna ncy. Three embryos per uterine horn, a total of six embryos per recipient m ouse at days 1-6, were transferred to the endometrium or uterine cavity and implantation and pregnancy rates were calculated. The implantation rate fo r intraendometrial embryo transfer to recipients of days 3, 5 and 6 was sig nificantly higher for uterotubal junction-ligated mice (72.2, 20.8 and 9.7% , respectively) than for pseudopregnant mice (55.0, 8.3 and 0.0%, respectiv ely). The implantation rate for intrauterine embryo transfer to recipients at days 2, 5 and 6 was significantly higher for uterotubal junction-ligated mice (11.1, 25.0 and 8.3%, respectively) than for pseudopregnant mice (0.0 , 3.3 and 0.0%, respectively). Uterotubal junction-ligated mice achieved im plantation and bore neonates by intrauterine embryo transfer on days 2 and 6, whereas no implantation was achieved in pseudopregnant mice. The differe nce in implantation rate could not be explained by a difference in progeste rone concentration between the groups. The distribution of proliferating ce lls in the endometrium was also studied immunohistochemically by use of ant i-proliferating cell nuclear antigen (PCNA) antibody in the recipient mice. PCNA-positive cells were more abundant in uterotubal junction-ligated mice and demonstrated a marked extension from the epithelium to the stroma over time, in contrast to those in pseudopregnant mice. These findings indicate that an intraoviductal embryo exerts a biological effect by sending a sign al to the endometrial epithelium and stroma, thus facilitating endometrial receptivity to the embryo and improving the rate of implantation..