Studies have shown a decrease in uterine receptivity after fresh IVF transfers compared to frozen transfers, however the mechanisms underlying this are currently unknown. A recently developed rat ovarian hyperstimulation (OH) model provides a novel mechanism to study changes in the endometrium caused by IVF drugs.
One important aspect of normal blastocyst implantation is the removal of uterine epithelial cells (UECs) to facilitate invasion of the blastocyst into the underlying stroma. At the time of fertilisation the basal plasma membrane is flattened and contains focal adhesions (FA) and associated proteins, paxillin and talin. At the time of implantation, there is a decrease in FA proteins, a loss of morphological FAs and an increase in tortuosity of the basal plasma membrane. This study aimed to investigate the changes in the basal plasma membrane, FAs and associated proteins and thus the ease at which luminal UECs are removed at the time of implantation in the rat OH model.
Ultrastructural studies on UECs at the time of implantation in OH rats shows that the basal plasma membrane is flattened and contains morphological FAs. This is in contrast to the tortuous basal plasma membrane lacking FAs seen at this time during normal pregnancy. An increase in UEC basal paxillin and talin staining was seen at the time of implantation in OH compared to normal pregnancy. This correlated with a statistically significant increase in paxillin delta, the isoform associated with inhibiting cell migration, in OH compared to normal pregnancy.
The dramatic alteration in the appearance and molecular composition of the basal plasma membrane of UECs at the time of implantation in OH compared to normal pregnancy suggests a phenotype that is more difficult to remove. This may provide a mechanism to explain the decrease in uterine receptivity immediately following fresh stimulated IVF cycles.