A decrease in uterine receptivity after fresh IVF transfers compared to frozen transfers has been documented, however the underlying mechanisms are unknown. A rat ovarian hyperstimulation (OH) model provides a novel mechanism to study endometrial changes caused by IVF drugs.
During normal pregnancy in rats, there are significant changes in the basal plasma membrane of uterine epithelial cells (UECs) at the time of receptivity; the basal lamina becomes thickened and highly tortuous and there is a loss of focal adhesions (FAs). This enables the UECs to become ‘unstuck’ in the implantation chamber surrounding the blastocyst, thus allowing invasion into the underlying stroma. This study investigated changes in morphological FAs and associated proteins at the time of implantation after ovarian hyperstimulation.
Results from this study demonstrate a flattened basal plasma membrane containing numerous FAs at the time of implantation during OH pregnancy, similar to that seen at times of non-receptivity. There is also an increase in paxillin, an integral focal adhesion protein and integrin β1, a membrane bound protein linking the actin cytoskeleton to the basal lamina. Phosphorylated focal adhesion kinase, which indicates activation of the FA complex, is also increased in the basal portion of UECs at the time of implantation during OH pregnancy.
The retention of morphological FAs and the increase in a number of key FA proteins at the time of implantation after OH compared to normal pregnancy suggests that basal FAs are retained and activated. Thus UECs remain ‘clinging on’ to the basal lamina which could prevent penetration of the blastocyst into the underlying stroma, providing a reason for the decrease in uterine receptivity immediately following fresh stimulated IVF cycles.