During early pregnancy the luminal uterine epithelial cells (UECs) are extensively remodelled, microvilli are lost from the apical plasma membrane which enables interaction with the blastocyst. These dynamic changes in microvilli are unique to UECs, however the underlying molecular mechanisms driving the growth and retraction of these microvilli are currently unknown.
During early pregnancy after ovarian hyperstimulation (OH) there is a significant reduction in blastocyst implantation rate and a corresponding disruption of UEC remodelling.
The present study utilised transmission electron microscopy and morphometry techniques to measure the length of microvilli at the time of fertilisation and implantation in normal and OH pregnancy. Two cytoskeletal proteins have also been examined in UECs; the actin nucleation factor “Cordon-Bleu” (COBL) as it regulates the length of microvilli and the intermediate filament protein cytokeratin 15 (CK15).
At the time of fertilisation in normal and OH pregnancy, UECs possess short, regular microvilli with COBL concentrated at their base. At the time of implantation during normal pregnancy, the UECs lose microvilli and COBL remains localised to the region of the terminal web, where it may sever actin microfilaments to reduce the length of microvilli.
At the time of implantation in OH pregnancy microvilli are retained on the surface of the UECs. Morphometric analysis demonstrates that these microvilli are significantly longer than those seen at the time of fertilisation and they have an irregular, branching appearance. At the same time COBL is lost from UECs and CK15 is concentrated in the apical region of the UECs.
This study demonstrates significant changes in microvillar length in OH pregnancy compared to normal pregnancy and shows that their cytoskeletal supporting structures are changed. The significantly longer microvilli present at the time of implantation in OH pregnancy may inhibit blastocyst implantation and explain the reduced implantation rate seen in this model.