The adherens junction (zonula adherens) is a lateral junction that links the actin cytoskeleton of adjacent uterine epithelial cells (UECs). This junction forms a band around the apical portion of the epithelial cells, providing lateral adhesion between the cells to resist mechanical stresses and prevent adjacent cells from separating. The terminal web anchors into the adherens junction on the lateral plasma membrane of UECs.
A major part of the plasma membrane transformation of UECs involves the loss of the terminal web. This cytoskeletal structure is lost at the time of implantation, along with the microvilli that anchor into it. However, there has been little study of the adherens junction during early pregnancy and its role in the plasma membrane transformation.
The adherens junction of UECs was examined in the rat during early pregnancy. Expression of PLEKHA7, a protein that has been reported to be specific to the adherens junction, has also been examined in these cells. Uteri of rats at the time of fertilisation and time of implantation were infiltrated with osmium tetroxide and potassium ferrocyanide (KFeCN) then examined by transmission electron microscopy (TEM). The combination of osmium and KFeCN enhanced the contrast of the plasma membrane, thereby improving resolution of the adherens junction.
At the time of fertilisation, the adherens junction was found to be present in UECs. However, at the time of implantation the adherens junction is lost. The expression of PLEKHA7 is also reduced at the time of implantation. These results suggest that the loss of the adherens junction may play a role in the disassembly of the terminal web that also occurs at this time. Furthermore, the loss of the adherens junction at the time of implantation is expected to reduce the lateral adhesion between UECs, which may be required to permit implantation of the blastocyst.