Securing the Connection via Receptor-Ligand Binding.
The trophoblast attaches to the uterine epithelium via the trophoblast’s microvilli. This is mediated by ligand-receptor interactions. The integrin family of receptors span the whole membrane: intracellularly they interact with the cytoskeleton and extracellularly integrins have receptors for matrix proteins such as collagen, laminin, fibronectin etc.
During implantation, the trophoblast expresses integrin receptors extracellularly and binds to the laminin within the endometrial decidual cells.
On days 20-24 of the menstrual cycle, the glandular epithelium expresses integrin receptors, consistent with the implantation window phenomenon, and may serve as markers of the ‘endometrial capacity of implantation’.
As well as integrin-matrix interactions, there are also contacts between heparin or heparan sulphate proteoglycans with surface receptors on the uterine epithelial cells. The number of endometrial proteoglycan receptors is increased as the time of implantation approaches.
Any of these ligand-receptor interactions lead to changes in cell architecture resulting in the movement of epithelia cells from their basal lamina to a mesechymcal structure, thus providing access for the trophoblast to penetrate the endometrium.
For epithelial to mesenchymal transition image: www.hopkinsmedicine.org/dmontell/research/index.htm