13 Abbildung 1: Implantation window Plate 1 depicts gradual apposition of blastocyst to endometrium during the onset of the implantation window, delineated here by putative biomarkers for endometrial receptivity: integrins (red), pinopodes (violet) and LIF (orange). Trophectodermal integrins (green) on the apical membrane surface of the blastocyst are necessary for adhesion (Plate 2). Adhesion induces reaction in trophoblast cells to differentiate into inner cytotrophoblast (dark pink) and outer syncytiotrophoblast (light pink) layers. Syncytiotrophoblasts are large, polymorph, possess several nuclei and initially invade the decidua (Plate 3). Trophoblasts now produce pregnancy-conserving β-hCG (not depicted in this scheme), which stimulates the corpus luteum to produce progesterone. In plate 4, implantation is complete. The implantation window is now closed.