Trophoblast outgrowth and invasion of villous explants on the extracellular matrix (ECM) gel accompanying elevated SERPINE2 expression. (A) Western blot analysis was used to detect the secreted SERPINE2 protein in the supernatant of explant cultures. Significant increases of the explants during culture were evidenced compared to the loading controls (BSA from FBS in the culture medium, partially removed by affinity chromatography). (B) After long-term (10-day) explant culture, the gels had shrunk and formed condensed 'balls' and probably detached from the plastic surface of the culture dish to float in the medium. (C) Before condensation of the ECM gel, a 3D mesh could be observed under an inverted microscope. An illustrative photo of a 7-day explant is shown. (D) Explants in shrunken gel were harvested and immunochemically stained with SERPINE2 antiserum. (E) The invading, lined trophoblasts were immunoreactive with SERPINE2 (arrowhead). (F) The invading cells were mostly CK-7-positive trophoblasts. Scale bars represent 500 μm (C, D), and 100 μm (E, F).