Localization of fluorescent beads in the spongiotrophoblast layer following ectoplacental cone microinjection. (A) Placement of the micropipette tip (arrow) near the center of the ectoplacental cone region (EPC) at E7.5. The EPC is demarcated by an echogenic 'V'-shape (arrowhead) , and is therefore easy to identify on ultrasound. (B-D) Histological images obtained from a conceptus at E11.5 following microinjection of green fluorescent beads into the EPC at E7.5. Frozen sections were counterstained with DAPI (nuclei stain blue) and immunofluorescence was used to detect collagen 4 in the basement membrane of the labyrinth capillaries (pink). The beads are localized in the placental spongiotrophoblast layer between the labyrinth and the decidua. Boxed regions in (B) and (C) are shown as higher power images in (C) and (D) respectively. AC, amniotic cavity; Emb, embryo; Sp, spongiotrophoblast layer; US, uterine stabilizer.