Green fluorescent protein and megalin expression in yolk sac visceral endoderm of mutant embryos. The cubilin gene targeting strategy (see Fig. 1) created a GFP reporter for cubilin locus expression. Shown in Ais an H&E stained section of a homozygous 8.0 dpc embryo. Shown in Bis anti-GFP fluorescence of a serial section to the one shown in A. Note that cells within the blood islands are expressing the GFP reporter. Shown in Cis an anti-megalin stained 8.0 dpc mutant embryo. Embryos in panels A-B are oriented such that anterior is to the left and posterior is to the right. The embryo in C is oriented with anterior to the right and posterior to the left. VE, yolk sac visceral endoderm; BI, blood island; Am, amnion; Asterisk, points to the boundary of the VE and presumed EE.