a,b, 10-kDa dextran-tracer injections at E15.5 revealed a defective BBB in mice lacking Mfsd2a. a, The tracer was confined to the capillaries (arrow) in wild-type littermates, whereas Mfsd2a−/− embryos showed large amounts of tracer leakage in the brain parenchyma (arrowheads). b, Capillaries (arrows) surrounded by tracer-filled brain parenchyma cells (arrowheads) in Mfsd2a−/− cortex. Quantification of tracer-filled parenchyma cells in control versus Mfsd2a−/− cortical plates (lower panel n=7 embryos/genotype). c, Spectrophotometric quantification of 10-kDa dextran-tracer from cortical extracts, 16hr post i.v. injection, indicating that BBB leakiness in Mfsd2a−/− mice persists into adulthood (n=3 mice/genotype). d, Mfsd2a−/− mice exhibit normal vascular patterning. No abnormalities were found in cortical vascular density, branching and capillary diameter (E15.5, green: PECAM). Quantification of wild-type and Mfsd2a−/− samples (n=4 embryos/per genotype, (P>0.5)). All data are mean±s.e.m.