During embryonic development new blood vessels invade the brain making up the blood brain barrier. The image shows a zebrafish embryo stained by immunohistochemistry labelling blood vessels (blue), pax neurons (yellow) and mature neurons (magenta). This image was produced during the project: “Contribution of maternal thyroid hormones during teleost embryonic development” financed by FCT (EXPL/MAR-BIO/430/2013).