Immunohistochemical localization of CD31 (A–C) and PV-1 (D) in 12 (A,D), 17 WG (B,E), and 21 WG (C,F) choroid. CD31 labels the choriocapillaris and developing intermediate vessels while no PV-1 immunolabeling is seen until 17 WG and more prominently at 21 WG. The structure of the choroid is shown with H & E at the three ages (G–I). Inner segments (IS) are starting to form at 21 WG (I). (arrowhead, RPE; scale bar 30 μm; A–F, APase immunoreactivity with bleaching).