The embryonic human choriocapillaris develops by hemo-vasculogenesis

The purpose of this study was to characterize normal human choroidal vascular development from 6-23 weeks gestation (WG). Markers of endothelial cells (EC) (CD34, CD31, vWf), angioblasts and EC (CD39), leukocytes (CD45), erythroblasts (epsilon chain of hemoglobin, Hb-e), proliferating cells (Ki67), and VEGFR-2 were employed. At 6-7 WG, many erythroblasts were observed within islands of precursor cells in the choriocapillaris layer and others were independent from the islands. Many erythroblasts (Hb-ε⁺) were also positive for EC markers and/or VEGFR-2. By 8-12 WG, most of the Hb-ε cells had disappeared and vascular lumens became apparent. At 14-23 WG, some EC were proliferating on the scleral side of choriocapillaris in association with forming deeper vessels. In conclusion, embryonic choriocapillaris appears to form initially by hemo-vasculogenesis (blood vessels and blood cells form simultaneously from common precursors) while angiogenesis appears to be the mode of intermediate and large choroidal vessel development in the fetus.