Role of CD44⁺ stem cells in mural cell formation in the human choroid: Evidence of vascular instability due to limited pericyte ensheathment

PURPOSE. To examine mural cell differentiation and pericyte ensheathment during human choroidal vascular formation and into adulthood. METHODS. Triple- and double-labeled immunohistochemistry (alpha-smooth muscle actin [αSMA], desmin, NG2, calponin, caldesmon, CD44, CD34, and CD39) were applied to human fetal (8-32 weeks' gestation) and adult choroidal and retinal wholemounts and histologic cross-sections. Transmission electron microscopy (TEM) was also undertaken. RESULTS. Early in development CD44⁺ stem cells also stained with αSMA and CD39, suggesting a common precursor. At 12 weeks' gestation, αSMA⁺ mural precursor cells, confirmed by TEM, were found scattered and isolated over the primordial vascular tree. During development, αSMA⁺ cells formed a continuous sheath around large arterioles; in veins there were gaps in αSMA expression. The choriocapillaris had an extensive vascular bed but limited coverage by αSMA⁺ and NG2⁺ mural cells. Calponin was expressed only on large vessels, and no caldesmon was detected. Pericyte ensheathment of adult capillaries was 11% for choroid versus 94% for retina. Remarkably, choroidal pericytes had no visible intermediate filaments (IFs) on TEM, though IFs were present in retinal pericytes. Neither retinal nor choroidal pericytes stained with desmin. CONCLUSIONS. CD44⁺ stem cells are involved in the formation of mural cells in the human choroidal vasculature. A marked reduction in pericyte ensheathment of human choroidal vessels suggests a permanently open "plasticity window" and a predisposition to vascular instability and poor autoregulatory ability.