DISTRIBUTION OF ANIONIC SITES ON MICROVASCULAR ENDOTHELIUM OF THE CHICK CHORIOALLANTOIC MEMBRANE

It is generally accepted that luminal surfaces of adult microvascular endothelia present an anionic barrier that limits passage of anionic m acromolecules. To assess the ontogeny of the barrier, temporal and spa tial expression of endothelial anionic sites was evaluated in the chor ioallantoic membrane of chicken embryos from days 4.5 to 18 of incubat ion, After an initial flush, the vessels were perfused with cationic f erritin (CF, 1.0 mg/ml in PBS) for 2 min, Following a second flush to remove unbound CF, the chick chorioallontoic membranes (CAMs) were fix ed and processed for electron microscopy, Continuous CF binding was re vealed on the luminal endothelium, the junctional clefts and the plasm alemmal vesicles from days 4.5 to 14, However, by day 18, anionic site s had become discontinuous. Prior perfusion with protamine sulfate abo lished CF binding and facilitated native ferritin binding, Further ult rastructural evaluation, using peroxidase labeled LFA lectin, revealed sialic acid moieties in patches on the CAM endothelium. Thus, in earl y chick embryogenesis, the CAM endothelium displays a continuous patte rn of luminal anionic sites comprised in part of sialic acid. As the C AM ages, endothelial anionic sites become reduced, That the expression of endothelial anionic domains remained constant despite changes in C AM microvascular permeability in early development (Rizzo et al., 1995 a) serves to suggest a minimal role for anionic domains in the develop ment of microvascular permselectivity during normal angiogenesis.