MACROMOLECULAR SELECTIVITY OF CHICK CHORIOALLANTOIC MEMBRANE MICROVESSELS DURING NORMAL ANGIOGENESIS AND ENDOTHELIAL DIFFERENTIATION

Progressive angiogenesis and endothelial differentiation in the chick chorioallantoic membrane (CAM) serve to accommodate oxygen demands of the growing embryo. The present study evaluated CAM microvascular endo thelial permselectivity during the most rapid phase of angiogenesis (d ay 10) and after initiation of endothelial cytodifferentiation (day 14 ). Chick embryos were incubated using established shell-less culture t echniques for intravital and ultrastructural observations. Systemic mi croinjections of FITC-dextrans (40, 70 and 150 KDa) provided an index of endothelial permselectivity after 2.5 min and 10 min perfusions. Ul trastructural examinations of the same dextran probes served to detect small, intermittent foci within the perivascular interstitium. Althou gh minor variations of dextran particle distributions around specific segments of the microcirculation were observed ultrastructurally, peri vascular accumulation was not sufficient to elicit a detectable fluore scent signal. Thus, substantial accumulation of the graded-dextran ser ies in the perivascular interstitium was not detected. Morphometric an alyses of the precapillary, capillary, and postcapillary microvascular segments served to demonstrate a continuous endothelium which display ed cytoplasmic attenuation at day 14. Plasmalemmal vesicles were few a nd uniform within the microvascular units at day 10. A three-fold incr ease in vesicle densities characterized the precapillary endothelia at day 14. Average widths of the endothelial junctional clefts were homo geneous within the segmental microvascular endothelia at both days 10 and 14. Junctional cleft lengths were also homogeneous, except the sig nificantly longer capillary endothelial clefts observed at day 10. The se results are consistent with the concept that, despite certain diffe rences in segmental vesicle densities and junctional cleft lengths, ne ovascularization of the CAM is achieved without excessive macromolecul ar efflux across the microvascular endothelia.