DIFFERENTIATION OF THE MICROVASCULAR ENDOTHELIUM DURING EARLY ANGIOGENESIS AND RESPIRATORY ONSET IN THE CHICK CHORIOALLANTOIC MEMBRANE

The present study served to determine the extent of microvascular endo thelial differentiation during early stages of morphogenesis (days 4.5 -5.5 of the 21-day incubation) in the chick chorioallantoic membrane ( CAM). CAM's, which serve as the embryonic lung, were prepared for intr avital injections of a graded series of FITC-dextrans and subsequent u ltrastructural morphometric analyses of the microvascular units. The p recapillary, capillary, and postcapillary microvascular segments prese nted a continuous endothelium that was substantially thicker than that of adult lung endothelia (DeFouw, 1988). Further, plasmalemmal vesicl es were uniformly sparse, while endothelial vacuoles, of variable diam eters, were present continuously in the proliferating microvascular un its. Average widths and depths of the interendothelial clefts were uni form and suggested complete structural differentiation from the onset of CAM morphogenesis. Based on our recent estimates of CAM microvascul ar permeability coefficients (Rizzo et al., 1995), the observed endoth elial ultrastructure was associated with microvascular selectivity com parable to that of adult pulmonary microvessels (Lanken et al., 1985). Therefore, despite incomplete ultrastructural differentiation of the early CAM microvascular endothelium, these angiogenic microvessels pre sented adult-like barrier properties. Further they were less permeable than (Wu et al., 1993; Yuan et al., 1993) and ultrastructurally disti nct from (Kohn et al., 1992) certain tumorigenic microvessels. Thus, a ngiogenesis is likely not a routinely homogeneous process, and CAM mic rovascular permeability characteristics may be teleologically signific ant.