Endothelial cell morphology in areas of in vivo Evans blue uptake in the aorta of young pigs. II. Ultrastructure of the intima in areas of differing permeability to proteins.

The fine structure of the intima of the pig aortic arch is described for areas of spontaneously differing in vivo endothelial permeability, as demarcated by uptake of the protein-binding azo dye Evans blue. Areas of enhanced permeability (blue areas) consistently show a variety of features not observed in areas devoid of dye accumulation (white areas). The subendothelial space of blue areas is markedly thickened and edematous, containing collagen, elastic tissue elements, and undifferentiated cells dispersed in an amorphous floccular matrix of low electron density. Endothelial cells in blue areas are generally cuboidal, with relatively short, frequently vacuolated junctions. In contrast, endothelial cells from white areas are flat and elongate, with long intercellular junctions exhibiting many interdigitations. Cytoplasmic differences include a well-developed rough endoplasmic reticulum and more frequent lysosomal bodies in blue areas and a prominent Golgi apparatus in the endothelium of white areas. Additionally, endothelial cell injury or death with and without denudation occurs with a significantly greater frequency in blue relative to white areas. An endothelial glycocalyx is some threefold thicker over the surface of white relative to blue areas. It is concluded that neither endothelial structure nor function are homogeneous within the aortic arch of the young pig and that areas of spontaneously differing permeability to proteins are associated with a spectrum of alterations in endothelial and intimal morphology.