Endothelial surface characteristics in pigeon coronary artery atherosclerosis. I. Cellular alterations during the initial stages of dietary cholesterol challenge.

The early events in coronary artery atherosclerosis in White Carneau pigeons were studied by combined scanning and transmission electron microscopy. Endothelial cells throughout the right coronary artery were elongate in shape, having an axial ratio (width + length) of 0.48 +/- 0.03, and were oriented with the long axis parallel to the direction of blood flow. Deviation in both orientation and morphology were found in control birds in zones of high probability for disease. Endothelial cells in the high predilection zones were less elongate in shape, having an axial ratio of 0.69 +/- 0.04, and had poorly defined margins and a paucity of microvilli. These less elongate cells were on the average 50 per cent larger (surface area of 147.9 +/- 13.3 versus 93.4 +/- 5.5 sq. microns for the typical cell) than the normal endothelial cell. Upon cholesterol challenge, the percentage of elongate cells decreased in the high propensity zones within the first few weeks, and following 8 weeks of cholesterol challenge the less elongate cells were associated with nascent atherosclerotic lesions. In addition to these large irregularly shaped endothelial cells, the early lesions characteristically were encircled by small cells having prominant nuclei and a large number of microvilli clustered over the nuclear area. Continuation of the cholesterol diet beyond 36 weeks resulted in progressive lesion enlargement and the selective binding of blood cells to the lesion periphery. This margination of blood cells was maximal at 36 weeks when the process was evident in 60 per cent of the cholesterol-fed animals. With further prolongation of cholesterol challenge, adherent cells were found in only 25 per cent of the birds. The surface events observed by scanning electron microscopy were correlated with ultrastructural changes in the endothelial glycocalyx. Cholesterol challenge resulted in a significant reduction in both the amount of ruthenium red stain and the number of cationized ferritin particles which were bound to luminal surfaces. In control diet animals, the glycocalyx thickness was in the range of 700 to 1000 A, with the thinnest areas being those in zones of high disease predilection and numerous nonelongate endothelial cells. When analyzed on an individual cell basis, the villus portions of the endothelial surface (glycocalyx thickness, 436 A) averaged 64 per cent thinner than the nonvillus portions of the cell. Upon cholesterol challenge, the glycocalyx was reduced in all arterial zones to 450 A, which approximated the thickness found for microvilli. Our results clearly document changes in endothelial morphology during the early stages of coronary atherosclerosis. These morphologic features are discussed with respect to the role the altered cells may play in disease progression.