The pathogenesis of giant cell arteritis: morphological aspects.

The light-microscopic, electron-microscopic and immunocytochemical characteristics of giant cell arteritis (GCA) have been investigated in a number of studies on temporal arteries. Arterial atrophy and calcification of the internal elastic membrane appear to be prerequisites for the evolution of the inflammatory process. Foreign body giant cells form close to calcifications, apparently without connection with other inflammatory cells and probably by the fusion of modified vascular smooth muscle cells. The foreign body giant cells attack the calcifications. Lymphocytes accumulate around them and may be found in pockets in their cell surface. This focal reaction is found in atrophic, calcified arterial segments in a minority of inflamed temporal artery biopsies. More commonly seen is a diffuse mononuclear attack of the vessel wall in atrophic as well as non-atrophic segments which leads to severe arterial dilatation. Langhans giant cells form by the fusion of macrophages in the diffuse inflammatory infiltrate. The fact that the diffusely inflamed arteries are markedly widened compared to the focally inflamed vessels suggests that the inflammatory process starts as a focal foreign body giant cell reaction directed at calcifications which in turn initiates a more diffuse and widespread inflammation.