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T he mechanism that may cause degenerative fibrotic skin lesions was studied in situ using skin biopsies from patients with systemic sclerosis (SSc), localized scleroderma, or keloids, and at the initial disease stage in the University of California at Davis (UCD) lines 200/206 chickens, which develop a hereditary systemic connective tissue disease resembling human SSc and permit study of disease stages not accessible in humans. Frozen skin sections were analyzed simultaneously for apoptosis by terminal deoxynucleotidyl transferase-mediated FITC-dUTP nick end labeling and indirect immunofluorescence staining of cell markers with tetramethylrhodamine isothiocyanate conjugates. The results showed that endothelial cells are clearly the first cells to undergo apoptosis in the skin of UCD-200/206 chickens, a process that seems to be induced by anti-endothelial cell antibodies. In human fibrotic skin diseases, apoptotic endothelial cells could only be detected in early inflammatory disease stages of SSc and localized scleroderma.