A 38-year-old woman is in her first pregnancy, which has been uneventful until the 34th week, when she develops swelling of feet and hands. An obstetric check-up reveals that she also has hypeension and proteinuria. Laboratory analysis shows elevated aspaate aminotransferase (AST) and alanine aminotransferase (ALT) and slightly decreased platelets. The initial event in the pathogenesis of her condition is thought to be which of the following?