A 22-year-old woman has experienced increasing dyspnea for the past 2 months. On physical examination, she is afebrile and normotensive. Inspiratory wheezes are noted on auscultation of the chest. A chest CT scan shows an 8x10 cm posterior mediastinal mass that impinges on the trachea and esophagus. A mediastinoscopy is performed, and a biopsy of the mass microscopically shows scattered large multinucleated cells, with prominent nucleoli that mark with CD15, and lymphocytes and macrophages separated by dense collagenous bands. Which of the following cells is most likely to be seen microscopically in this biopsy specimen?