Ovarian surface epithelial (OSE) cells in culture. The epithelial origin of primary OSE cultures is demonstrated for one responder (case 2) and one non-responder cultures (case 5). a, cytokeratin staining, b, DAPI counterstain, c, merge of cytokeratin and DAPI staining. Magnifications: 10 × 20. The majority of OSE cultures expressed keratin, confirming their epithelial origin.