Cell cluster staining for Sox9, Stro-1, and collagen type II. (a) Cells in clusters can be negative (white arrow) for Sox9 or show cytoplasmic and/or nuclear staining (black arrow) (10×). (b) Double staining with Stro-1 (brown) and Sox9 (red) indicate cells that are single (white arrow) or double positive (black arrow) (40×). (c) Collagen type II and Sox9 immunostaining of osteoarthritis (OA) cartilage. Clusters are surrounded by collagen type II matrix and not all cells in these clusters are Sox9 positive (black arrow positive; white arrow negative) (10× and 40×).