Immune cytochemical detection of MMP-9 protein in bone marrow. Bone marrow biopsies from a healthy donor (A-D) or MDS patients (E-H) were fixed in B5 fixative, antigen-retrieved, and incubated with a monoclonal antibody against MMP-9 (clone REGA 2D9; B-H) or an isotype-matched control antibody (A). The bound antibody was detected with HRP-conjugated secondary antibodies and Vector ABC stain kit. Positive staining of MMP-9 is brown; nuclei counterstained with hematoxylin are blue. Mature myeloid cells are MMP-9⁺ in both the healthy donor and MDS patients. Fibroblastic cells (stroma) were negative (nuclei of the fibroblasts are indicated by arrowheads in panel D). Megakaryocytes, evident in panel H, are also negative, as are endothelial cells and blood cells in sinusoids shown in panel E. Specific patient samples shown include: E, patient no. 9; F, patient no. 20; G, patient no. 12; H, patient no. 22 (Table S1 provides patient data). The images were captured and flattened prior to leveling contrast and brightness. Original objective × 40 for panels A-B; × 100 for panels C-H.