mAb CTA 157-2 staining of the endothelial lining of proliferating collateral vessels but not of control vessels ((a) and (c) parallel sections of proliferating collateral vessel; (a) BrdU staining (green fluorescence), (c) mAb CTA 157-2 staining (green fluorescence); (b) and (d) parallel sections of control vessel; (b) absence of BrdU staining (green fluorescence), (d) absence of mAb CTA 157-2 staining (green fluorescence); scale bar 40 μm). All slides were counterstained with propidium iodide as nuclear staining (red fluorescence). (e) Flow cytometric analysis of nonpermeabilized VR-EPCs after staining with mAb CTA 157-2 demonstrating specific extracellular staining of all cells (filled curve). Open curve shows staining with control antibody. (f) Fluorescence microscopy of BrdU treated cells stained with CTA 157-2 (red fluorescence) before permeabilization followed by permeabilization and staining of BrdU (green fluorescence) demonstrating focal distribution of the antigen on the cell membrane of proliferating VR-EPCs. (g) Light microscopic image of a vascular resident progenitor cell with immunohistochemical staining for total (intra- and extracellular) 20S proteasome (brown color; hematoxylin counterstain: blue): most 20S proteasome is found intracellularly, predominantly, and perinuclearly. (h) Light microscopic image of total 20S proteasome (brown color; hematoxylin counterstain: blue): most total (presumably intracellular) 20S proteasome is found in perinuclear cells.