CD45 and α-SMA expression on BM-CFU-Fs and colocalization of BM-CFU-Fs with CD34⁺ cells. (A) Human bone marrow paraffin sections were stained with antibodies against CD271 and CD45. White arrow indicates a vessel with perivascular CD271⁺/CD45⁻ cells (left). White arrowheads indicate round, hematopoietic CD271^low/CD45⁺ cells (right). Bone-lining cells in the right photograph are CD271⁺/CD45⁻. (B) A larger vessel costained with CD271 and α-SMA is shown. White arrowheads indicate CD271⁺/α-SMA^low perivascular cells. (C-D) Bone marrow sections were stained against CD34 to detect hematopoietic stem/progenitor cells and against CD271 to detect CFU-Fs. (C) Round CD34⁺ cells (green) were found in proximity to the reticular extensions of perivascular CD271⁺ cells (red). (D) In rare instances, round CD34⁺ cells (green) were found neighboring CD271⁺ cells in endosteal regions. White arrowheads indicate CD34⁺ cells. Nuclei were stained with TO-PRO3 (blue). Scale bars indicate 25 μm. Asterisks indicate the lumen of a vessel. Trabecular bone (b) is indicated. Photographs were taken with a confocal microscope (DMRE; Leica). For the expression of α-SMA and perivascular localization of CD271⁺ cells, see supplemental Figure 5.