Human mast cell development in the mouse skin. (A) RT-PCR analysis for tryptase and chymase mRNA expression. The skin of NOG mice 12 weeks after the transplantation of human CD34⁺ cells expressed human mast cell-specific tryptase and chymase mRNA. CB-hCMC indicates cord blood-derived human cultured mast cells. (B-C) Acetone-fixed frozen sections of NOG mouse skin 12 weeks (B) and 20 weeks (C) after the transplantation of human cord blood CD34⁺ cells were stained for human CD45 (red fluorescent with Cy3), mast cells (yellowish green with FITC-avidin), and nuclei (blue with Hoechst 33342). Arrows indicate human CD45⁺ mast cells, which are stained orange. Magnification, × 200. (D-F) Human MC specific chymase⁺ cells in the mouse skin. Acetone-fixed frozen sections of NOG mouse skin 24 weeks after the transplantation were stained with antihuman chymase mAb. Human chymase⁺ cells proliferated focally in the upper dermis (e), represented by the bar bellows, whereas in other lesions on the same samples nonstained granulated cells were located in the upper dermis (f). Magnification, × 12.5 (D) and × 200 (E-F). (G) The number of chymase⁺ human mast cells and nonstained granulated mouse mast cells in NOG mouse skin 24 weeks after the transplantation. The number of human and mouse mast cells supported by mouse dermis was almost identical. Bar graphs display mean ± SD values from 5 different preparations.