Human mast cell development in the mouse lung and gastric stomach. (A) RT-PCR analysis for tryptase and chymase mRNA expression. The lung and gastric stomach of NOG mice 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) Histologic findings for lung (B) and gastric stomach (C). Very small numbers of formalin-resistant toluidine blue⁺ cells appeared in the lung 20 weeks after the transplantation and, in sequential sections, were almost identical to tryptase⁺ cells (arrows). In gastric stomach, formalinresistant toluidine blue⁺ cells were identified in both the mucosa and submucosa. In the acetone-fixed frozen thin sections stained with antichymase mAb, chymase⁺ cells (white arrowheads) were located only in submucosal lesions. Magnification, × 200 (toluidine blue and tryptase) and × 100 (chymase).