Skin biopsies from sites that had been injected with rhSCF, obtained from patients no. 2 and no. 3 who had received rhSCF for 13 days (A) or 1 day (B through D), respectively. The biopsy in (A) was obtained 1 hour 45 minutes after the SC injection of 5 μg/kg of rhSCF, whereas the biopsy in (B), (C), and (D) was obtained 1 hour 40 minutes after the SC injection of 25 μg/kg of rhSCF. A large amount of DAO-gold label is present on the electron-dense contents of cytoplasmic granules in the mast cell in (A; solid arrowhead); the cell exhibits evidence of secretory activity, and an extruded, non–membrane-bound granule with altered matrix (which is located external to the plasma membrane) retains very little DAO-gold label for histamine (open arrowhead). However, DAO-gold heavily labels adjacent collagen fibers, which are seen in cross-section (arrows). In (B), multiple extruded, membrane-free mast cell granules (G), which exhibit altered matrix materials, are present in the interstitium near the degranulating mast cell; these extruded granules exhibit relatively little DAO-gold labeling for histamine, except in regions of the granules that contain the most electron-dense matrix material (solid arrowheads). In (C), the DAO-gold was absorbed with solid-phase histamine before being used to stain the specimen. Nearly all DAO-gold staining of intracellular mast cell granules (arrowhead), as well as of the extruded, membrane-free granules (G), was abolished by this specificity control. (D) shows that the extensive fibrin deposits in the extracellular space in this biopsy are heavily labeled with DAO-gold. Original magnifications: (A) ×52,500; (B) ×28,000; (C) ×46,500; and (D) ×45,000.