Mast cell granules in skin biopsies of rhSCF-injected sites from patients who had received rhSCF for 6 days (A and B; obtained 2.5 hours after the SC injection of 5 μg/kg of rhSCF in patient no. 2) or 13 days (C and D, obtained 2 hours after the SC injection of 5 μg/kg of rhSCF in patient no. 1). The photomicrographs illustrate the ultrastructure of electron-dense granules in nonsecretory mast cells. Parallel crystalline arrays, oriented either in several directions (A) or all in one direction (C), are typical for the cytoplasmic granules of human skin mast cells. The DAO-gold staining (A) was largely abrogated by absorption of the DAO-gold with solid-phase histamine before staining (C). The granule in (B) is filled with a membranous scroll pattern that is also heavily labeled with the DAO-gold. The DAO-gold staining is not seen in the scroll granule in (D) from a section that was digested on the grid with DAO before DAO-gold staining. Original magnifications: (A) ×110,000; (B) ×136,500; (C) ×93,000; and (D) ×72,000.