Dissociated human foreskin mast cells degranulate in response to anti-IgE and substance P

Human skin mast cells release histamine in response to both immunologic stimulation mediated by anti-IgE and IgE-independent mechanisms of which substance P is a prototypical secretagogue. We compared the ultrastructural changes produced in dissociated foreskin mast cells by these two stimuli with histamine release. Mast cells were isolated and pooled from the foreskins of 2- to 7-year-old boys in four separate experiments and comprised 25 to 60% of the total dissociated cells. The secretory granules in resting mast cells comprised 47.5% of the extranuclear cell volume and contained crystalline structures, namely, scrolls, gratings, and lattices, in an electron-dense matrix. Stimulation with either anti-IgE or substance P resulted in a net histamine release of 10.2 +/- 1.7% or 21.4 +/- 4.0%, respectively. After either secretagogue, about 75% of the cells underwent compound exocytosis, with fusion of the granule membranes with one another and with the plasma membrane to produce large degranulation channels that opened to the extracellular space. The granules lost their crystalline structure and electron density during secretion but retained the round shape of the original granule as a core that subsequently formed a fibrillar residue. Degranulation channels occupied 30 to 60% of the cytoplasmic volume after substance P stimulation and 10 to 40% after anti-IgE, which compared well with the greater histamine release measured after substance P. The rapid increase in the volume of the degranulation channels after substance P was accompanied by a decrease in cytoplasmic volume, suggesting water moved from the cytoplasm into the granules after stimulation. This study shows that secretion produced in dissociated human foreskin mast cells by two different stimuli, anti-IgE and substance P, which act through different membrane receptors and have distinct secretory characteristics, is similar morphologically.