Scanning electron microscopy of mast cell degranulation.

Scanning and transmission electron microscopy were used to study the morphologic changes occurring in isolated mast cells after stimulation by compound 48/80. Two approaches were used to obtain cells for scanning electron microscopy. Cells incubated in suspension were collected on Millipore filters and air-dried from isopropyl alcohol. Other cells were allowed to attach to cover slips, stimulated, fix in situ, and dried from Freon at its critical point. Resting mast cells prepared by the latter method were covered by a tracery of shelflike ridges, whereas cells prepared by the first method had no ridges but presented a bumpy surface disclosing the underlying granules. Stimulation by compound 48/80 caused marked changes in the surface configuration of the cells, with loss of the regular pattern of ridges, extrusion of granules, and the appearance of openings leading to the interior of the cell. Correlation with transmission electron microscopy indicates that these openings lead to channels containing granules and lined by an intact membrane. These openings give extracellular cations access to granules within the confines of the cell, where the cations may exchange for histamine.