A filamentous network surrounding secretory granules from mast cells.

A freezing-thawing method for rupturing mast cells has made it possible to show that each of the secretory granules is surrounded by a regular network of filaments forming a basket-like structure. The filaments attach to the membrane with hook-like structures. Immunoelectron microscopy of intact mast cells shows that actin occurs mainly in the subplasmalemmal space, whereas in ruptured cells it is seen around granules, indicating that the granule network might consist of actin.