Acid phosphatase demonstrated ultrastructurally in mast cell granules altered by pinocytosis

Rat peritoneal mast cells concentrate colloidal gold particles from the peritoneal fluid in subsurface vacuole-like spaces from which the particles are pinocytosed into small vesicles, into cytoplasmic granules that appear smaller than the specific granules and possess in homogeneous content, and into the specific cytoplasmic granules. Acid phosphatase activity was not consistently or unequivocally demonstrated in the specific granules of mast cells unexposed to colloidal gold. However, mast cells pinocytosing gold exhibited strong activity almost exclusively in the infrequent specific granules which contained gold particles, further indicating that these granules function as heterophagic secondary lysosomes. The pinocytic vesicles and small granules containing endocytosed gold spherules lacked acid phosphatase and apparently derived from an exogenous rather than a Golgi source. The small gold-laden granules appeared, therefore, not to represent an immature or precondensation stage of the specific cytoplasmic granule.