The cytoplasm of acinar cells shows numerous electron dense zymogen granules (a), which are positive for trypsin (b). Some cells showed in the cytoplasm, zymogen granules and alpha-type secretory granules (c), with typical electron dense core and clear halo (inset). Double label electron microscopy immunocytochemistry (d) demonstrated that alpha-type secretory granules were glucagon-positive (glucagon immunoreactivity identified with 12 nm colloidal gold), while zymogen granules were positive for trypsin (trypsin-immunoreactivity is evidenced with 18 nm colloidal gold). Moreover, other cells presented in the cytoplasm zymogen granules were crystalline beta-type secretory granules (some indicated with arrows) (e). Double label electron microscopy immunocytochemistry (f) demonstrated that beta-type secretory granules were insulin-positive (insulin immunoreactivity identified with 12 nm colloidal gold), while zymogen granules were positive for trypsin (trypsin-immunoreactivity is evidenced with 18 nm colloidal gold). In tissues processed for electron microscopy immunocytochemistry (embedded in London White Resin, without post-fixation in 1% osmium tetroxide) zymogen granules appear clearer than that observed in tissue processed for conventional electron microscopy.