Human neutrophils contain a novel intracellular compartment that is distinct from the previously characterized azurophil and specific granules. This compartment is distinguished by the presence of cytochemically detectable alkaline phosphatase activity. The alkaline phosphatase-containing compartments are short rod-shaped organelles that rapidly undergo a dramatic reorganization upon cell stimulation with either a chemoattractant or an active phorbol ester. Biochemical analysis shows that in unstimulated neutrophils the majority of the alkaline phosphatase activity is intracellular, but after stimulation essentially all of this activity becomes associated with the cell surface. The exocytotic pathway is unusual in that these small organelles fuse to form elongated tubular structures before their association with the plasmalemma.

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