Alkaline phosphatase activity in phagocytizing granulocytes: a backscattered electron imaging study.

The scanning electron microscope (SEM) in the backscattered electron imaging (BEI) mode has been used to detect alkaline phosphatase activity in normal human PMNs using a lead method. Unfortunately commercial scanning electron microscopes do not allow resolution of the small and dispersed lead depositions in the secondary granules. However, polymorphonuclear leukocytes (PMNs) previously incubated with latex particles, show well evident lead precipitates allowing identification of different phases of the phagocytic sequence. In fact the finding of irregular depositions arranged around a central not-emitting zone might be due to the secondary granules gathering near the phagocytic vacuole. Moreover, homogeneous, round, well defined precipitates, having a diameter slightly larger than latex particles, probably corresponding to vacuoles filled with the enzyme after granules coaslescence and fusion.