Platelets secrete lysosmal enzymes during the "platelet release reaction" early in clot formation. This study was undertaken to identify primary lysosomes of platelets and to detemine their origin in megakaryocytes. Using electron microscopy and cytochemistry, we localized two lysosomal enzymes, arylsulfatase and acid phosphatase, in megakaryocytes and platelets of normal and thrombocytopenic rats. In platelets and mature megakaryocytes, reaction product for both enzymes is confined to vesicles measuring 175-250 nm. These vesicles, which are primary lysosmes, first appear in the earliest recognizable megakaryocytes and increase in number during cellular maturation. In immature and maturing megakaryocytes, arylsulfatase and acid phosphatase can also be demonstrated in an organell similar to GERL (Golgi-endoplasmic reticulumlysosome), i.e., single smooth-surfaced cisternal with associated vesicles near the stacked Golgi cisternae. Scant reaction product for acid phosphatase is also sometimes seen in Golgi cisternae and endoplasmic reticulum. No reaction product was found in alpha-granules at any stage of megakaryocyte maturation, nor in alpha- or serotonin granules of platelets. Thus, our findings indicate that the primay lysosomes of megakaryocytes and platelets are small vesicles derived from GERL early in megakaryocyte differentiation. They can be indentified only after cytochemical staining and are distinct from both alpha- and serotonin granules.
M E Bentfeld, D F Bainton