Non-identity of the α-Granules of Human Blood Platelets with Typical Lysosomes

Abstract

PLATELETS are known to be important in blood coagulation, in haemostasis and in thrombosis, and during this process they undergo a series of profound morphological and biochemical changes termed viscous metamorphosis. Platelets contain a wide variety of subcellular organelles^1–5, and some of them are altered during viscous metamorphosis^6,7. The most prominent organelles are the very osmiophilic α-granules, which disappear shortly after the onset of viscous metamorphosis⁶. The α-granules are believed by some to be the chief source of platelet factor 3, the pro-coagulant phospholipoprotein of platelets^4,8,9; others regard them as storage organelles for serotonin^10,11 and adenosine triphosphate (ATP)¹². On the other hand, several authors^13–15 have believed that platelets contain granules with lysosomal activity, and recently Marcus et al.¹⁶ claimed to have identified as lysosomes α-granules isolated from platelet homogenates. Evidence from electron microscopy has encouraged¹⁷ a comparison of the α-granules with the enzyme-secreting vesicles of the pancreas, and similar particles have been described with the same typical striated internal structure in endothelial cells¹⁸. Thus there are discrepancies in the interpretation of the nature and function of the α-granules and a reinvestigation of the problem seemed justified.