Platelet Alpha-granular Fibrinogen

Platelets contain approximately 3% of the total circulating pool of fibrinogen (fg). Immunofluorescence and subcellular fractionation reveal that platelet fg is stored within intracellular structures which have been confirmed as α-granules by immunoelectronmicroscopy. The nature, origin(s) and possible function(s) of α-granular fg have long been a point of controversy. Many early investigations into the properties of platelet fg were constrained by proteolysis, plasma fg contamination and lack of knowledge of γ-chain heterogeneity. Later studies on the biochemistry and origins of α-granular proteins resulted in the general consensus that platelet fg was not only identical to the major form within plasma but was almost certainly biosynthesised within the platelet precursor cell, the megakaryocyte (MK). However, the recent discovery of endocytic and pinocytic pathways within both MKs and platelets has requestioned the origin of many α-granular proteins, including fg, resulting in reappraisal of early data, and a series of experiments which culminated in the study of fg mRNA expression in purified MKs and platelets. These studies suggest that α-granular fg is derived from the plasma pool via a Gp IIb/IIIa mediated endocytic mechanism.