Gray platelet syndrome: Immunoelectron microscopic localization of fibrinogen and von Willebrand factor in platelets and megakaryocytes

Abstract

An immunogold method was used for investigating the subcellular localization of von Willebrand factor (vWF) and fibrinogen (Fg) in platelets and cultured megakaryocytes from normal subjects and from three patients with the gray platelet syndrome (GPS), a rare congenital disorder characterized by the absence of a-granules. In normal platelets at rest, vWF was detected exclusively in a-granules, with a characteristic distribution: gold particles were localized at one pole of each labeled granule, outlining the inner face of its membrane. vWF was distributed similarly in the a-granules of megakaryocytes at day 12 of culture, where it was also found in small vesicles near the Golgi complex. In contrast, Fg was observed in the whole matrix of all platelet a-granules but not in the nucleoids. In platelets from three patients with GPS, vWF and Fg were distributed homogeneously in the rare normal a-granules, which could be recognized by their size, and also in small granules identified as abnormal a-granules, which were similar in size to the small, possibly immature granules present in normal megakaryocytes. In addition, in some unstimulated platelets, Fg labeling was associated with dense material in the lumen of the surface-connected canalicular system (SCCS). At day 12 of culture, megakaryocytes from the patients with GPS contained some small a-granules labeled for Fg and vWF identical to those found in mature platelets. The majority of a-granules of normal size appeared partially or completely empty. Thus, we conclude that vWF is distributed differently from Fg in normal a-granules, and that unstimulated platelets from patients with GPS contain Fg and vWF in a population of small granules identifiable as abnormal a-granules only by immunoelectron microscopy. In addition, the presence of Fg in the SCCS of gray platelets suggests a spontaneous release of the a-granule content.