Induction of GPIb/IX-vWF receptor-ligand translocation on surface-activated platelets

Platelets are covercytes, not phagocytes: Uptake of bacteria involves channels of the open canalicular system

Recent studies have suggested that uptake of bacteria by human platelets is more similar to ingestion of the same organisms by neutrophils than previously considered. Platelet pseudopods surround a bacterium by a process of circumferential adherence until the organism is fully enclosed in a vacuole consisting of internalized extracellular space surrounded by plasma membrane. The surface connected, open canalicular system (OCS) is not involved in the process of platelet phagocytosis. The present investigation has reviewed the literature on platelet-particle and platelet-bacterial interaction, and added a new study of the process of bacterial engulfment employing tannic acid as an electron dense tracer. The osmium black reaction product formed by tannic and osmic acids during fixation stained the glycocalyx on the platelet exterior surface and lining channels of the OCS. It also stained the vacuoles containing bacteria in platelets as well as channels of the OCS connected to the engulfment vacuole and exterior surface of the cell. Alpha granules releasing their contents into the OCS and into engulfment vacuoles were also stained by osmium black. The only way that tannic acid could reach and stain the interior surface of engulfment vacuoles during fixation was through patent channels of the OCS. In contrast, neutrophils do not contain an OCS, and phagocytic vacuoles containing organisms are never stained by tannic acid. Thus the platelet is a covercyte, not a phagocyte, and uptake of the bacteria does involve channels of the OCS.

Platelet pathology in carriers of the X-linked GATA-1 macrothrombocytopenia

Previous investigations from our laboratory have characterized the ultrastructural and cytochemical pathology of platelets in male members of a family with X-linked GATA-1 G208S macrothrombocytoenia. A large proportion of their circulating platelets were hypogranular macrothrombocytes, resembling cells from patients with the Gray Platelet Syndrome. However, most of the GATA-1 macrothrombocytes contained some alpha granules, and a small number had as many as are present in normal platelets. GATA-1 macrothrombocytes also contained tubular inclusions formed from elements of the dense tubular system wrapped around each other like scrolls. Many macrothrombocytes contained flat tubular membrane sheets connected to channels of the open canalicular system, platelets in platelets and platelets attached to platelets forming very large macrothrombocytes. The present study has examined one obligate and three potential female carriers in this family. Thin sections of their platelets examined in the electron microscope revealed features consistent with the pathology observed in male family members. Most of their platelets were normal-sized, discoid cells containing the usual complement of alpha and delta storage organelles and channels of the dense tubular system and OCS. However, a significant number of giant platelets containing the usual frequency of alpha and delta granules and hypogranular and agranular giant platelets were observed. The frequency of the macrothrombocytes varied in each of the four women studied, but were present in all. The ability of their platelets to bind multimers of vWF, in contrast to male family members, did not differ from normal controls. Near normal as well as normal platelet counts and the ability of their platelets to bind vWF multimers may protect them from the serious bleeding problems of males with the X-linked GATA-1 G208S mutation. Our findings indicate that obligate female carriers of the GATA-1 gene can be detected by examination of their platelets in the electron microscope and distinguished from the pathology of other giant platelet disorders.

Platelet pathology in sex-linked GATA-1 dyserythropoietic macrothrombocytopenia II. Cytochemistry

A previous investigation detailed the pathology of platelets in a family with the X-linked GATA-1 G208S mutation causing dyserythropoiesis and megathrombocytopenia. The present study has used ultrastructural immunocytochemistry, cytochemistry, and tannic acid staining to answer questions raised in the original investigation. Earlier studies, as well as ours, had shown that GATA-1 megathrombocytes are hypogranular, but did not definitively determine which organelles are decreased. Cytochemical localization of aryl sulfatase revealed that lysosomes were present in normal numbers, and the whole mount technique showed a normal frequency of dense bodies rich in arlenine nucleotides and serotonin. Thus alpha granules were the only organelles deficient in GATA-1 platelets. Tannic acid staining confirmed that the membranes wrapped around each other to form tubular inclusions come from elements of the dense tubular system. The unique tubular membrane inclusions in GATA-1 megathrombocytes, thought originally to derive from endoplasmic reticulum in the parent cell, were shown to be in direct continuity with elements of the surface connected open canalicular system (OCS), and to drive from the demarcation membrane system (DMS) of the megakaryocyte. Platelets in platelets and platelets in platelets in platelets were independent cells, and not derived by cytoplasmic sequestration in the enclosing macrothrombocytes. Fully spread GATA-1 platelets incubated with fibrinogen coated gold (Fgn/Au) particles before or after fixation bound as many Fgn/Au particles as normal spread platelets and moved the Fgn/Au- GPIIb/IIIa complexes from peripheral margins to cell centers and into channels of the OCS as efficiently. Exposure of spread normal platelets to bovine vWF resulted in coverage of the surface from edge to edge with multimers detected by anti-vWF antibody and protein A gold. Spread GATA-1 platelets bound very few vWF multimers, which were much smaller in size than those on normal spread cells, but were able to move then to cell centers. These findings support the concept that GATA-1 platelets are macrothrombocytes because they are not able to detach normally from each other during separation from megakaryocyte proplatelets. The marked decrease in the number and abnormal distribution of GPIb/IX receptors may play a role in GATA-1 megathrombocyte formation.