[Show abstract][Hide abstract] ABSTRACT: Background:
Extracellular vesicles (EVs) are naturally occurring membrane particles that mediate intercellular communication by delivering molecular information between cells. In this study, we investigated the effectiveness of two different populations of EVs (microvesicle- and exosome-enriched) as carriers of Paclitaxel to autologous prostate cancer cells.
EVs were isolated from LNCaP- and PC-3 prostate cancer cell cultures using differential centrifugation and characterized by electron microscopy, nanoparticle tracking analysis, and Western blot. The uptake of microvesicles and exosomes by the autologous prostate cancer cells was assessed by flow cytometry and confocal microscopy. The EVs were loaded with Paclitaxel and the effectiveness of EV-mediated drug delivery was assessed with viability assays. The distribution of EVs and EV-delivered Paclitaxel in cells was inspected by confocal microscopy.
Our main finding was that the loading of Paclitaxel to autologous prostate cancer cell-derived EVs increasedits cytotoxic effect. This capacity was independent of the EV population and the cell line tested. Although the EVs without the drug increased cancer cell viability, the net effect of enhanced cytotoxicity remained. Both EV populations deliveredPaclitaxel to the recipient cells through endocytosis, leading to the release of the drug from within the cells. The removal of EV surface proteins did not affect exosomes, while the drug delivery mediated by microvesicles was partially inhibited.
Cancer cell-derived EVs can be used as effective carriers of Paclitaxel to their parental cells, bringing the drug into the cells through an endocytic pathway and increasing its cytotoxicity. However,due to the increased cell viability, theuseof cancer cell-derived EVs must be further investigated before any clinical applications can be designed.
[Show abstract][Hide abstract] ABSTRACT: Introduction: The poor penetration of anticancer drugs is a challenge for oncological treatments. Extracellular vesicles (EVs) have gained interest as drug vehicles due to their natural capacity for cargo delivery. In this study, prostate cancer (PCa) cell line-derived EVs were studied as carries of Paclitaxel, an anti-mitotic cancer drug.
Methods: EV populations from LNCaP and PC3 cell lines were isolated by differential centrifugations. EVs were
characterized by TEM, NTA and Western blotting. The importance of EV surface proteins was studied by trypsin treated control EVs. Uptake of DiD-labeled EVs to PCa cells was determined in time by confocal microscopy and quantified by flow cytometry and a novel fluorescence image analysis (FIA). EVs were loaded with 5-50 nM Paclitaxel, measured by spectrophotometry and confirmed with UPLC-MS. The cytotoxic effect of the Paclitaxel-loaded EVs was assessed by AlamarBlue viability assay after 24 and 48 hrs. The cellular localization of the drug and the EVs was examined by live cell confocal microscopy using Oregon green-labeled Paclitaxel and DiD-labeled EVs. Results: The three different methods (microscopy, flow cytometry and FIA) showed similar efficacy of EV uptake during 24 hrs irrespective of the used EV population or the cell line. Significant uptake was observed already by 9 hrs. EVs on their own EVs increased the viability of the PCa cells, whereas the Paclitaxel-loaded EVs had a cytotoxic effect at 48 hrs. This effect was reduced by trypsin digestion of the EVs to remove surface proteins as shown by Western blotting. Interestingly, the localization of the OG-labeled Paclitaxel in the recipient cells differed depending on whether the drug was introduced by the EVs or by media. Summary/Conclusion: Uptake of multiple EV subtypes and origins can be quickly compared with FIA. EVs may be used as an efficient drug delivery system to tumor cells, which is influenced by the EV surface receptors.
[Show abstract][Hide abstract] ABSTRACT: Motivation: Extracellular vesicles are spherical bilayered proteolipids, harboring various bioactive molecules. Due to the complexity of the vesicular nomenclatures and components, online searches for extracellular vesicle-related publications and vesicular components are currently challenging.
Results: We present an improved version of EVpedia, a public database for extracellular vesicles research. This community web portal contains a database of publications and vesicular components, identification of orthologous vesicular components, bioinformatic tools, and a personalized function. EVpedia includes 6,879 publications, 172,080 vesicular components from 263 high-throughput datasets, and has been accessed >65,000 times from >750 cities. In addition, about 350 members from 73 international research groups have participated in developing EVpedia. This free web-based database might serve as a useful resource to stimulate the emerging field of extracellular vesicle research.
Availability and implementation: The web site was implemented in PHP, Java, MySQL and Apache, and is freely available at http://evpedia.info.
[Show abstract][Hide abstract] ABSTRACT: Background
Platelet-derived extracellular vesicles (EVs) participate, for example, in haemostasis, immunity and development. Most studies of platelet EVs have targeted microparticles, whereas exosomes and EV characterization under various conditions have been less analyzed. Studies have been hampered by the difficulty in obtaining EVs free from contaminating cells and platelet remnants. Therefore, we optimized an EV isolation protocol and compared the quantity and protein content of EVs induced by different agonists.
Platelets isolated with iodixanol gradient were activated by thrombin and collagen, lipopolysaccharide (LPS) or Ca2+ ionophore. Microparticles and exosomes were isolated by differential centrifugations. EVs were quantitated by nanoparticle tracking analysis (NTA) and total protein. Size distributions were determined by NTA and electron microscopy. Proteomics was used to characterize the differentially induced EVs.
The main EV populations were 100–250 nm and over 90% were <500 nm irrespective of the activation. However, activation pathways differentially regulated the quantity and the quality of EVs, which also formed constitutively. Thrombogenic activation was the most potent physiological EV-generator. LPS was a weak inducer of EVs, which had a selective protein content from the thrombogenic EVs. Ca2+ ionophore generated a large population of protein-poor and unselectively packed EVs. By proteomic analysis, EVs were highly heterogeneous after the different activations and between the vesicle subpopulations.
Although platelets constitutively release EVs, vesiculation can be increased, and the activation pathway determines the number and the cargo of the formed EVs. These activation-dependent variations render the use of protein content in sample normalization invalid. Since most platelet EVs are 100–250 nm, only a fraction has been analyzed by previously used methods, for example, flow cytometry. As the EV subpopulations could not be distinguished and large vesicle populations may be lost by differential centrifugation, novel methods are required for the isolation and the differentiation of all EVs.
[Show abstract][Hide abstract] ABSTRACT: Introduction: Extracellular vesicles (EVs) have an important role in the intercellular transfer of genetic information. EVs have been shown to contain nucleic acids such as mRNA, microRNAs, ncRNAs and even DNA. However, less is known of the genomic DNA (gDNA) packed into EVs. It is also unknown, whether the gDNA cargo is randomly
sorted to the different EV subpopulations, or if it is preferably packed into specific vesicle types. The aim of this study was to analyse whether different prostate cancer (PCa) cell-derived EV subpopulations (apoptotic bodies, microvesicles and exosomes) carry different gDNA fragments. Methods: EV subpopulations from 3 PCa cell lines
(LNCaP, PC-3 and RC92a/h) were separated by differential ultracentrifugation (1,200�g, 20,000�g and 110,000�g). The different EV subpopulations were verified through transmission electron microscopy and characterized by total protein content and nanoparticle tracking analysis (NTA). gDNA fragments of different genes were
detected by qPCR and confirmed by DNA sequencing. Results: We report that the PCa EV subpopulations were different in terms of total protein and DNA content. Although the particle concentration of microvesicles and exosomes by NTA were similar, the total protein content was significantly different. Particle concentration and total protein content correlated with each other for some, but not all PCa cell-derived microvesicles and exosomes. Analysis of the gDNA content of TP53, PTEN and MLH1 fragments in the EV populations from the different PCa cell lines showed, that different EV subpopulations carry different gDNA content, which could indicate a
selective mechanism of nucleic acid packing depending on the cell and the EV subtype. Summary/conclusion: PCa EV subpopulations carry different gDNA sequences, which could potentially be used as diagnostic and prognostic biomarkers.
[Show abstract][Hide abstract] ABSTRACT: Mesenchymal stromal cells (MSC) are shown to have a great therapeutic potential in many immunological disorders. Currently the therapeutic effect of MSCs is considered to be mediated via paracrine interactions with immune cells. Umbilical cord blood is an attractive but still less studied source of MSCs. We investigated the production of extracellular membrane vesicles (MVs) from human umbilical cord blood derived MSCs (hUCBMSC) in the presence (MVstim) or absence (MVctrl) of inflammatory stimulus.
hUCBMSCs were cultured in serum free media with or without IFN-γ and MVs were collected from conditioned media by ultracentrifugation. The protein content of MVs were analyzed by mass spectrometry. Hypoxia induced acute kidney injury rat model was used to analyze the in vivo therapeutic potential of MVs and T-cell proliferation and induction of regulatory T cells were analyzed by co-culture assays.
Both MVstim and MVctrl showed similar T-cell modulation activity in vitro, but only MVctrls were able to protect rat kidneys from reperfusion injury in vivo. To clarify this difference in functionality we made a comparative mass spectrometric analysis of the MV protein contents. The IFN-γ stimulation induced dramatic changes in the protein content of the MVs. Complement factors (C3, C4A, C5) and lipid binding proteins (i.e apolipoproteins) were only found in the MVctrls, whereas the MVstim contained tetraspanins (CD9, CD63, CD81) and more complete proteasome complex accompanied with MHCI. We further discovered that differently produced MV pools contained specific Rab proteins suggesting that same cells, depending on external signals, produce vesicles originating from different intracellular locations.
We demonstrate by both in vitro and in vivo models accompanied with a detailed analysis of molecular characteristics that inflammatory conditioning of MSCs influence on the protein content and functional properties of MVs revealing the complexity of the MSC paracrine regulation.
[Show abstract][Hide abstract] ABSTRACT: High Mobility Group B1 (HMGB1) is a 30 kDa protein widely expressed in mammalian
cells. HMGB1 has a high content of charged amino acids and has a bipolar structure
consisting of two highly positive amino terminal HMG-box domains and an acidic carboxy
terminal tail. HMGB1 has nuclear functions regulating chromatin structure and gene
expression and extracellular functions regulating immune response and cell motility.
Biochemical and cell biological studies have revealed that HMGB1 binds to various kinds
of biomolecules and these interactions are crucial for determining the in vivo functions of
HMGB1. Albeit several different biochemical methods have been used to detect HMGB1-
binding components, HMGB1-affinity column chromatography has rarely been applied in
such studies. Here, we describe an affinity chromatography method that we have applied
to isolation and identification of HMGB1-binding molecules from different cell types.
Biomolecules recovered with HMGB1-affinity chromatography include proinflammatory
bacterial DNA and glioblastoma cell histones H1 and H3 which all have previously been
reported as HMGB1-binding molecules by other methods. Furthermore, an entirely new
HMGB1-binding protein, Multimerin-1 containing complex, was identified from platelet
lysates by HMGB1-affinity chromatography. Endogenous Multimerin-1 and HMGB1 were
shown to associate on the surface of endothelial cells and activated platelets, and endogenous
Multimerin-1 also regulated the release of HMGB1 from activated platelets. In
conclusion, HMGB1-affinity chromatography can be used to isolate and characterize novel
HMGB1-binding partners from a variety of cellular sources. Such new interactions reveal
further complexity in the multi-faceted biology of the HMGB1.
[Show abstract][Hide abstract] ABSTRACT: Extracellular vesicles (EVs) are membraneous vesicles released by a variety of cells into their microenvironment. Recent studies have elucidated the role of EVs in intercellular communication, pathogenesis, drug, vaccine and gene-vector delivery, and as possible reservoirs of biomarkers. These findings have generated immense interest, along with an exponential increase in molecular data pertaining to EVs. Here, we describe Vesiclepedia, a manually curated compendium of molecular data (lipid, RNA, and protein) identified in different classes of EVs from more than 300 independent studies published over the past several years. Even though databases are indispensable resources for the scientific community, recent studies have shown that more than 50% of the databases are not regularly updated. In addition, more than 20% of the database links are inactive. To prevent such database and link decay, we have initiated a continuous community annotation project with the active involvement of EV researchers. The EV research community can set a gold standard in data sharing with Vesiclepedia, which could evolve as a primary resource for the field.
[Show abstract][Hide abstract] ABSTRACT: Platelets can release a heterogeneous pool of vesicles which include plasma membrane-derived microparticles (PMPs) and multivesicular body-derived exosomes. As both vesicle types are generated upon activation and their distinction is complicated due to an overlap in their molecular properties and sizes, they are best discussed as an entity, the platelet-derived microvesicles (PMVs). PMPs can be formed through several induction pathways, which determine their different molecular profiles and facilitate tailor-made participation in intercellular communication. This dynamic variability may lie behind the multifaceted and sometimes very different observations of the PMPs in physiological and pathological settings. Currently, little is known of platelet-derived exosomes. In all, PMVs not only participate in several homeostatic multicellular processes, such as hemostasis, maintenance of vascular health, and immunity, but they also play a role in thrombotic and inflammatory diseases and cancer progression. In the past few years, the number of original articles and reviews on microvesicles has dramatically increased, but the data simultaneously raise further questions, the answers to which depend on forthcoming analytical improvements. In this article, the differential activation pathways and the molecular and functional properties of PMVs are reviewed in context with their sometimes paradoxical role in health and in disease. Also, the methodological issues of PMV detection and analysis are discussed in the light of recent advances within the field.
No preview · Article · Feb 2012 · Seminars in Thrombosis and Hemostasis
[Show abstract][Hide abstract] ABSTRACT: Comprehensive mapping of protein-binding sites within human collagen III has allowed the recognition motifs for integrin alpha(2)beta(1) and VWF A3 domain to be identified. Glycoprotein VI-binding sites are understood, although less well defined. This information, together with recent developments in understanding collagen fiber architecture, and crystal structures of the receptor collagen-binding domains, allows a coherent model for the interaction of collagen with the platelet surface to be developed. This complements our understanding of the orchestration of receptor presentation by membrane microdomains, such that the polyvalent collagen surface may stabilize signaling complexes within the heterogeneous receptor composition of the lipid raft. The ensuing interactions lead to the convergence of signals from each of the adhesive receptors, mediated by FcR gamma-chain and/or FcgammaRIIa, leading to concerted and co-operative platelet activation. Each receptor has a shear-dependent role, VWF/GpIb essential at high shear, and alpha(2)beta(1) at low and intermediate shear, whilst GpVI provides core signals that contribute to enhanced integrin affinity, tighter binding to collagen and consequent platelet activation.
Full-text · Article · Aug 2007 · Journal of Thrombosis and Haemostasis
[Show abstract][Hide abstract] ABSTRACT: A set of 57 synthetic peptides encompassing the entire triplehelical domain of human collagen III was used to locate binding sites for the collagen-binding integrin alpha(2)beta(1). The capacity of the peptides to support Mg(2+)-dependent binding of several integrin preparations was examined. Wild-type integrins (recombinant alpha(2) I-domain, alpha(2)beta(1) purified from platelet membranes, and recombinant soluble alpha(2)beta(1) expressed as an alpha(2)-Fos/beta(1)-Jun heterodimer) bound well to only three peptides, two containing GXX'GER motifs (GROGER and GMOGER, where O is hydroxyproline) and one containing two adjacent GXX'GEN motifs (GLKGEN and GLOGEN). Two mutant alpha(2) I-domains were tested: the inactive T221A mutant, which recognized no peptides, and the constitutively active E318W mutant, which bound a larger subset of peptides. Adhesion of activated human platelets to GER-containing peptides was greater than that of resting platelets, and HT1080 cells bound well to more of the peptides compared with platelets. Binding of cells and recombinant proteins was abolished by anti-alpha(2) monoclonal antibody 6F1 and by chelation of Mg(2+). We describe two novel high affinity integrin-binding motifs in human collagen III (GROGER and GLOGEN) and a third motif (GLKGEN) that displays intermediate activity. Each motif was verified using shorter synthetic peptides.
No preview · Article · Mar 2006 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Only three recognition motifs, GFOGER, GLOGER, and GASGER, all present in type I collagen, have been identified to date for collagen-binding integrins, such as alpha(2)beta(1). Sequence alignment was used to investigate the occurrence of related motifs in other human fibrillar collagens, and located a conserved array of novel GER motifs within their triple helical domains. We compared the integrin binding properties of synthetic triple helical peptides containing examples of such sequences (GLSGER, GMOGER, GAOGER, and GQRGER) or the previously identified motifs. Recombinant inserted (I) domains of integrin subunits alpha(1), alpha(2) and alpha(11) all bound poorly to all motifs other than GFOGER and GLOGER. Similarly, alpha(2)beta(1) -containing resting platelets adhered well only to GFOGER and GLOGER, while ADP-activated platelets, HT1080 cells and two active alpha(2)I domain mutants (E318W, locked open) bound all motifs well, indicating that affinity modulation determines the sequence selectivity of integrins. GxO/SGER peptides inhibited platelet adhesion to collagen monomers with order of potency F >/= L >/= M > A. These results establish GFOGER as a high affinity sequence, which can interact with the alpha(2)I domain in the absence of activation and suggest that integrin reactivity of collagens may be predicted from their GER content.
No preview · Article · Nov 2004 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The platelet glycoproteins (GPs) Ib, integrin alpha(2)beta(1), and GPVI are considered central to thrombus formation. Recently, their relative importance has been re-evaluated based on data from murine knockout models. To examine their relationship during human thrombus formation on collagen type I fibers at high shear (1000 s(-1)), we tested a novel antibody against GPVI, an immunoglobulin single-chain variable fragment, 10B12, together with specific antagonists for GPIb alpha (12G1 Fab(2)) and alpha(2)beta(1) (6F1 mAb or GFOGER-GPP peptide). GPVI was found to be crucial for aggregate formation, Ca(2+) signaling, and phosphatidylserine (PS) exposure, but not for primary adhesion, even with more than 97% receptor blockade. Inhibiting alpha(2)beta(1) revealed its involvement in regulating Ca(2+) signaling, PS exposure, and aggregate size. Both GPIb alpha and alpha(2)beta(1) contributed to primary adhesion, showing overlapping function. The coinhibition of receptors revealed synergism in thrombus formation: the coinhibition of adenosine diphosphate (ADP) receptors with collagen receptors further decreased adhesion and aggregation, and, crucially, the complete eradication of thrombus formation required the coinhibition of GPVI with either GPIb alpha or alpha(2)beta(1). In summary, human platelet deposition on collagen depends on the concerted interplay of several receptors: GPIb in synergy with alpha(2)beta(1) mediating primary adhesion, reinforced by activation through GPVI, which further regulates the thrombus formation.
[Show abstract][Hide abstract] ABSTRACT: Collagen is a potent adhesive substrate for cells, an event essentially mediated by the integrins alpha 1 beta 1 and alpha 2 beta 1. Collagen fibrils also bind to the integrin alpha 2 beta 1 and the platelet receptor glycoprotein VI to activate and aggregate platelets. The distinct triple helical recognition motifs for these receptors, GXOGER and (GPO)n, respectively, all contain hydroxyproline. Using unhydroxylated collagen I produced in transgenic plants, we investigated the role of hydroxyproline in the receptor-binding properties of collagen. We show that alpha 2 beta 1 but not alpha 1 beta 1 mediates cell adhesion to unhydroxylated collagen. Soluble recombinant alpha 1 beta 1 binding to unhydroxylated collagen is considerably reduced compared with bovine collagens, but binding can be restored by prolyl hydroxylation of recombinant collagen. We also show that platelets use alpha 2 beta 1 to adhere to the unhydroxylated recombinant molecules, but the adhesion is weaker than on fully hydroxylated collagen, and the unhydroxylated collagen fibrils fail to aggregate platelets. Prolyl hydroxylation is thus required for binding of collagen to platelet glycoprotein VI and to cells by alpha 1 beta 1. These observations give new insights into the molecular basis of collagen-receptor interactions and offer new selective applications for the recombinant unhydroxylated collagen I.
[Show abstract][Hide abstract] ABSTRACT: The collagen-platelet interaction is central to haemostasis and may be a critical determinant of arterial thrombosis, where subendothelium is exposed after rupture of atherosclerotic plaque. Recent research has capitalized on the cloning of an important signalling receptor for collagen, glycoprotein VI, which is expressed only on platelets, and on the use of collagen-mimetic peptides as specific tools for both glycoprotein VI and integrin alpha 2 beta 1. We have identified sequences, GPO and GFOGER (where O denotes hydroxyproline), within collagen that are recognized by the collagen receptors glycoprotein VI and integrin alpha 2 beta 1 respectively, allowing their signalling properties and specific functional roles to be examined. Triple-helical peptides containing these sequences were used to show the signalling potential of integrin alpha 2 beta 1, and to confirm its important contribution to platelet adhesion. Glycoprotein VI appears to operate functionally on the platelet surface as a dimer, which recognizes GPO motifs that are separated by four triplets of collagen sequence. These advances will allow the relationship between the structure of collagen and its haemostatic activity to be established.
Preview · Article · Feb 2003 · Biochemical Society Symposium
[Show abstract][Hide abstract] ABSTRACT: The purpose of this study was to determine if the results obtained in platelet function tests and whole blood perfusions are associated with those in platelet function analyser (PFA)-100. We used collagen type I monomers and fibrils to analyse the distinct roles of glycoprotein (GP) Ia/IIa and other collagen receptors in flowing blood under a high shear rate (1600/s) and in aggregation studies. Also, anticoagulation [citrate vs. D-phenylalanyl-1-prolyl-1 arginine chloromethyl ketone (PPACK)] was varied to enhance the functions of GP Ia/IIa, since it has been shown that the cation-poor environment of citrated blood impairs GP Ia/IIa-dependent platelet recruitment. Large interindividual variability (45-fold) was detected in deposition of platelets in whole blood perfusions over collagen monomers, whereas this variation was only fourfold in fibrils. In PFA, this variation was reduced to 2.5-fold. However, platelet deposition on monomers is associated with epinephrine-enhanced PFA (r=-.49, P<.03), whereas platelet deposition on fibrils is correlated with adenosine diphosphate (ADP)-enhanced PFA (r=-.47, P<.05), suggesting a distinct synergism between epinephrine and monomers (GP Ia/IIa) as well as ADP with fibrils (other collagen receptors). Donors with 807 C/C polymorphism of GP Ia (n=14) had longer lag phase in aggregation experiments compared with C/T (n=7) both by monomers and fibrils (P<.04), but these polymorphisms with their mild impact on GP Ia/IIa activity did not markedly differ in other tests. In conclusion, the results obtained in perfusion studies and PFA experiments correlated, but PFA fails to reveal the large-scale variability related to collagen-induced platelet responses.
No preview · Article · Aug 2001 · Thrombosis Research
[Show abstract][Hide abstract] ABSTRACT: In the final stages of activation, platelets express coagulation-promoting activity by 2 simultaneous processes: exposure of aminophospholipids, eg, phosphatidylserine (PS), at the platelet surface, and formation of membrane blebs, which may be shed as microvesicles. Contact with collagen triggers both processes via platelet glycoprotein VI (GPVI). Here, we studied the capacity of 2 GPVI ligands, collagen-related peptide (CRP) and the snake venom protein convulxin (CVX), to elicit the procoagulant platelet response. In platelets in suspension, either ligand induced full aggregation and high Ca(2+) signals but little microvesiculation or PS exposure. However, most of the platelets adhering to immobilized CRP or CVX had exposed PS and formed membrane blebs after a prolonged increase in cytosolic [Ca(2+)](i). Platelets adhering to fibrinogen responded similarly but only when exposed to soluble CRP or CVX. By scanning electron microscopic analysis, the bleb-forming platelets were detected as either round, spongelike structures with associated microparticles or as arrays of vesicular cell fragments. The phosphorylation of p38 mitogen-activated protein kinase (MAPK) elicited by CRP and CVX was enhanced in fibrinogen-adherent platelets compared with that in platelets in suspension. The p38 inhibitor SB203580 and the calpain protease inhibitor calpeptin reduced only the procoagulant bleb formation, having no effect on PS exposure. Inhibition of p38 also downregulated calpain activity. We conclude that the procoagulant response evoked by GPVI stimulation is potentiated by platelet adhesion. The sequential activation of p38 MAPK and calpain appears to regulate procoagulant membrane blebbing but not PS exposure.
No preview · Article · May 2001 · Arteriosclerosis Thrombosis and Vascular Biology
[Show abstract][Hide abstract] ABSTRACT: Visualisation of the procoagulant transformation of human platelets has recently become possible through use of an in vitro approach combined with fluorescence and phase contrast microscopy. Here, we extended these studies to the ultrastructural level by employing both rapid freezing/freeze-substitution and conventional ambient-temperature chemical fixation for transmission and scanning electron microscopy. Procoagulant transformation was only inducible by adhering platelets to collagen fibrils or to the collagen-related peptide and exposing them to physiological extracellular Ca2+ levels. Under these conditions prominent, 2- to 4-m-wide balloon-like structures were regularly observed, regardless of the specimen fixation protocol. In strong contrast to normal platelets in their vicinity, the balloons' subcellular architecture proved remarkably poor: dilute cytoplasm, no cytoskeleton, only a few, randomly distributed organelles and/or their remnants. Cryofixed balloons displayed intact and smooth surfaces whereas conventional specimen processing caused plasma membrane perforations and shrinkage of the balloons. Our results clearly show that neither the balloons themselves, nor their simple ultrastructure reflect fixation artefacts caused by inadequate membrane stabilisation. The balloons are interpreted as to be transformed and/or fragmented procoagulant platelets. Thus, the generation of balloons represents a genuine, final stage of platelet ontogenesis, presumably occurring alternatively to aggregate formation.