Weibel-Palade bodies

Specialized secretory vesicles found in endothelial cells that line blood vessels throughout the body.

WPb store von Willebrand factor other proteins that participate in inflammation, angiogenesis and tissue repair.

Cigar-shaped, diameter 0.1-0.3 µm and the length of 1-5 µm.

Surrounded by lipid bilayer membrane.

Containing tubules almost entirely of von Willebrand factor a large multimeric protein secreted in response to a number of stimulants.

Von Willebrand factor propeptide acts as a pH depending clamp keeping the von Willebrand factor in tubules within the Weibel-Palade bodies.

Von Willebrand factor is a long fibrillar protein compressed into WPb in a helical array.

Von Willebrand factor compressed into the WPb has a compression factor of approximately 50:1 in terms of length.

Von Willebrand factor and other contents of the Weibel-Palade bodies are surrounded by a membrane protein, P-selectin that binds to white blood cells.

WPb exocytosis caused by thrombin, other agonists injury results in secretion of von Willebrand factor and then binds to endothelial cells were it can attract platelets.

A fusion WPb with the plasma membrane results in an increase in P-selectin concentration, and the lack and mediate increased adhesion leukocytes promoting an inflammatory response.

The density WPb per cell varies among different vascular beds and can affect the likelihood for thrombosis or leukocyte recruitment at those sites.

In some locations such as the lung endothelial cells stored factor VIII along with von Willebrand factor in WPb and both may be secreted.

Desmopressin is used to treat bleeding in patients with mild hemophilia or von Willebrand disease and causes a secretion of both von Willebrand factor and factor VIII from Weibel-Palade bodies.

Abnormalities in the assembly and packing of von Willebrand factor multiper within WPb can cause von Willebrand disease by impaired secretion of the von Willebrand factor even if the total amount of von Willebrand factor synthesized is not decreased significantly.

In severe von Willebrand disease, like that seen in children, no von Willebrand factor is synthesized and WPb are absent, and Factor VIII concentrations are relatively low.

Approximately 95% or more of the proteins in WPb are Von Willebrand factor but other proteins stored there include: Exotaxin-3, endothelin, angiopoietin-2, Rab3D, Rab-27A, Calcitonin, gene-related peptide, endothelin converting enzyme, alpha1,3-fucosyltransferase VI, tissue-type plasminogen activator, osteoprotegrin, P-selectin.

These proteins are released with von Willebrand and can influence angiogenesis and inflammation following vascular injury.

WP secrete contents by 3 types of exocytosis: normal, lingering kiss, and multigranular exocytosis.

Normally the WPb fuses with the plasma membrne and the vWF and is secreted all together with ither proteins.

After fusion with the plasma membrane a ring of actin filiaments and myosin II contracts around the Wibel-Palade bodies and squeezes out its contents.

In the so-called lingering kiss type of exocytosis refers to a transient small pore, 10-12 nm, that forms between the Weibel-Palade body and the plasma membrane and allows exit of cytokines but not vWF or P-selectin.

In the multigranular exocytosis a number of WPb coalesce and they secree their contents all at once.

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