Polarity of VWF release. In the TGN, a small proportion of newly synthesized VWF is selected for entry into WPBs. WPB release primarily occurs at the apical side of the cell, either in the absence of a stimulus (basal release) or in the presence of a stimulus (regulated release). Apically released VWF is composed of high-molecular weight (HMW) multimers; regulated release of VWF has been shown to result in the formation of VWF fibers on the surface of endothelial cells. Constitutive released VWF, containing LMW multimers, is targeted to the basolateral side of endothelial cells.