The release of secretory vesicle in encysting Giardia lamblia.

Giardia is an intestinal parasite that undergoes adaptation for survival outside the host. It secretes an extracellular cyst wall using a poorly understood process. An encystation-specific secretory vesicle (ESV) was previously described containing cyst wall proteins. The process of release of these vesicles has been suggested to occur after fragmentation of large ESV in small secretory vesicles, followed by exocytosis, but it was not demonstrated. The release of the ESV was studied by transmission electron microscopy. It was observed: (1) the moment of vesicle release; (2) that a large vesicle is exocytosed and does not fragment into small vesicles; (3) membrane fusion is distinct from traditional exocytosis since it is incomplete; (4) the occurrence of membrane fragmentation and that those membranes reseal to form ghosts; (5) these membrane ghosts may be endocytosed, adhered to flagellar surface or/and form empty vesicles in the extracellular medium.